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Title: The inventions, researches and writings of Nikola Tesla

Author: Thomas Commerford Martin

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*** START OF THE PROJECT GUTENBERG EBOOK THE INVENTIONS, RESEARCHES
AND WRITINGS OF NIKOLA TESLA ***

THE INVENTIONS
RESEARCHES AND WRITINGS
OF

NIKOLA TESLA

TO HIS COUNTRYMEN
IN EASTERN EUROPE THIS RECORD OF
THE WORK ALREADY ACCOMPLISHED BY

NIKOLA TESLA
IS RESPECTFULLY DEDICATED

THE INVENTIONS

RESEARCHES AND WRITINGS
OF

Nikola Tesla
WITH SPECIAL REFERENCE TO HIS WORK IN
POLYPHASE
CURRENTS AND HIGH POTENTIAL LIGHTING

BY

THOMAS COMMERFORD MARTIN
Editor The Electrical Engineer; Past-President
American Institute Electrical Engineers

1894
THE ELECTRICAL ENGINEER
NEW YORK
D. VAN NOSTRAND COMPANY,
NEW YORK.

Entered according to Act of Congress in the year 1893 by
T. C. MARTIN
in the office of the Librarian of Congress at Washington

Press of McIlroy & Emmet, 36 Cortlandt St., N. Y.

PREFACE.
The electrical problems of the present day lie largely in the economical transmission of
power and in the radical improvement of the means and methods of illumination. To many
workers and thinkers in the domain of electrical invention, the apparatus and devices that are
familiar, appear cumbrous and wasteful, and subject to severe limitations. They believe that the
principles of current generation must be changed, the area of current supply be enlarged, and the
appliances used by the consumer be at once cheapened and simplified. The brilliant successes of
the past justify them in every expectancy of still more generous fruition.
The present volume is a simple record of the pioneer work done in such departments up to
date, by Mr. Nikola Tesla, in whom the world has already recognized one of the foremost of
modern electrical investigators and inventors. No attempt whatever has been made here to
emphasize the importance of his researches and discoveries. Great ideas and real inventions win
their own way, determining their own place by intrinsic merit. But with the conviction that Mr.
Tesla is blazing a path that electrical development must follow for many years to come, the
compiler has endeavored to bring together all that bears the impress of Mr. Tesla's genius, and is
worthy of preservation. Aside from its value as showing the scope of his inventions, this volume
may be of service as indicating the range of his thought. There is intellectual profit in studying
the push and play of a vigorous and original mind.
Although the lively interest of the public in Mr. Tesla's work is perhaps of recent growth,
this volume covers the results of full ten years. It includes his lectures, miscellaneous articles
and discussions, and makes note of all his inventions thus far known, particularly those bearing
on polyphase motors and the effects obtained with currents of high potential and high frequency.
It will be seen that Mr. Tesla has ever pressed forward, barely pausing for an instant to work out
in detail the utilizations that have at once been obvious to him of the new principles he has
elucidated. Wherever possible his own language has been employed.
It may be added that this volume is issued with Mr. Tesla's sanction and approval, and that
permission has been obtained for the re-publication in it of such papers as have been read before
various technical societies of this country and Europe. Mr. Tesla has kindly favored the author
by looking over the proof sheets of the sections embodying his latest researches. The work has
also enjoyed the careful revision of the author's friend and editorial associate, Mr. Joseph
Wetzler, through whose hands all the proofs have passed.
December, 1893.
T. C. M.

CONTENTS.

PART I.
POLYPHASE CURRENTS.

CHAPTER I.
Biographical and Introductory. 3

CHAPTER II.
A New System of Alternating Current Motors and Transformers. 7

CHAPTER III.
The Tesla Rotating Magnetic Field.—Motors with Closed
Conductors.—Synchronizing Motors.—Rotating Field 9
Transformers.

CHAPTER IV.
Modifications and Expansions of the Tesla Polyphase Systems. 26

CHAPTER V.
Utilizing Familiar Types of Generators of the Continuous Current
31
Type.

CHAPTER VI.
Method of Obtaining Desired Speed of Motor Or Generator. 36

CHAPTER VII.
Regulator for Rotary Current Motors. 45

CHAPTER VIII.
Single Circuit, Self-starting Synchronizing Motors. 50

CHAPTER IX.
Change from Double Current to Single Current Motors. 56

CHAPTER X.
Motor with "Current Lag" Artificially Secured. 58

CHAPTER XI.
Another Method of Transformation from a Torque to a
62
Synchronizing Motor.

CHAPTER XII.
"Magnetic Lag" Motor. 67

CHAPTER XIII.
Method of Obtaining Difference of Phase by Magnetic Shielding. 71

CHAPTER XIV.
Type of Tesla Single-Phase Motor. 76

CHAPTER XV.
Motors With Circuits of Different Resistance. 79

CHAPTER XVI.
Motor with Equal Magnetic Energies in Field and Armature. 81

CHAPTER XVII.
Motors with Coinciding Maxima of Magnetic Effect in Armature and
83
Field.

CHAPTER XVIII.
Motor Based on the Difference of Phase in the Magnetization of the
88
Inner and Outer Parts of an Iron Core.

CHAPTER XIX.
Another Type of Tesla Induction Motor. 92

CHAPTER XX.
Combinations of Synchronizing Motor and Torque Motor. 95

CHAPTER XXI.
Motor with a Condenser in the Armature Circuit. 101

CHAPTER XXII.
Motor with Condenser in One of the Field Circuits. 106

CHAPTER XXIII.
Tesla Polyphase Transformer. 109

CHAPTER XXIV.
A Constant Current Transformer with Magnetic Shield Between
113
Coils of Primary and Secondary.

PART II.
THE TESLA EFFECTS WITH HIGH FREQUENCY AND HIGH

POTENTIAL CURRENTS.

CHAPTER XXV.
Introductory.—The Scope of The Tesla Lectures. 119

CHAPTER XXVI.
The New York Lecture. Experiments with Alternate Currents of
Very High Frequency, and Their Application to Methods of Artificial 145
Illumination, May 20, 1891.

CHAPTER XXVII.
The London Lecture. Experiments with Alternate Currents of High
198
Potential and High Frequency, February 3, 1892.

CHAPTER XXVIII.
The Philadelphia and St. Louis Lecture. On Light and Other High
294
Frequency Phenomena, February and March, 1893.

CHAPTER XXIX.
Tesla Alternating Current Generators for High Frequency. 374

CHAPTER XXX.
Alternate Current Electrostatic Induction Apparatus. 392

CHAPTER XXXI.
"Massage" with Currents of High Frequency. 394

CHAPTER XXXII.
Electric Discharge in Vacuum Tubes. 396

PART III.
MISCELLANEOUS INVENTIONS AND WRITINGS.

CHAPTER XXXIII.
Method of Obtaining Direct from Alternating Currents. 409

CHAPTER XXXIV.
Condensers with Plates in Oil. 418

CHAPTER XXXV.
Electrolytic Registering Meter. 420

CHAPTER XXXVI.
Thermo-Magnetic Motors and Pyro-Magnetic Generators. 424

CHAPTER XXXVII.
Anti-sparking Dynamo Brush and Commutator. 432

CHAPTER XXXVIII.
Auxiliary Brush Regulation of Direct Current Dynamos. 438

CHAPTER XXXIX.
Improvement in Dynamo and Motor Construction. 448

CHAPTER XL.
Tesla Direct Current Arc Lighting System. 451

CHAPTER XLI.
Improvement in Unipolar Generators. 465

PART IV.
APPENDIX: EARLY PHASE MOTORS AND THE TESLA
OSCILLATORS.

CHAPTER XLII.
Mr. Tesla's Personal Exhibit at the World's Fair. 477

CHAPTER XLIII.
The Tesla Mechanical and Electrical Oscillators. 486

PART I.

POLYPHASE CURRENTS.

CHAPTER I.
Biographical and Introductory.

As an introduction to the record contained in this volume of Mr. Tesla's investigations and
discoveries, a few words of a biographical nature will, it is deemed, not be out of place, nor
other than welcome.
Nikola Tesla was born in 1857 at Smiljan, Lika, a borderland region of Austro-Hungary, of
the Serbian race, which has maintained against Turkey and all comers so unceasing a struggle
for freedom. His family is an old and representative one among these Switzers of Eastern
Europe, and his father was an eloquent clergyman in the Greek Church. An uncle is to-day
Metropolitan in Bosnia. His mother was a woman of inherited ingenuity, and delighted not only
in skilful work of the ordinary household character, but in the construction of such mechanical
appliances as looms and churns and other machinery required in a rural community. Nikola was
educated at Gospich in the public school for four years, and then spent three years in the Real
Schule. He was then sent to Carstatt, Croatia, where he continued his studies for three years in
the Higher Real Schule. There for the first time he saw a steam locomotive. He graduated in
1873, and, surviving an attack of cholera, devoted himself to experimentation, especially in
electricity and magnetism. His father would have had him maintain the family tradition by
entering the Church, but native genius was too strong, and he was allowed to enter the
Polytechnic School at Gratz, to finish his studies, and with the object of becoming a professor of
mathematics and physics. One of the machines there experimented with was a Gramme dynamo,
used as a motor. Despite his instructor's perfect demonstration of the fact that it was impossible
to operate a dynamo without commutator or brushes, Mr. Tesla could not be convinced that such
accessories were necessary or desirable. He had already seen with quick intuition that a way
could be found to dispense with them; and from that time he may be said to have begun work on
the ideas that fructified ultimately in his rotating field motors.
In the second year of his Gratz course, Mr. Tesla gave up the notion of becoming a teacher,
and took up the engineering curriculum. His studies ended, he returned home in time to see his
father die, and then went to Prague and Buda-Pesth to study languages, with the object of
qualifying himself broadly for the practice of the engineering profession. For a short time he
served as an assistant in the Government Telegraph Engineering Department, and then became
associated with M. Puskas, a personal and family friend, and other exploiters of the telephone in
Hungary. He made a number of telephonic inventions, but found his opportunities of benefiting
by them limited in various ways. To gain a wider field of action, he pushed on to Paris and there
secured employment as an electrical engineer with one of the large companies in the new
industry of electric lighting.
It was during this period, and as early as 1882, that he began serious and continued efforts to
embody the rotating field principle in operative apparatus. He was enthusiastic about it; believed
it to mark a new departure in the electrical arts, and could think of nothing else. In fact, but for
the solicitations of a few friends in commercial circles who urged him to form a company to
exploit the invention, Mr. Tesla, then a youth of little worldly experience, would have sought an

immediate opportunity to publish his ideas, believing them to be worthy of note as a novel and
radical advance in electrical theory as well as destined to have a profound influence on all
dynamo electric machinery.
At last he determined that it would be best to try his fortunes in America. In France he had
met many Americans, and in contact with them learned the desirability of turning every new
idea in electricity to practical use. He learned also of the ready encouragement given in the
United States to any inventor who could attain some new and valuable result. The resolution
was formed with characteristic quickness, and abandoning all his prospects in Europe, he at
once set his face westward.
Arrived in the United States, Mr. Tesla took off his coat the day he arrived, in the Edison
Works. That place had been a goal of his ambition, and one can readily imagine the benefit and
stimulus derived from association with Mr. Edison, for whom Mr. Tesla has always had the
strongest admiration. It was impossible, however, that, with his own ideas to carry out, and his
own inventions to develop, Mr. Tesla could long remain in even the most delightful employ;
and, his work now attracting attention, he left the Edison ranks to join a company intended to
make and sell an arc lighting system based on some of his inventions in that branch of the art.
With unceasing diligence he brought the system to perfection, and saw it placed on the market.
But the thing which most occupied his time and thoughts, however, all through this period, was
his old discovery of the rotating field principle for alternating current work, and the application
of it in motors that have now become known the world over.
Strong as his convictions on the subject then were, it is a fact that he stood very much alone,
for the alternating current had no well recognized place. Few electrical engineers had ever used
it, and the majority were entirely unfamiliar with its value, or even its essential features. Even
Mr. Tesla himself did not, until after protracted effort and experimentation, learn how to
construct alternating current apparatus of fair efficiency. But that he had accomplished his
purpose was shown by the tests of Prof. Anthony, made in the of winter 1887-8, when Tesla
motors in the hands of that distinguished expert gave an efficiency equal to that of direct current
motors. Nothing now stood in the way of the commercial development and introduction of such
motors, except that they had to be constructed with a view to operating on the circuits then
existing, which in this country were all of high frequency.
The first full publication of his work in this direction—outside his patents—was a paper
read before the American Institute of Electrical Engineers in New York, in May, 1888 (read at
the suggestion of Prof. Anthony and the present writer), when he exhibited motors that had been
in operation long previous, and with which his belief that brushes and commutators could be
dispensed with, was triumphantly proved to be correct. The section of this volume devoted to
Mr. Tesla's inventions in the utilization of polyphase currents will show how thoroughly from
the outset he had mastered the fundamental idea and applied it in the greatest variety of ways.
Having noted for years the many advantages obtainable with alternating currents, Mr. Tesla
was naturally led on to experiment with them at higher potentials and higher frequencies than
were common or approved of. Ever pressing forward to determine in even the slightest degree
the outlines of the unknown, he was rewarded very quickly in this field with results of the most
surprising nature. A slight acquaintance with some of these experiments led the compiler of this
volume to urge Mr. Tesla to repeat them before the American Institute of Electrical Engineers.
This was done in May, 1891, in a lecture that marked, beyond question, a distinct departure in

electrical theory and practice, and all the results of which have not yet made themselves fully
apparent. The New York lecture, and its successors, two in number, are also included in this
volume, with a few supplementary notes.
Mr. Tesla's work ranges far beyond the vast departments of polyphase currents and high
potential lighting. The "Miscellaneous" section of this volume includes a great many other
inventions in arc lighting, transformers, pyro-magnetic generators, thermo-magnetic motors,
third-brush regulation, improvements in dynamos, new forms of incandescent lamps, electrical
meters, condensers, unipolar dynamos, the conversion of alternating into direct currents, etc. It
is needless to say that at this moment Mr. Tesla is engaged on a number of interesting ideas and
inventions, to be made public in due course. The present volume deals simply with his work
accomplished to date.

CHAPTER II.
A New System of Alternating Current Motors
and Transformers.

The present section of this volume deals with polyphase currents, and the inventions by Mr.
Tesla, made known thus far, in which he has embodied one feature or another of the broad
principle of rotating field poles or resultant attraction exerted on the armature. It is needless to
remind electricians of the great interest aroused by the first enunciation of the rotating field
principle, or to dwell upon the importance of the advance from a single alternating current, to
methods and apparatus which deal with more than one. Simply prefacing the consideration here
attempted of the subject, with the remark that in nowise is the object of this volume of a polemic
or controversial nature, it may be pointed out that Mr. Tesla's work has not at all been fully
understood or realized up to date. To many readers, it is believed, the analysis of what he has
done in this department will be a revelation, while it will at the same time illustrate the beautiful
flexibility and range of the principles involved. It will be seen that, as just suggested, Mr. Tesla
did not stop short at a mere rotating field, but dealt broadly with the shifting of the resultant
attraction of the magnets. It will be seen that he went on to evolve the "multiphase" system with
many ramifications and turns; that he showed the broad idea of motors employing currents of
differing phase in the armature with direct currents in the field; that he first described and
worked out the idea of an armature with a body of iron and coils closed upon themselves; that
he worked out both synchronizing and torque motors; that he explained and illustrated how
machines of ordinary construction might be adapted to his system; that he employed condensers
in field and armature circuits, and went to the bottom of the fundamental principles, testing,
approving or rejecting, it would appear, every detail that inventive ingenuity could hit upon.
Now that opinion is turning so emphatically in favor of lower frequencies, it deserves
special note that Mr. Tesla early recognized the importance of the low frequency feature in
motor work. In fact his first motors exhibited publicly—and which, as Prof. Anthony showed in
his tests in the winter of 1887-8, were the equal of direct current motors in efficiency, output and
starting torque—were of the low frequency type. The necessity arising, however, to utilize these
motors in connection with the existing high frequency circuits, our survey reveals in an
interesting manner Mr. Tesla's fertility of resource in this direction. But that, after exhausting all
the possibilities of this field, Mr. Tesla returns to low frequencies, and insists on the superiority
of his polyphase system in alternating current distribution, need not at all surprise us, in view of
the strength of his convictions, so often expressed, on this subject. This is, indeed, significant,
and may be regarded as indicative of the probable development next to be witnessed.
Incidental reference has been made to the efficiency of rotating field motors, a matter of
much importance, though it is not the intention to dwell upon it here. Prof. Anthony in his
remarks before the American Institute of Electrical Engineers, in May, 1888, on the two small
Tesla motors then shown, which he had tested, stated that one gave an efficiency of about 50 per
cent. and the other a little over sixty per cent. In 1889, some tests were reported from Pittsburgh,
made by Mr. Tesla and Mr. Albert Schmid, on motors up to 10 h. p. and weighing about 850
pounds. These machines showed an efficiency of nearly 90 per cent. With some larger motors it

was then found practicable to obtain an efficiency, with the three wire system, up to as high as
94 and 95 per cent. These interesting figures, which, of course, might be supplemented by others
more elaborate and of later date, are cited to show that the efficiency of the system has not had
to wait until the present late day for any demonstration of its commercial usefulness. An
invention is none the less beautiful because it may lack utility, but it must be a pleasure to any
inventor to know that the ideas he is advancing are fraught with substantial benefits to the
public.

CHAPTER III.
The Tesla Rotating Magnetic Field.—Motors
With Closed Conductors.—Synchronizing
Motors.—Rotating Field Transformers.

The best description that can be given of what he attempted, and succeeded in doing, with
the rotating magnetic field, is to be found in Mr. Tesla's brief paper explanatory of his rotary
current, polyphase system, read before the American Institute of Electrical Engineers, in New
York, in May, 1888, under the title "A New System of Alternate Current Motors and
Transformers." As a matter of fact, which a perusal of the paper will establish, Mr. Tesla made
no attempt in that paper to describe all his work. It dealt in reality with the few topics
enumerated in the caption of this chapter. Mr. Tesla's reticence was no doubt due largely to the
fact that his action was governed by the wishes of others with whom he was associated, but it
may be worth mention that the compiler of this volume—who had seen the motors running, and
who was then chairman of the Institute Committee on Papers and Meetings—had great
difficulty in inducing Mr. Tesla to give the Institute any paper at all. Mr. Tesla was overworked
and ill, and manifested the greatest reluctance to an exhibition of his motors, but his objections
were at last overcome. The paper was written the night previous to the meeting, in pencil, very
hastily, and under the pressure just mentioned.
In this paper casual reference was made to two special forms of motors not within the group
to be considered. These two forms were: 1. A motor with one of its circuits in series with a
transformer, and the other in the secondary of the transformer. 2. A motor having its armature
circuit connected to the generator, and the field coils closed upon themselves. The paper in its
essence is as follows, dealing with a few leading features of the Tesla system, namely, the
rotating magnetic field, motors with closed conductors, synchronizing motors, and rotating field
transformers:—
The subject which I now have the pleasure of bringing to your notice is a novel system of
electric distribution and transmission of power by means of alternate currents, affording peculiar
advantages, particularly in the way of motors, which I am confident will at once establish the
superior adaptability of these currents to the transmission of power and will show that many
results heretofore unattainable can be reached by their use; results which are very much desired
in the practical operation of such systems, and which cannot be accomplished by means of
continuous currents.
Before going into a detailed description of this system, I think it necessary to make a few
remarks with reference to certain conditions existing in continuous current generators and
motors, which, although generally known, are frequently disregarded.
In our dynamo machines, it is well known, we generate alternate currents which we direct by
means of a commutator, a complicated device and, it may be justly said, the source of most of
the troubles experienced in the operation of the machines. Now, the currents so directed cannot
be utilized in the motor, but they must—again by means of a similar unreliable device—be

reconverted into their original state of alternate currents. The function of the commutator is
entirely external, and in no way does it affect the internal working of the machines. In reality,
therefore, all machines are alternate current machines, the currents appearing as continuous only
in the external circuit during their transit from generator to motor. In view simply of this fact,
alternate currents would commend themselves as a more direct application of electrical energy,
and the employment of continuous currents would only be justified if we had dynamos which
would primarily generate, and motors which would be directly actuated by, such currents.
But the operation of the commutator on a motor is twofold; first, it reverses the currents
through the motor, and secondly, it effects automatically, a progressive shifting of the poles of
one of its magnetic constituents. Assuming, therefore, that both of the useless operations in the
systems, that is to say, the directing of the alternate currents on the generator and reversing the
direct currents on the motor, be eliminated, it would still be necessary, in order to cause a
rotation of the motor, to produce a progressive shifting of the poles of one of its elements, and
the question presented itself—How to perform this operation by the direct action of alternate
currents? I will now proceed to show how this result was accomplished.

Fig. 1. Fig. 1a.

In the first experiment a drum-armature was provided with two coils at right angles to each
other, and the ends of these coils were connected to two pairs of insulated contact-rings as usual.
A ring was then made of thin insulated plates of sheet-iron and wound with four coils, each two
opposite coils being connected together so as to produce free poles on diametrically opposite
sides of the ring. The remaining free ends of the coils were then connected to the contact-rings
of the generator armature so as to form two independent circuits, as indicated in Fig. 9. It may
now be seen what results were secured in this combination, and with this view I would refer to
the diagrams, Figs. 1 to 8a. The field of the generator being independently excited, the rotation
of the armature sets up currents in the coils C C1, varying in strength and direction in the well-
known manner. In the position shown in Fig. 1, the current in coil C is nil, while coil C1 is
traversed by its maximum current, and the connections may be such that the ring is magnetized
by the coils c1 c1, as indicated by the letters N S in Fig. 1a, the magnetizing effect of the coils c c
being nil, since these coils are included in the circuit of coil C.
In Fig. 2, the armature coils are shown in a more advanced position, one-eighth of one
revolution being completed. Fig. 2a illustrates the corresponding magnetic condition of the ring.

At this moment the coil C1 generates a
current of the same direction as
previously, but weaker, producing the
poles n1 s1 upon the ring; the coil C also
generates a current of the same
direction, and the connections may be
such that the coils c c produce the poles
n s, as shown in Fig. 2a. The resulting
polarity is indicated by the letters N S,
and it will be observed that the poles of
the ring have been shifted one-eighth of
the periphery of the same.

Fig. 2. In Fig. 3 the armature has
completed one quarter of one
re
vo
lut
io
n.
In
thi
s
ph
as
e
th
Fig. 3. e
cu Fig. 2a.
rrent in coil C is a maximum, and of such direction as
to produce the poles N S in Fig. 3a, whereas the
current in coil C1 is nil, this coil being at its neutral
position. The poles N S in Fig. 3a are thus shifted
one quarter of the circumference of the ring.
Fig. 4 shows the coils C C in a still more
advanced position, the armature having completed
three-eighths of one revolution. At that moment the
coil C still generates a current of the same direction
as before, but of less strength, producing the
comparatively weaker poles n s in Fig. 4a. The
current in the coil C1 is of the same strength, but
opposite direction. Its effect is, therefore, to Fig. 3a.
produce upon the ring the poles n1 s1, as indicated,
and a polarity, N S, results, the poles now being shifted three-eighths of the periphery of the ring.

Fig. 4. Fig. 4a.

Fig. 5. Fig. 5a.

In Fig. 5 one half of one revolution of the armature is completed, and the resulting magnetic
condition of the ring is indicated in Fig. 5a. Now the current in coil C is nil, while the coil C1
yields its maximum current, which is of the same direction as previously; the magnetizing effect
is, therefore, due to the coils, c1 c1 alone, and, referring to Fig. 5a, it will be observed that the
poles N S are shifted one half of the circumference of the ring. During the next half revolution
the operations are repeated, as represented in the Figs. 6 to 8a.
A reference to the diagrams will make it clear that during one revolution of the armature the
poles of the ring are shifted once around its periphery, and, each revolution producing like
effects, a rapid whirling of the poles in harmony with the rotation of the armature is the result. If
the connections of either one of the circuits in the ring are reversed, the shifting of the poles is
made to progress in the opposite direction, but the operation is identically the same. Instead of
using four wires, with like result, three wires may be used, one forming a common return for
both circuits.
This rotation or whirling of the poles manifests itself in a series of curious phenomena. If a
delicately pivoted disc of steel or other magnetic metal is approached to the ring it is set in rapid
rotation, the direction of rotation varying with the position of the disc. For instance, noting the

Fig. 6. Fig. 6a.

Fig. 7. Fig. 7a.

direction outside of the ring it will be found that inside the ring it turns in an opposite direction,
while it is unaffected if placed in a position symmetrical to the ring. This is easily explained.
Each time that a pole approaches, it induces an opposite pole in the nearest point on the disc,
and an attraction is produced upon that point; owing to this, as the pole is shifted further away
from the disc a tangential pull is exerted upon the same, and the action being constantly
repeated, a more or less rapid rotation of the disc is the result. As the pull is exerted mainly upon
that part which is nearest to the ring, the rotation outside and inside, or right and left,
respectively, is in opposite directions, Fig. 9. When placed symmetrically to the ring, the pull on
the opposite sides of the disc being equal, no rotation results. The action is based on the
magnetic inertia of iron; for this reason a disc of hard steel is much more affected than a disc of
soft iron, the latter being capable of very rapid variations of magnetism. Such a disc has proved
to be a very useful instrument in all these investigations, as it has enabled me to detect any
irregularity in the action. A curious effect is also produced upon iron filings. By placing some
upon a paper and holding them externally quite close to the ring, they are set in a vibrating
motion, remaining in the same place, although the paper may be moved back and forth; but in
lifting the paper to a certain height which seems to be dependent on the intensity of the poles
and the speed of rotation, they are thrown away in a direction always opposite to the supposed

movement of the poles. If a paper with filings is put flat upon the ring and the current turned on
suddenly, the existence of a magnetic whirl may easily be observed.
To demonstrate the complete analogy between the ring and a revolving magnet, a strongly
energized electro-magnet was rotated by mechanical power, and phenomena identical in every
particular to those mentioned above were observed.
Obviously, the rotation of the poles produces corresponding inductive effects and may be
utilized to generate currents in a closed conductor placed within the influence of the poles. For
this purpose it is convenient to wind a ring with two sets of superimposed coils forming
respectively the primary and secondary circuits, as shown in Fig. 10. In order to secure the most
economical results the magnetic circuit should be completely closed, and with this object in
view the construction may be modified at will.

Fig. 8. Fig. 8a.

The inductive effect exerted upon the secondary coils will be mainly due to the shifting or
movement of the magnetic action; but there may also be currents set up in the circuits in
consequence of the variations in the intensity of the poles. However, by properly designing the
generator and determining the magnetizing effect of the primary coils, the latter element may be
made to disappear. The intensity of the poles being maintained constant, the action of the
apparatus will be perfect, and the same result will be secured as though the shifting were
effected by means of a commutator with an infinite number of bars. In such case the theoretical
relation between the energizing effect of each set of primary coils and their resultant
magnetizing effect may be expressed by the equation of a circle having its centre coinciding
with that of an orthogonal system of axes, and in which the radius represents the resultant and
the co-ordinates both of its components. These are then respectively the sine and cosine of the
angle α between the radius and one of the axes (O X). Referring to Fig. 11, we have r2 = x2 + y2;
where x = r cos α, and y = r sin α.
Assuming the magnetizing effect of each set of coils in the transformer to be proportional to
the current—which may be admitted for weak degrees of magnetization—then x = Kc and y =
Kc1, where K is a constant and c and c1 the current in both sets of coils respectively. Supposing,
further, the field of the generator to be uniform, we have for constant speed c1 = K1 sin α and c =
K1 sin (90° + α) = K1 cos α, where K1 is a constant. See Fig. 12.

Therefore,
x = K c = K K1 cos α;
y = K c1 = K K1 sin α; and
K K1 = r.

Fig. 9.
That is, for a uniform field the disposition of the two coils at right angles will secure the
theoretical result, and the intensity of the shifting poles will be constant. But from r2 = x2 + y2 it
follows that for y = 0, r = x; it follows that the joint magnetizing effect of both sets of coils
should be equal to the effect of one set when at its maximum action. In transformers and in a
certain class of motors the fluctuation of the poles is not of great importance, but in another
class of these motors it is desirable to obtain the theoretical result.
In applying this principle to the construction of motors, two typical forms of motor have
been developed. First, a form having a comparatively small rotary effort at the start but
maintaining a perfectly uniform speed at all loads, which motor has been termed synchronous.
Second, a form possessing a great rotary effort at the start, the speed being dependent on the
load.
These motors may be operated in three different ways: 1. By the alternate currents of the
source only. 2. By a combined action of these and of induced currents. 3. By the joint action of
alternate and continuous currents.

Fig. 10.
The simplest form of a synchronous motor is obtained by winding a laminated ring provided
with pole projections with four coils, and connecting the same in the manner before indicated.
An iron disc having a segment cut away on each side may be used as an armature. Such a motor
is shown in Fig. 9. The disc being arranged to rotate freely within the ring in close proximity to
the projections, it is evident that as the poles are shifted it will, owing to its tendency to place
itself in such a position as to embrace the greatest number of the lines of force, closely follow
the movement of the poles, and its motion will be synchronous with that of the armature of the
generator; that is, in the peculiar disposition shown in Fig. 9, in which the armature produces by
one revolution two current impulses in each of the circuits. It is evident that if, by one revolution
of the armature, a greater number of impulses is produced, the speed of the motor will be
correspondingly increased. Considering that the attraction exerted upon the disc is greatest when
the same is in close proximity to the poles, it follows that such a motor will maintain exactly the
same speed at all loads within the limits of its capacity.
To facilitate the starting, the disc may be provided with a coil closed upon itself. The
advantage secured by such a coil is evident. On the start the currents set up in the coil strongly
energize the disc and increase the attraction exerted upon the same by the ring, and currents
being generated in the coil as long as the speed of the armature is inferior to that of the poles,
considerable work may be performed by such a motor even if the speed be below normal. The
intensity of the poles being constant, no currents will be generated in the coil when the motor is
turning at its normal speed.
Instead of closing the coil upon itself, its ends may be connected to two insulated sliding
rings, and a continuous current supplied to these from a suitable generator. The proper way to
start such a motor is to close the coil upon itself until the normal speed is reached, or nearly so,

and then turn on the continuous current. If the disc be very strongly energized by a continuous
current the motor may not be able to start, but if it be weakly energized, or generally so that the
magnetizing effect of the ring is preponderating, it will start and reach the normal speed. Such a
motor will maintain absolutely the same speed at all loads. It has also been found that if the
motive power of the generator is not excessive, by checking the motor the speed of the generator
is diminished in synchronism with that of the motor. It is characteristic of this form of motor
that it cannot be reversed by reversing the continuous current through the coil.

Fig. 11. Fig. 12.

The synchronism of these motors may be demonstrated experimentally in a variety of ways.
For this purpose it is best to employ a motor consisting of a stationary field magnet and an
armature arranged to rotate within the same, as indicated in Fig. 13. In this case the shifting of
the poles of the armature produces a rotation of the latter in the opposite direction. It results
therefrom that when the normal speed is reached, the poles of the armature assume fixed
positions relatively to the field magnet, and the same is magnetized by induction, exhibiting a
distinct pole on each of the pole-pieces. If a piece of soft iron is approached to the field magnet,
it will at the start be attracted with a rapid vibrating motion produced by the reversals of polarity
of the magnet, but as the speed of the armature increases, the vibrations become less and less
frequent and finally entirely cease. Then the iron is weakly but permanently attracted, showing
that synchronism is reached and the field magnet energized by induction.
The disc may also be used for the experiment. If held quite close to the armature it will turn
as long as the speed of rotation of the poles exceeds that of the armature; but when the normal
speed is reached, or very nearly so, it ceases to rotate and is permanently attracted.

Fig. 13.
A crude but illustrative experiment is made with an incandescent lamp. Placing the lamp in
circuit with the continuous current generator and in series with the magnet coil, rapid
fluctuations are observed in the light in consequence of the induced currents set up in the coil at
the start; the speed increasing, the fluctuations occur at longer intervals, until they entirely
disappear, showing that the motor has attained its normal speed. A telephone receiver affords a
most sensitive instrument; when connected to any circuit in the motor the synchronism may be
easily detected on the disappearance of the induced currents.
In motors of the synchronous type it is desirable to maintain the quantity of the shifting
magnetism constant, especially if the magnets are not properly subdivided.
To obtain a rotary effort in these motors was the subject of long thought. In order to secure
this result it was necessary to make such a disposition that while the poles of one element of the
motor are shifted by the alternate currents of the source, the poles produced upon the other
elements should always be maintained in the proper relation to the former, irrespective of the
speed of the motor. Such a condition exists in a continuous current motor; but in a synchronous
motor, such as described, this condition is fulfilled only when the speed is normal.

Fig. 14.
The object has been attained by placing within the ring a properly subdivided cylindrical
iron core wound with several independent coils closed upon themselves. Two coils at right
angles as in Fig. 14, are sufficient, but a greater number may be advantageously employed. It
results from this disposition that when the poles of the ring are shifted, currents are generated in
the closed armature coils. These currents are the most intense at or near the points of the greatest
density of the lines of force, and their effect is to produce poles upon the armature at right
angles to those of the ring, at least theoretically so; and since this action is entirely independent
of the speed—that is, as far as the location of the poles is concerned—a continuous pull is
exerted upon the periphery of the armature. In many respects these motors are similar to the
continuous current motors. If load is put on, the speed, and also the resistance of the motor, is
diminished and more current is made to pass through the energizing coils, thus increasing the
effort. Upon the load being taken off, the counter-electromotive force increases and less current
passes through the primary or energizing coils. Without any load the speed is very nearly equal
to that of the shifting poles of the field magnet.

Fig. 15. Fig. 16. Fig. 17.

It will be found that the rotary effort in these motors fully equals that of the continuous
current motors. The effort seems to be greatest when both armature and field magnet are without
any projections; but as in such dispositions the field cannot be concentrated, probably the best

results will be obtained by leaving pole projections on one of the elements only. Generally, it
may be stated the projections diminish the torque and produce a tendency to synchronism.
A characteristic feature of motors of this kind is their property of being very rapidly
reversed. This follows from the peculiar action of the motor. Suppose the armature to be rotating
and the direction of rotation of the poles to be reversed. The apparatus then represents a dynamo
machine, the power to drive this machine being the momentum stored up in the armature and its
speed being the sum of the speeds of the armature and the poles.

Fig. 18. Fig. 19. Fig. 20. Fig. 21.

If we now consider that the power to drive such a dynamo would be very nearly proportional
to the third power of the speed, for that reason alone the armature should be quickly reversed.
But simultaneously with the reversal another element is brought into action, namely, as the
movement of the poles with respect to the armature is reversed, the motor acts like a transformer
in which the resistance of the secondary circuit would be abnormally diminished by producing
in this circuit an additional electromotive force. Owing to these causes the reversal is
instantaneous.
If it is desirable to secure a constant speed, and at the same time a certain effort at the start,
this result may be easily attained in a variety of ways. For instance, two armatures, one for
torque and the other for synchronism, may be fastened on the same shaft and any desired
preponderance may be given to either one, or an armature may be wound for rotary effort, but a
more or less pronounced tendency to synchronism may be given to it by properly constructing
the iron core; and in many other ways.
As a means of obtaining the required phase of the currents in both the circuits, the
disposition of the two coils at right angles is the simplest, securing the most uniform action; but
the phase may be obtained in many other ways, varying with the machine employed. Any of the
dynamos at present in use may be easily adapted for this purpose by making connections to
proper points of the generating coils. In closed circuit armatures, such as used in the continuous
current systems, it is best to make four derivations from equi-distant points or bars of the
commutator, and to connect the same to four insulated sliding rings on the shaft. In this case
each of the motor circuits is connected to two diametrically opposite bars of the commutator. In
such a disposition the motor may also be operated at half the potential and on the three-wire
plan, by connecting the motor circuits in the proper order to three of the contact rings.
In multipolar dynamo machines, such as used in the converter systems, the phase is
conveniently obtained by winding upon the armature two series of coils in such a manner that
while the coils of one set or series are at their maximum production of current, the coils of the
other will be at their neutral position, or nearly so, whereby both sets of coils may be subjected
simultaneously or successively to the inducing action of the field magnets.

Generally the circuits in the motor will be similarly disposed, and various arrangements may
be made to fulfill the requirements; but the simplest and most practicable is to arrange primary
circuits on stationary parts of the motor, thereby obviating, at least in certain forms, the
employment of sliding contacts. In such a case the magnet coils are connected alternately in
both the circuits; that is, 1, 3, 5 ... in one, and 2, 4, 6 ... in the other, and the coils of each set of
series may be connected all in the same manner, or alternately in opposition; in the latter case a
motor with half the number of poles will result, and its action will be correspondingly modified.
The Figs. 15, 16, and 17, show three different phases, the magnet coils in each circuit being
connected alternately in opposition. In this case there will be always four poles, as in Figs. 15
and 17; four pole projections will be neutral; and in Fig. 16 two adjacent pole projections will
have the same polarity. If the coils are connected in the same manner there will be eight
alternating poles, as indicated by the letters n' s' in Fig. 15.
The employment of multipolar motors secures in this system an advantage much desired and
unattainable in the continuous current system, and that is, that a motor may be made to run
exactly at a predetermined speed irrespective of imperfections in construction, of the load, and,
within certain limits, of electromotive force and current strength.
In a general distribution system of this kind the following plan should be adopted. At the
central station of supply a generator should be provided having a considerable number of poles.
The motors operated from this generator should be of the synchronous type, but possessing
sufficient rotary effort to insure their starting. With the observance of proper rules of
construction it may be admitted that the speed of each motor will be in some inverse proportion
to its size, and the number of poles should be chosen accordingly. Still, exceptional demands
may modify this rule. In view of this, it will be advantageous to provide each motor with a
greater number of pole projections or coils, the number being preferably a multiple of two and
three. By this means, by simply changing the connections of the coils, the motor may be adapted
to any probable demands.
If the number of the poles in the motor is even, the action will be harmonious and the proper
result will be obtained; if this is not the case, the best plan to be followed is to make a motor
with a double number of poles and connect the same in the manner before indicated, so that half
the number of poles result. Suppose, for instance, that the generator has twelve poles, and it
would be desired to obtain a speed equal to 12/7 of the speed of the generator. This would
require a motor with seven pole projections or magnets, and such a motor could not be properly
connected in the circuits unless fourteen armature coils would be provided, which would
necessitate the employment of sliding contacts. To avoid this, the motor should be provided with
fourteen magnets and seven connected in each circuit, the magnets in each circuit alternating
among themselves. The armature should have fourteen closed coils. The action of the motor will
not be quite as perfect as in the case of an even number of poles, but the drawback will not be of
a serious nature.
However, the disadvantages resulting from this unsymmetrical form will be reduced in the
same proportion as the number of the poles is augmented.
If the generator has, say, n, and the motor n1 poles, the speed of the motor will be equal to
that of the generator multiplied by n/n1.

The speed of the motor will generally be dependent on the number of the poles, but there
may be exceptions to this rule. The speed may be modified by the phase of the currents in the
circuit or by the character of the current impulses or by intervals between each or between
groups of impulses. Some of the possible cases are indicated in the diagrams, Figs. 18, 19, 20
and 21, which are self-explanatory. Fig. 18 represents the condition generally existing, and
which secures the best result. In such a case, if the typical form of motor illustrated in Fig. 9 is
employed, one complete wave in each circuit will produce one revolution of the motor. In Fig.
19 the same result will be effected by one wave in each circuit, the impulses being successive;
in Fig. 20 by four, and in Fig. 21 by eight waves.
By such means any desired speed may be attained, that is, at least within the limits of
practical demands. This system possesses this advantage, besides others, resulting from
simplicity. At full loads the motors show an efficiency fully equal to that of the continuous
current motors. The transformers present an additional advantage in their capability of operating
motors. They are capable of similar modifications in construction, and will facilitate the
introduction of motors and their adaptation to practical demands. Their efficiency should be
higher than that of the present transformers, and I base my assertion on the following:
In a transformer, as constructed at present, we produce the currents in the secondary circuit
by varying the strength of the primary or exciting currents. If we admit proportionality with
respect to the iron core the inductive effect exerted upon the secondary coil will be proportional
to the numerical sum of the variations in the strength of the exciting current per unit of time;
whence it follows that for a given variation any prolongation of the primary current will result in
a proportional loss. In order to obtain rapid variations in the strength of the current, essential to
efficient induction, a great number of undulations are employed; from this practice various
disadvantages result. These are: Increased cost and diminished efficiency of the generator; more
waste of energy in heating the cores, and also diminished output of the transformer, since the
core is not properly utilized, the reversals being too rapid. The inductive effect is also very small
in certain phases, as will be apparent from a graphic representation, and there may be periods of
inaction, if there are intervals between the succeeding current impulses or waves. In producing a
shifting of the poles in a transformer, and thereby inducing currents, the induction is of the ideal
character, being always maintained at its maximum action. It is also reasonable to assume that
by a shifting of the poles less energy will be wasted than by reversals.

CHAPTER IV.
Modifications and Expansions of the Tesla
Polyphase Systems.

In his earlier papers and patents relative to polyphase currents, Mr. Tesla devoted himself
chiefly to an enunciation of the broad lines and ideas lying at the basis of this new work; but he
supplemented this immediately by a series of other striking inventions which may be regarded
as modifications and expansions of certain features of the Tesla systems. These we shall now
proceed to deal with.
In the preceding chapters we have thus shown and described the Tesla electrical systems for
the transmission of power and the conversion and distribution of electrical energy, in which the
motors and the transformers contain two or more coils or sets of coils, which were connected up
in independent circuits with corresponding coils of an alternating current generator, the
operation of the system being brought about by the co-operation of the alternating currents in
the independent circuits in progressively moving or shifting the poles or points of maximum
magnetic effect of the motors or converters. In these systems two independent conductors are
employed for each of the independent circuits connecting the generator with the devices for
converting the transmitted currents into mechanical energy or into electric currents of another
character. This, however, is not always necessary. The two or more circuits may have a single
return path or wire in common, with a loss, if any, which is so extremely slight that it may be
disregarded entirely. For the sake of illustration, if the generator have two independent coils and
the motor two coils or two sets of coils in corresponding relations to its operative elements one
terminal of each generator coil is connected to the corresponding terminals of the motor coils
through two independent conductors, while the opposite terminals of the respective coils are
both connected to one return wire. The following description deals with the modification. Fig.
22 is a diagrammatic illustration of a generator and single motor constructed and electrically
connected in accordance with the invention. Fig. 23 is a diagram of the system as it is used in
operating motors or converters, or both, in parallel, while Fig. 24 illustrates diagrammatically
the manner of operating two or more motors or converters, or both, in series. Referring to Fig.
22, A A designate the poles of the field magnets of an alternating-current generator, the armature
of which, being in this case cylindrical in form and mounted on a shaft, C, is wound
longitudinally with coils B B'. The shaft C carries three insulated contact-rings, a b c, to two of
which, as b c, one terminal of each coil, as e d, is connected. The remaining terminals, f g, are
both connected to the third ring, a.
A motor in this case is shown as composed of a ring, H, wound with four coils, I I J J,
electrically connected, so as to co-operate in pairs, with a tendency to fix the poles of the ring at
four points ninety degrees apart. Within the magnetic ring H is a disc or cylindrical core wound
with two coils, G G', which may be connected to form two closed circuits. The terminals j k of
the two sets or pairs of coils are connected, respectively, to the binding-posts E' F', and the other
terminals, h i, are connected to a single binding-post, D'. To operate the motor, three line-wires
are used to connect the terminals of the generator with those of the motor.

Fig. 22. Fig. 24.

Fig. 23.
So far as the apparent action or mode of operation of this arrangement is concerned, the
single wire D, which is, so to speak, a common return-wire for both circuits, may be regarded as
two independent wires. In the illustration, with the order of connection shown, coil B' of the
generator is producing its maximum current and coil B its minimum; hence the current which
passes through wire e, ring b, brush b', line-wire E, terminal E', wire j, coils I I, wire or terminal
D', line-wire D, brush a', ring a, and wire f, fixes the polar line of the motor midway between the
two coils I I; but as the coil B' moves from the position indicated it generates less current, while
coil B, moving into the field, generates more. The current from coil B passes through the devices
and wires designated by the letters d, c, c' F, F' k, J J, i, D', D, a', a, and g, and the position of the
poles of the motor will be due to the resultant effect of the currents in the two sets of coils—that
is, it will be advanced in proportion to the advance or forward movement of the armature coils.
The movement of the generator-armature through one-quarter of a revolution will obviously
bring coil B' into its neutral position and coil B into its position of maximum effect, and this
shifts the poles ninety degrees, as they are fixed solely by coils B. This action is repeated for
each quarter of a complete revolution.

When more than one motor or other device is employed, they may be run either in parallel
or series. In Fig. 23 the former arrangement is shown. The electrical device is shown as a
converter, L, of which the two sets of primary coils p r are connected, respectively, to the mains
F E, which are electrically connected with the two coils of the generator. The cross-circuit wires l
m, making these connections, are then connected to the common return-wire D. The secondary
coils p' p'' are in circuits n o, including, for example, incandescent lamps. Only one converter is
shown entire in this figure, the others being illustrated diagrammatically.
When motors or converters are to be run in series, the two wires E F are led from the
generator to the coils of the first motor or converter, then continued on to the next, and so on
through the whole series, and are then joined to the single wire D, which completes both circuits
through the generator. This is shown in Fig. 24, in which J I represent the two coils or sets of
coils of the motors.
There are, of course, other conditions under which the same idea may be carried out. For
example, in case the motor and generator each has three independent circuits, one terminal of
each circuit is connected to a line-wire, and the other three terminals to a common return-
conductor. This arrangement will secure similar results to those attained with a generator and
motor having but two independent circuits, as above described.
When applied to such machines and motors as have three or more induced circuits with a
common electrical joint, the three or more terminals of the generator would be simply connected
to those of the motor. Mr. Tesla states, however, that the results obtained in this manner show a
lower efficiency than do the forms dwelt upon more fully above.

CHAPTER V.
Utilizing Familiar Types of Generator of the
Continuous Current Type.

The preceding descriptions have assumed the use of alternating current generators in which,
in order to produce the progressive movement of the magnetic poles, or of the resultant
attraction of independent field magnets, the current generating coils are independent or separate.
The ordinary forms of continuous current dynamos may, however, be employed for the same
work, in accordance with a method of adaptation devised by Mr. Tesla. As will be seen, the
modification involves but slight changes in their construction, and presents other elements of
economy.
On the shaft of a given generator, either in place of or in addition to the regular commutator,
are secured as many pairs of insulated collecting-rings as there are circuits to be operated. Now,
it will be understood that in the operation of any dynamo electric generator the currents in the
coils in their movement through the field of force undergo different phases—that is to say, at
different positions of the coils the currents have certain directions and certain strengths—and
that in the Tesla motors or transformers it is necessary that the currents in the energizing coils
should undergo a certain order of variations in strength and direction. Hence, the further step—
viz., the connection between the induced or generating coils of the machine and the contact-
rings from which the currents are to be taken off—will be determined solely by what order of
variations of strength and direction in the currents is desired for producing a given result in the
electrical translating device. This may be accomplished in various ways; but in the drawings we
give typical instances only of the best and most practicable ways of applying the invention to
three of the leading types of machines in widespread use, in order to illustrate the principle.
Fig. 25 is a diagram illustrative of the mode of applying the invention to the well-known
type of "closed" or continuous circuit machines. Fig. 26 is a similar diagram embodying an
armature with separate coils connected diametrically, or what is generally called an "open-
circuit" machine. Fig. 27 is a diagram showing the application of the invention to a machine the
armature-coils of which have a common joint.

Fig. 25.
Referring to Fig. 25, let A represent a Tesla motor or transformer which, for convenience, we
will designate as a "converter." It consists of an annular core, B, wound with four independent
coils, C and D, those diametrically opposite being connected together so as to co-operate in pairs
in establishing free poles in the ring, the tendency of each pair being to fix the poles at ninety
degrees from the other. There may be an armature, E, within the ring, which is wound with coils
closed upon themselves. The object is to pass through coils C D currents of such relative strength
and direction as to produce a progressive shifting or movement of the points of maximum
magnetic effect around the ring, and to thereby maintain a rotary movement of the armature.
There are therefore secured to the shaft F of the generator, four insulated contact-rings, a b c d,
upon which bear the collecting-brushes a' b' c' d', connected by wires G G H H, respectively, with
the terminals of coils C and D.
Assume, for sake of illustration, that the coils D D are to receive the maximum and coils C C
at the same instant the minimum current, so that the polar line may be midway between the coils
D D. The rings a b would therefore be connected to the continuous armature-coil at its neutral
points with respect to the field, or the point corresponding with that of the ordinary commutator
brushes, and between which exists the greatest difference of potential; while rings c d would be
connected to two points in the coil, between which exists no difference of potential. The best

results will be obtained by making these connections at points equidistant from one another, as
shown. These connections are easiest made by using wires L between the rings and the loops or
wires J, connecting the coil I to the segments of the commutator K. When the converters are
made in this manner, it is evident that the phases of the currents in the sections of the generator
coil will be reproduced in the converter coils. For example, after turning through an arc of
ninety degrees the conductors L L, which before conveyed the maximum current, will receive the
minimum current by reason of the change in the position of their coils, and it is evident that for
the same reason the current in these coils has gradually fallen from the maximum to the
minimum in passing through the arc of ninety degrees. In this special plan of connections, the
rotation of the magnetic poles of the converter will be synchronous with that of the armature
coils of the generator, and the result will be the same, whether the energizing circuits are
derivations from a continuous armature coil or from independent coils, as in Mr. Tesla's other
devices.
In Fig. 25, the brushes M M are shown in dotted lines in their proper normal position. In
practice these brushes may be removed from the commutator and the field of the generator
excited by an external source of current; or the brushes may be allowed to remain on the
commutator and to take off a converted current to excite the field, or to be used for other
purposes.
In a certain well-known class of machines known as the "open circuit," the armature
contains a number of coils the terminals of which connect to commutator segments, the coils
being connected across the armature in pairs. This type of machine is represented in Fig. 26. In
this machine each pair of coils goes through the same phases as the coils in some of the
generators already shown, and it is obviously only necessary to utilize them in pairs or sets to
operate a Tesla converter by extending the segments of the commutators belonging to each pair
of coils and causing a collecting brush to bear on the continuous portion of each segment. In this
way two or more circuits may be taken off from the generator, each including one or more pairs
or sets of coils as may be desired.

Fig. 26. Fig. 27.

In Fig. 26 I I represent the armature coils, T T the poles of the field magnet, and F the shaft
carrying the commutators, which are extended to form continuous portions a b c d. The brushes
bearing on the continuous portions for taking off the alternating currents are represented by a' b'
c' d'. The collecting brushes, or those which may be used to take off the direct current, are
designated by M M. Two pairs of the armature coils and their commutators are shown in the
figure as being utilized; but all may be utilized in a similar manner.
There is another well-known type of machine in which three or more coils, A' B' C', on the
armature have a common joint, the free ends being connected to the segments of a commutator.
This form of generator is illustrated in Fig. 27. In this case each terminal of the generator is
connected directly or in derivation to a continuous ring, a b c, and collecting brushes, a' b' c',
bearing thereon, take off the alternating currents that operate the motor. It is preferable in this
case to employ a motor or transformer with three energizing coils, A'' B'' C'', placed symmetrically
with those of the generator, and the circuits from the latter are connected to the terminals of such
coils either directly—as when they are stationary—or by means of brushes e' and contact rings
e. In this, as in the other cases, the ordinary commutator may be used on the generator, and the
current taken from it utilized for exciting the generator field-magnets or for other purposes.

CHAPTER VI.
Method of Obtaining Desired Speed of Motor or
Generator.

With the object of obtaining the desired speed in motors operated by means of alternating
currents of differing phase, Mr. Tesla has devised various plans intended to meet the practical
requirements of the case, in adapting his system to types of multipolar alternating current
machines yielding a large number of current reversals for each revolution.
For example, Mr. Tesla has pointed out that to adapt a given type of alternating current
generator, you may couple rigidly two complete machines, securing them together in such a way
that the requisite difference in phase will be produced; or you may fasten two armatures to the
same shaft within the influence of the same field and with the requisite angular displacement to
yield the proper difference in phase between the two currents; or two armatures may be attached
to the same shaft with their coils symmetrically disposed, but subject to the influence of two sets
of field magnets duly displaced; or the two sets of coils may be wound on the same armature
alternately or in such manner that they will develop currents the phases of which differ in time
sufficiently to produce the rotation of the motor.
Another method included in the scope of the same idea, whereby a single generator may run
a number of motors either at its own rate of speed or all at different speeds, is to construct the
motors with fewer poles than the generator, in which case their speed will be greater than that of
the generator, the rate of speed being higher as the number of their poles is relatively less. This
may be understood from an example, taking a generator that has two independent generating
coils which revolve between two pole pieces oppositely magnetized; and a motor with
energizing coils that produce at any given time two magnetic poles in one element that tend to
set up a rotation of the motor. A generator thus constructed yields four reversals, or impulses, in
each revolution, two in each of its independent circuits; and the effect upon the motor is to shift
the magnetic poles through three hundred and sixty degrees. It is obvious that if the four
reversals in the same order could be produced by each half-revolution of the generator the motor
would make two revolutions to the generator's one. This would be readily accomplished by
adding two intermediate poles to the generator or altering it in any of the other equivalent ways
above indicated. The same rule applies to generators and motors with multiple poles. For
instance, if a generator be constructed with two circuits, each of which produces twelve
reversals of current to a revolution, and these currents be directed through the independent
energizing-coils of a motor, the coils of which are so applied as to produce twelve magnetic
poles at all times, the rotation of the two will be synchronous; but if the motor-coils produce but
six poles, the movable element will be rotated twice while the generator rotates once; or if the
motor have four poles, its rotation will be three times as fast as that of the generator.

Fig. 28. Fig. 29.

These features, so far as necessary to an understanding of the principle, are here illustrated.
Fig. 28 is a diagrammatic illustration of a generator constructed in accordance with the
invention. Fig. 29 is a similar view of a correspondingly constructed motor. Fig. 30 is a diagram
of a generator of modified construction. Fig. 31 is a diagram of a motor of corresponding
character. Fig. 32 is a diagram of a system containing a generator and several motors adapted to
run at various speeds.
In Fig. 28, let C represent a cylindrical armature core wound longitudinally with insulated
coils A A, which are connected up in series, the terminals of the series being connected to
collecting-rings a a on the shaft G. By means of this shaft the armature is mounted to rotate
between the poles of an annular field-magnet D, formed with polar projections wound with coils
E, that magnetize the said projections. The coils E are included in the circuit of a generator F, by
means of which the field-magnet is energized. If thus constructed, the machine is a well-known
form of alternating-current generator. To adapt it to his system, however, Mr. Tesla winds on
armature C a second set of coils B B intermediate to the first, or, in other words, in such positions
that while the coils of one set are in the relative positions to the poles of the field-magnet to
produce the maximum current, those of the other set will be in the position in which they
produce the minimum current. The coils B are connected, also, in series and to two connecting-
rings, secured generally to the shaft at the opposite end of the armature.
The motor shown in Fig. 29 has an annular field-magnet H, with four pole-pieces wound
with coils I. The armature is constructed similarly to the generator, but with two sets of two coils
in closed circuits to correspond with the reduced number of magnetic poles in the field. From
the foregoing it is evident that one revolution of the armature of the generator producing eight
current impulses in each circuit will produce two revolutions of the motor-armature.
The application of the principle of this invention is not, however, confined to any particular
form of machine. In Figs. 30 and 31 a generator and motor of another well-known type are

Fig. 30. Fig. 31.

shown. In Fig. 30, J J are magnets disposed in a circle and wound with coils K, which are in
circuit with a generator which supplies the current that maintains the field of force. In the usual
construction of these machines the armature-conductor L is carried by a suitable frame, so as to
be rotated in face of the magnets J J, or between these magnets and another similar set in front of
them. The magnets are energized so as to be of alternately opposite polarity throughout the
series, so that as the conductor C is rotated the current impulses combine or are added to one
another, those produced by the conductor in any given position being all in the same direction.
To adapt such a machine to his system, Mr. Tesla adds a second set of induced conductors M, in
all respects similar to the first, but so placed in reference to it that the currents produced in each
will differ by a quarter-phase. With such relations it is evident that as the current decreases in
conductor L it increases in conductor M, and conversely, and that any of the forms of Tesla motor
invented for use in this system may be operated by such a generator.
Fig. 31 is intended to show a motor corresponding to the machine in Fig. 30. The
construction of the motor is identical with that of the generator, and if coupled thereto it will run
synchronously therewith. J' J' are the field-magnets, and K' the coils thereon. L' is one of the
armature-conductors and M' the other.
Fig. 32 shows in diagram other forms of machine. The generator N in this case is shown as
consisting of a stationary ring O, wound with twenty-four coils P P', alternate coils being
connected in series in two circuits. Within this ring is a disc or drum Q, with projections Q'
wound with energizing-coils included in circuit with a generator R. By driving this disc or
cylinder alternating currents are produced in the coils P and P', which are carried off to run the
several motors.
The motors are composed of a ring or annular field-magnet S, wound with two sets of
energizing-coils T T', and armatures U, having projections U' wound with coils V, all connected in
series in a closed circuit or each closed independently on itself.
Suppose the twelve generator-coils P are wound alternately in opposite directions, so that
any two adjacent coils of the same set tend to produce a free pole in the ring O between them
and the twelve coils P' to be similarly wound. A single revolution of the disc or cylinder Q, the
twelve polar projections of which are of opposite polarity, will therefore produce twelve current
impulses in each of the circuits W W'. Hence the motor X, which has sixteen coils or eight free
poles, will make one and a half turns to the generator's one. The motor Y, with twelve coils or
six poles, will rotate with twice the speed of the generator, and the motor Z, with eight coils or

four poles, will revolve three times as fast as the generator. These multipolar motors have a
peculiarity which may be often utilized to great advantage. For example, in the motor X, Fig. 32,
the eight poles may be either alternately opposite or there may be at any given time alternately
two like and two opposite poles. This is readily attained by making the proper electrical
connections. The effect of such a change, however, would be the same as reducing the number
of poles one-half, and thereby doubling the speed of any given motor.

Fig. 32.
It is obvious that the Tesla electrical transformers which have independent primary currents
may be used with the generators described. It may also be stated with respect to the devices we
now describe that the most perfect and harmonious action of the generators and motors is
obtained when the numbers of the poles of each are even and not odd. If this is not the case,
there will be a certain unevenness of action which is the less appreciable as the number of poles
is greater; although this may be in a measure corrected by special provisions which it is not here
necessary to explain. It also follows, as a matter of course, that if the number of the poles of the
motor be greater than that of the generator the motor will revolve at a slower speed than the
generator.

In this chapter, we may include a method devised by Mr. Tesla for avoiding the very high
speeds which would be necessary with large generators. In lieu of revolving the generator
armature at a high rate of speed, he secures the desired result by a rotation of the magnetic poles
of one element of the generator, while driving the other at a different speed. The effect is the
same as that yielded by a very high rate of rotation.
In this instance, the generator which supplies the current for operating the motors or
transformers consists of a subdivided ring or annular core wound with four diametrically-
opposite coils, E E', Fig. 33. Within the ring is mounted a cylindrical armature-core wound
longitudinally with two independent coils, F F', the ends of which lead, respectively, to two pairs
of insulated contact or collecting rings, D D' G G', on the armature shaft. Collecting brushes d d' g
g' bear upon these rings, respectively, and convey the currents through the two independent line-
circuits M M'. In the main line there may be included one or more motors or transformers, or
both. If motors be used, they are of the usual form of Tesla construction with independent coils
or sets of coils J J', included, respectively, in the circuits M M'. These energizing-coils are wound
on a ring or annular field or on pole pieces thereon, and produce by the action of the alternating
currents passing through them a progressive shifting of the magnetism from pole to pole. The
cylindrical armature H of the motor is wound with two coils at right angles, which form
independent closed circuits.
If transformers be employed, one set of the primary coils, as N N, wound on a ring or annular
core is connected to one circuit, as M', and the other primary coils, N N', to the circuit M. The
secondary coils K K' may then be utilized for running groups of incandescent lamps P P'.

Fig. 33.
With this generator an exciter is employed. This consists of two poles, A A, of steel
permanently magnetized, or of iron excited by a battery or other generator of continuous
currents, and a cylindrical armature core mounted on a shaft, B, and wound with two
longitudinal coils, C C'. One end of each of these coils is connected to the collecting-rings b c,
respectively, while the other ends are both connected to a ring, a. Collecting-brushes b' c' bear
on the rings b c, respectively, and conductors L L convey the currents therefrom through the coils
E and E of the generator. L' is a common return-wire to brush a'. Two independent circuits are
thus formed, one including coils C of the exciter and E E of the generator, the other coils C' of the
exciter and E' E' of the generator. It results from this that the operation of the exciter produces a
progressive movement of the magnetic poles of the annular field-core of the generator, the

shifting or rotary movement of the poles being synchronous with the rotation of the exciter
armature. Considering the operative conditions of a system thus established, it will be found that
when the exciter is driven so as to energize the field of the generator, the armature of the latter,
if left free to turn, would rotate at a speed practically the same as that of the exciter. If under
such conditions the coils F F' of the generator armature be closed upon themselves or short-
circuited, no currents, at least theoretically, will be generated in these armature coils. In practice
the presence of slight currents is observed, the existence of which is attributable to more or less
pronounced fluctuations in the intensity of the magnetic poles of the generator ring. So, if the
armature-coils F F' be closed through the motor, the latter will not be turned as long as the
movement of the generator armature is synchronous with that of the exciter or of the magnetic
poles of its field. If, on the contrary, the speed of the generator armature be in any way checked,
so that the shifting or rotation of the poles of the field becomes relatively more rapid, currents
will be induced in the armature coils. This obviously follows from the passing of the lines of
force across the armature conductors. The greater the speed of rotation of the magnetic poles
relatively to that of the armature the more rapidly the currents developed in the coils of the latter
will follow one another, and the more rapidly the motor will revolve in response thereto, and
this continues until the armature generator is stopped entirely, as by a brake, when the motor, if
properly constructed, runs at the speed with which the magnetic poles of the generator rotate.
The effective strength of the currents developed in the armature coils of the generator is
dependent upon the strength of the currents energizing the generator and upon the number of
rotations per unit of time of the magnetic poles of the generator; hence the speed of the motor
armature will depend in all cases upon the relative speeds of the armature of the generator and
of its magnetic poles. For example, if the poles are turned two thousand times per unit of time
and the armature is turned eight hundred, the motor will turn twelve hundred times, or nearly so.
Very slight differences of speed may be indicated by a delicately balanced motor.
Let it now be assumed that power is applied to the generator armature to turn it in a direction
opposite to that in which its magnetic poles rotate. In such case the result would be similar to
that produced by a generator the armature and field magnets of which are rotated in opposite
directions, and by reason of these conditions the motor armature will turn at a rate of speed
equal to the sum of the speeds of the armature and magnetic poles of the generator, so that a
comparatively low speed of the generator armature will produce a high speed in the motor.
It will be observed in connection with this system that on diminishing the resistance of the
external circuit of the generator armature by checking the speed of the motor or by adding
translating devices in multiple arc in the secondary circuit or circuits of the transformer the
strength of the current in the armature circuit is greatly increased. This is due to two causes:
first, to the great differences in the speeds of the motor and generator, and, secondly, to the fact
that the apparatus follows the analogy of a transformer, for, in proportion as the resistance of the
armature or secondary circuits is reduced, the strength of the currents in the field or primary
circuits of the generator is increased and the currents in the armature are augmented
correspondingly. For similar reasons the currents in the armature-coils of the generator increase
very rapidly when the speed of the armature is reduced when running in the same direction as
the magnetic poles or conversely.
It will be understood from the above description that the generator-armature may be run in
the direction of the shifting of the magnetic poles, but more rapidly, and that in such case the

speed of the motor will be equal to the difference between the two rates.

CHAPTER VII.
Regulator for Rotary Current Motors.

An interesting device for regulating and reversing has been devised by Mr. Tesla for the
purpose of varying the speed of polyphase motors. It consists of a form of converter or
transformer with one element capable of movement with respect to the other, whereby the
inductive relations may be altered, either manually or automatically, for the purpose of varying
the strength of the induced current. Mr. Tesla prefers to construct this device in such manner that
the induced or secondary element may be movable with respect to the other; and the invention,
so far as relates merely to the construction of the device itself, consists, essentially, in the
combination, with two opposite magnetic poles, of an armature wound with an insulated coil
and mounted on a shaft, whereby it may be turned to the desired extent within the field
produced by the poles. The normal position of the core of the secondary element is that in which
it most completely closes the magnetic circuit between the poles of the primary element, and in
this position its coil is in its most effective position for the inductive action upon it of the
primary coils; but by turning the movable core to either side, the induced currents delivered by
its coil become weaker until, by a movement of the said core and coil through 90°, there will be
no current delivered.
Fig. 34 is a view in side elevation of the regulator. Fig. 35 is a broken section on line x x of
Fig. 34. Fig. 36 is a diagram illustrating the most convenient manner of applying the regulator to
ordinary forms of motors, and Fig. 37 is a similar diagram illustrating the application of the
device to the Tesla alternating-current motors. The regulator may be constructed in many ways
to secure the desired result; but that which is, perhaps, its best form is shown in Figs. 34 and 35.
A represents a frame of iron. B B are the cores of the inducing or primary coils C C. D is a
shaft mounted on the side bars, D', and on which is secured a sectional iron core, E, wound with
an induced or secondary coil, F, the convolutions of which are parallel with the axis of the shaft.
The ends of the core are rounded off so as to fit closely in the space between the two poles and
permit the core E to be turned to and held at any desired point. A handle, G, secured to the
projecting end of the shaft D, is provided for this purpose.
In Fig. 36 let H represent an ordinary alternating current generator, the field-magnets of
which are excited by a suitable source of current, I. Let J designate an ordinary form of
electromagnetic motor provided with an armature, K, commutator L, and field-magnets M. It is
well known that such a motor, if its field-magnet cores be divided up into insulated sections,
may be practically operated by an alternating current; but in using this regulator with such a
motor, Mr. Tesla includes one element of the motor only—say the armature-coils—in the main
circuit of the generator, making the connections through the brushes and the commutator in the
usual way. He also includes one of the elements of the regulator—say the stationary coils—in
the same circuit, and in the circuit with the secondary or movable coil of the regulator he
connects up the field-coils of the motor. He also prefers to use flexible conductors to make the
connections from the secondary coil of the regulator, as he thereby avoids the use of sliding
contacts or rings without interfering with the requisite movement of the core E.

Fig. 34. Fig. 35.

If the regulator be in its normal position, or that in which its magnetic circuit is most nearly
closed, it delivers its maximum induced current, the phases of which so correspond with those
of the primary current that the motor will run as though both field and armature were excited by
the main current.

Fig. 36.
To vary the speed of the motor to any rate between the minimum and maximum rates, the
core E and coils F are turned in either direction to an extent which produces the desired result,
for in its normal position the convolutions of coil F embrace the maximum number of lines of
force, all of which act with the same effect upon the coil; hence it will deliver its maximum
current; but by turning the coil F out of its position of maximum effect the number of lines of
force embraced by it is diminished. The inductive effect is therefore impaired, and the current
delivered by coil F will continue to diminish in proportion to the angle at which the coil F is
turned until, after passing through an angle of ninety degrees, the convolutions of the coil will
be at right angles to those of coils C C, and the inductive effect reduced to a minimum.

Incidentally to certain constructions, other causes may influence the variation in the strength
of the induced currents. For example, in the present case it will be observed that by the first
movement of coil F a certain portion of its convolutions are carried beyond the line of the direct
influence of the lines of force, and that the magnetic path or circuit for the lines is impaired;
hence the inductive effect would be reduced. Next, that after moving through a certain angle,
which is obviously determined by the relative dimensions of the bobbin or coil F, diagonally
opposite portions of the coil will be simultaneously included in the field, but in such positions
that the lines which produce a current-impulse in one portion of the coil in a certain direction
will produce in the diagonally opposite portion a corresponding impulse in the opposite
direction; hence portions of the current will neutralize one another.
As before stated, the mechanical construction of the device may be greatly varied; but the
essential conditions of the principle will be fulfilled in any apparatus in which the movement of
the elements with respect to one another effects the same results by varying the inductive
relations of the two elements in a manner similar to that described.

Fig. 37.
It may also be stated that the core E is not indispensable to the operation of the regulator; but
its presence is obviously beneficial. This regulator, however, has another valuable property in its
capability of reversing the motor, for if the coil F be turned through a half-revolution, the
position of its convolutions relatively to the two coils C C and to the lines of force is reversed,
and consequently the phases of the current will be reversed. This will produce a rotation of the
motor in an opposite direction. This form of regulator is also applied with great advantage to
Mr. Tesla's system of utilizing alternating currents, in which the magnetic poles of the field of a
motor are progressively shifted by means of the combined effects upon the field of magnetizing
coils included in independent circuits, through which pass alternating currents in proper order
and relations to each other.

In Fig. 37, let P represent a Tesla generator having two independent coils, P' and P'', on the
armature, and T a diagram of a motor having two independent energizing coils or sets of coils, R
R'. One of the circuits from the generator, as S' S', includes one set, R' R', of the energizing coils of
the motor, while the other circuit, as S S, includes the primary coils of the regulator. The
secondary coil of the regulator includes the other coils, R R, of the motor.
While the secondary coil of the regulator is in its normal position, it produces its maximum
current, and the maximum rotary effect is imparted to the motor; but this effect will be
diminished in proportion to the angle at which the coil F of the regulator is turned. The motor
will also be reversed by reversing the position of the coil with reference to the coils C C, and
thereby reversing the phases of the current produced by the generator. This changes the direction
of the movement of the shifting poles which the armature follows.
One of the main advantages of this plan of regulation is its economy of power. When the
induced coil is generating its maximum current, the maximum amount of energy in the primary
coils is absorbed; but as the induced coil is turned from its normal position the self-induction of
the primary-coils reduces the expenditure of energy and saves power.
It is obvious that in practice either coils C C or coil F may be used as primary or secondary,
and it is well understood that their relative proportions may be varied to produce any desired
difference or similarity in the inducing and induced currents.

CHAPTER VIII.
Single Circuit, Self-Starting Synchronizing
Motors.

In the first chapters of this section we have, bearing in mind the broad underlying principle,
considered a distinct class of motors, namely, such as require for their operation a special
generator capable of yielding currents of differing phase. As a matter of course, Mr. Tesla
recognizing the desirability of utilizing his motors in connection with ordinary systems of
distribution, addressed himself to the task of inventing various methods and ways of achieving
this object. In the succeeding chapters, therefore, we witness the evolution of a number of ideas
bearing upon this important branch of work. It must be obvious to a careful reader, from a
number of hints encountered here and there, that even the inventions described in these chapters
to follow do not represent the full scope of the work done in these lines. They might, indeed, be
regarded as exemplifications.
We will present these various inventions in the order which to us appears the most helpful to
an understanding of the subject by the majority of readers. It will be naturally perceived that in
offering a series of ideas of this nature, wherein some of the steps or links are missing, the
descriptions are not altogether sequential; but any one who follows carefully the main drift of
the thoughts now brought together will find that a satisfactory comprehension of the principles
can be gained.
As is well known, certain forms of alternating-current machines have the property, when
connected in circuit with an alternating current generator, of running as a motor in synchronism
therewith; but, while the alternating current will run the motor after it has attained a rate of
speed synchronous with that of the generator, it will not start it. Hence, in all instances
heretofore where these "synchronizing motors," as they are termed, have been run, some means
have been adopted to bring the motors up to synchronism with the generator, or approximately
so, before the alternating current of the generator is applied to drive them. In some instances
mechanical appliances have been utilized for this purpose. In others special and complicated
forms of motor have been constructed. Mr. Tesla has discovered a much more simple method or
plan of operating synchronizing motors, which requires practically no other apparatus than the
motor itself. In other words, by a certain change in the circuit connections of the motor he
converts it at will from a double circuit motor, or such as have been already described, and
which will start under the action of an alternating current, into a synchronizing motor, or one
which will be run by the generator only when it has reached a certain speed of rotation
synchronous with that of the generator. In this manner he is enabled to extend very greatly the
applications of his system and to secure all the advantages of both forms of alternating current
motor.
The expression "synchronous with that of the generator," is used here in its ordinary
acceptation—that is to say, a motor is said to synchronize with the generator when it preserves a
certain relative speed determined by its number of poles and the number of alternations
produced per revolution of the generator. Its actual speed, therefore, may be faster or slower

than that of the generator; but it is said to be synchronous so long as it preserves the same
relative speed.
In carrying out this invention Mr. Tesla constructs a motor which has a strong tendency to
synchronism with the generator. The construction preferred is that in which the armature is
provided with polar projections. The field-magnets are wound with two sets of coils, the
terminals of which are connected to a switch mechanism, by means of which the line-current
may be carried directly through these coils or indirectly through paths by which its phases are
modified. To start such a motor, the switch is turned on to a set of contacts which includes in
one motor circuit a dead resistance, in the other an inductive resistance, and, the two circuits
being in derivation, it is obvious that the difference in phase of the current in such circuits will
set up a rotation of the motor. When the speed of the motor has thus been brought to the desired
rate the switch is shifted to throw the main current directly through the motor-circuits, and
although the currents in both circuits will now be of the same phase the motor will continue to
revolve, becoming a true synchronous motor. To secure greater efficiency, the armature or its
polar projections are wound with coils closed on themselves.
In the accompanying diagrams, Fig. 38 illustrates the details of the plan above set forth, and
Figs. 39 and 40 modifications of the same.

Figs. 38, 39 and 40.
Referring to Fig. 38, let A designate the field-magnets of a motor, the polar projections of
which are wound with coils B C included in independent circuits, and D the armature with polar
projections wound with coils E closed upon themselves, the motor in these respects being
similar in construction to those described already, but having on account of the polar projections
on the armature core, or other similar and well-known features, the properties of a
synchronizing-motor. L L' represents the conductors of a line from an alternating current
generator G.
Near the motor is placed a switch the action of which is that of the one shown in the
diagrams, which is constructed as follows: F F' are two conducting plates or arms, pivoted at
their ends and connected by an insulating cross-bar, H, so as to be shifted in parallelism. In the
path of the bars F F' is the contact 2, which forms one terminal of the circuit through coils C, and
the contact 4, which is one terminal of the circuit through coils B. The opposite end of the wire
of coils C is connected to the wire L or bar F', and the corresponding end of coils B is connected
to wire L' and bar F; hence if the bars be shifted so as to bear on contacts 2 and 4 both sets of
coils B C will be included in the circuit L L' in multiple arc or derivation. In the path of the levers
F F' are two other contact terminals, 1 and 3. The contact 1 is connected to contact 2 through an
artificial resistance, I, and contact 3 with contact 4 through a self-induction coil, J, so that when

the switch levers are shifted upon the points 1 and 3 the circuits of coils B and C will be
connected in multiple arc or derivation to the circuit L L', and will include the resistance and self-
induction coil respectively. A third position of the switch is that in which the levers F and F' are
shifted out of contact with both sets of points. In this case the motor is entirely out of circuit.
The purpose and manner of operating the motor by these devices are as follows: The normal
position of the switch, the motor being out of circuit, is off the contact points. Assuming the
generator to be running, and that it is desired to start the motor, the switch is shifted until its
levers rest upon points 1 and 3. The two motor-circuits are thus connected with the generator
circuit; but by reason of the presence of the resistance I in one and the self-induction coil J in the
other the coincidence of the phases of the current is disturbed sufficiently to produce a
progression of the poles, which starts the motor in rotation. When the speed of the motor has run
up to synchronism with the generator, or approximately so, the switch is shifted over upon the
points 2 and 4, thus cutting out the coils I and J, so that the currents in both circuits have the
same phase; but the motor now runs as a synchronous motor.
It will be understood that when brought up to speed the motor will run with only one of the
circuits B or C connected with the main or generator circuit, or the two circuits may be
connected in series. This latter plan is preferable when a current having a high number of
alternations per unit of time is employed to drive the motor. In such case the starting of the
motor is more difficult, and the dead and inductive resistances must take up a considerable
proportion of the electromotive force of the circuits. Generally the conditions are so adjusted
that the electromotive force used in each of the motor circuits is that which is required to operate
the motor when its circuits are in series. The plan followed in this case is illustrated in Fig. 39.
In this instance the motor has twelve poles and the armature has polar projections D wound with
closed coils E. The switch used is of substantially the same construction as that shown in the
previous figure. There are, however, five contacts, designated as 5, 6, 7, 8, and 9. The motor-
circuits B C, which include alternate field-coils, are connected to the terminals in the following
order: One end of circuit C is connected to contact 9 and to contact 5 through a dead resistance,
I. One terminal of circuit B is connected to contact 7 and to contact 6 through a self-induction
coil, J. The opposite terminals of both circuits are connected to contact 8.
One of the levers, as F, of the switch is made with an extension, f, or otherwise, so as to
cover both contacts 5 and 6 when shifted into the position to start the motor. It will be observed
that when in this position and with lever F' on contact 8 the current divides between the two
circuits B C, which from their difference in electrical character produce a progression of the
poles that starts the motor in rotation. When the motor has attained the proper speed, the switch
is shifted so that the levers cover the contacts 7 and 9, thereby connecting circuits B and C in
series. It is found that by this disposition the motor is maintained in rotation in synchronism
with the generator. This principle of operation, which consists in converting by a change of
connections or otherwise a double-circuit motor, or one operating by a progressive shifting of
the poles, into an ordinary synchronizing motor may be carried out in many other ways. For
instance, instead of using the switch shown in the previous figures, we may use a temporary
ground circuit between the generator and motor, in order to start the motor, in substantially the
manner indicated in Fig. 40. Let G in this figure represent an ordinary alternating-current
generator with, say, two poles, M M', and an armature wound with two coils, N N', at right angles
and connected in series. The motor has, for example, four poles wound with coils B C, which are
connected in series, and an armature with polar projections D wound with closed coils E E. From

the common joint or union between the two circuits of both the generator and the motor an earth
connection is established, while the terminals or ends of these circuits are connected to the line.
Assuming that the motor is a synchronizing motor or one that has the capability of running in
synchronism with the generator, but not of starting, it may be started by the above-described
apparatus by closing the ground connection from both generator and motor. The system thus
becomes one with a two-circuit generator and motor, the ground forming a common return for
the currents in the two circuits L and L'. When by this arrangement of circuits the motor is
brought to speed, the ground connection is broken between the motor or generator, or both,
ground-switches P P' being employed for this purpose. The motor then runs as a synchronizing
motor.
In describing the main features which constitute this invention illustrations have necessarily
been omitted of the appliances used in conjunction with the electrical devices of similar systems
—such, for instance, as driving-belts, fixed and loose pulleys for the motor, and the like; but
these are matters well understood.
Mr. Tesla believes he is the first to operate electro-magnetic motors by alternating currents
in any of the ways herein described—that is to say, by producing a progressive movement or
rotation of their poles or points of greatest magnetic attraction by the alternating currents until
they have reached a given speed, and then by the same currents producing a simple alternation
of their poles, or, in other words, by a change in the order or character of the circuit connections
to convert a motor operating on one principle to one operating on another.

CHAPTER IX.
Change From Double Current to Single Current
Motor.

A description is given elsewhere of a method of operating alternating current motors by first
rotating their magnetic poles until they have attained synchronous speed, and then alternating
the poles. The motor is thus transformed, by a simple change of circuit connections from one
operated by the action of two or more independent energizing currents to one operated either by
a single current or by several currents acting as one. Another way of doing this will now be
described.
At the start the magnetic poles of one element or field of the motor are progressively shifted
by alternating currents differing in phase and passed through independent energizing circuits,
and short circuit the coils of the other element. When the motor thus started reaches or passes
the limit of speed synchronous with the generator, Mr. Tesla connects up the coils previously
short-circuited with a source of direct current and by a change of the circuit connections
produces a simple alternation of the poles. The motor then continues to run in synchronism with
the generator. The motor here shown in Fig. 41 is one of the ordinary forms, with field-cores
either laminated or solid and with a cylindrical laminated armature wound, for example, with
the coils A B at right angles. The shaft of the armature carries three collecting or contact rings C D
E. (Shown, for better illustration, as of different diameters.)

One end of coil A connects to one ring, as C, and one end of coil B connects with ring D. The
remaining ends are connected to ring E. Collecting springs or brushes F G H bear upon the rings
and lead to the contacts of a switch, to be presently described. The field-coils have their
terminals in binding-posts K K, and may be either closed upon themselves or connected with a
source of direct current L, by means of a switch M. The main or controlling switch has five
contacts a b c d e and two levers f g, pivoted and connected by an insulating cross-bar h, so as to
move in parallelism. These levers are connected to the line wires from a source of alternating
currents N. Contact a is connected to brush G and coil B through a dead resistance R and wire P.
Contact b is connected with brush F and coil A through a self-induction coil S and wire O.
Contacts c and e are connected to brushes G F, respectively, through the wires P O, and contact d
is directly connected with brush H. The lever f has a widened end, which may span the contacts
a b. When in such position and with lever g on contact d, the alternating currents divide between
the two motor-coils, and by reason of their different self-induction a difference of current-phase
is obtained that starts the motor in rotation. In starting, the field-coils are short circuited.

Fig. 41.
When the motor has attained the desired speed, the switch is shifted to the position shown in
dotted lines—that is to say, with the levers f g resting on points c e. This connects up the two
armature coils in series, and the motor will then run as a synchronous motor. The field-coils are
thrown into circuit with the direct current source when the main switch is shifted.

CHAPTER X.
Motor With "Current Lag" Artificially Secured.

One of the general ways followed by Mr. Tesla in developing his rotary phase motors is to
produce practically independent currents differing primarily in phase and to pass these through the
motor-circuits. Another way is to produce a single alternating current, to divide it between the
motor-circuits, and to effect artificially a lag in one of these circuits or branches, as by giving to
the circuits different self-inductive capacity, and in other ways. In the former case, in which the
necessary difference of phase is primarily effected in the generation of currents, in some instances,
the currents are passed through the energizing coils of both elements of the motor—the field and
armature; but a further result or modification may be obtained by doing this under the conditions
hereinafter specified in the case of motors in which the lag, as above stated, is artificially secured.
Figs. 42 to 47, inclusive, are diagrams of different ways in which the invention is carried out;
and Fig. 48, a side view of a form of motor used by Mr. Tesla for this purpose.

Figs. 42, 43 and 44.

A B in Fig. 42 indicate the two energizing circuits of a motor, and C D two circuits on the
armature. Circuit or coil A is connected in series with circuit or coil C, and the two circuits B D are
similarly connected. Between coils A and C is a contact-ring e, forming one terminal of the latter,
and a brush a, forming one terminal of the former. A ring d and brush c similarly connect coils B
and D. The opposite terminals of the field-coils connect to one binding post h of the motor, and
those of the armature coils are similarly connected to the opposite binding post i through a contact-
ring f and brush g. Thus each motor-circuit while in derivation to the other includes one armature
and one field coil. These circuits are of different self-induction, and may be made so in various
ways. For the sake of clearness, an artificial resistance R is shown in one of these circuits, and in
the other a self-induction coil S. When an alternating current is passed through this motor it divides
between its two energizing-circuits. The higher self-induction of one circuit produces a greater
retardation or lag in the current therein than in the other. The difference of phase between the two
currents effects the rotation or shifting of the points of maximum magnetic effect that secures the
rotation of the armature. In certain respects this plan of including both armature and field coils in
circuit is a marked improvement. Such a motor has a good torque at starting; yet it has also
considerable tendency to synchronism, owing to the fact that when properly constructed the
maximum magnetic effects in both armature and field coincide—a condition which in the usual
construction of these motors with closed armature coils is not readily attained. The motor thus
constructed exhibits too, a better regulation of current from no load to load, and there is less
difference between the apparent and real energy expended in running it. The true synchronous
speed of this form of motor is that of the generator when both are alike—that is to say, if the
number of the coils on the armature and on the field is x, the motor will run normally at the same
speed as a generator driving it if the number of field magnets or poles of the same be also x.

Figs. 45, 46 and 47.
Fig. 43 shows a somewhat modified arrangement of circuits. There is in this case but one
armature coil E, the winding of which maintains effects corresponding to the resultant poles
produced by the two field-circuits.
Fig. 44 represents a disposition in which both armature and field are wound with two sets of
coils, all in multiple arc to the line or main circuit. The armature coils are wound to correspond
with the field-coils with respect to their self-induction. A modification of this plan is shown in Fig.
45—that is to say, the two field coils and two armature coils are in derivation to themselves and in
series with one another. The armature coils in this case, as in the previous figure, are wound for
different self-induction to correspond with the field coils.
Another modification is shown in Fig. 46. In this case only one armature-coil, as D, is included
in the line-circuit, while the other, as C, is short-circuited.
In such a disposition as that shown in Fig. 43, or where only one armature-coil is employed,
the torque on the start is somewhat reduced, while the tendency to synchronism is somewhat
increased. In such a disposition as shown in Fig. 46, the opposite conditions would exist. In both
instances, however, there is the advantage of dispensing with one contact-ring.

Fig. 48.
In Fig. 46 the two field-coils and the armature-coil D are in multiple arc. In Fig. 47 this
disposition is modified, coil D being shown in series with the two field-coils.
Fig. 48 is an outline of the general form of motor in which this invention is embodied. The
circuit connections between the armature and field coils are made, as indicated in the previous
figures, through brushes and rings, which are not shown.

CHAPTER XI.
Another Method of Transformation from a
Torque to a Synchronizing Motor.

In a preceding chapter we have described a method by which Mr. Tesla accomplishes the
change in his type of rotating field motor from a torque to a synchronizing motor. As will be
observed, the desired end is there reached by a change in the circuit connections at the proper
moment. We will now proceed to describe another way of bringing about the same result. The
principle involved in this method is as follows:—
If an alternating current be passed through the field coils only of a motor having two
energizing circuits of different self-induction and the armature coils be short-circuited, the motor
will have a strong torque, but little or no tendency to synchronism with the generator; but if the
same current which energizes the field be passed also through the armature coils the tendency to
remain in synchronism is very considerably increased. This is due to the fact that the maximum
magnetic effects produced in the field and armature more nearly coincide. On this principle Mr.
Tesla constructs a motor having independent field circuits of different self-induction, which are
joined in derivation to a source of alternating currents. The armature is wound with one or more
coils, which are connected with the field coils through contact rings and brushes, and around the
armature coils a shunt is arranged with means for opening or closing the same. In starting this
motor the shunt is closed around the armature coils, which will therefore be in closed circuit.
When the current is directed through the motor, it divides between the two circuits, (it is not
necessary to consider any case where there are more than two circuits used), which, by reason of
their different self-induction, secure a difference of phase between the two currents in the two
branches, that produces a shifting or rotation of the poles. By the alternations of current, other
currents are induced in the closed—or short-circuited—armature coils and the motor has a strong
torque. When the desired speed is reached, the shunt around the armature-coils is opened and the
current directed through both armature and field coils. Under these conditions the motor has a
strong tendency to synchronism.

Figs. 49, 50 and 51.
In Fig. 49, A and B designate the field coils of the motor. As the circuits including these coils
are of different self-induction, this is represented by a resistance coil R in circuit with A, and a self-
induction coil S in circuit with B. The same result may of course be secured by the winding of the
coils. C is the armature circuit, the terminals of which are rings a b. Brushes c d bear on these rings
and connect with the line and field circuits. D is the shunt or short circuit around the armature. E is
the switch in the shunt.
It will be observed that in such a disposition as is illustrated in Fig. 49, the field circuits A and
B being of different self-induction, there will always be a greater lag of the current in one than the
other, and that, generally, the armature phases will not correspond with either, but with the
resultant of both. It is therefore important to observe the proper rule in winding the armature. For
instance, if the motor have eight poles—four in each circuit—there will be four resultant poles,
and hence the armature winding should be such as to produce four poles, in order to constitute a
true synchronizing motor.

Fig. 52.
The diagram, Fig. 50, differs from the previous one only in respect to the order of connections.
In the present case the armature-coil, instead of being in series with the field-coils, is in multiple
arc therewith. The armature-winding may be similar to that of the field—that is to say, the
armature may have two or more coils wound or adapted for different self-induction and adapted,
preferably, to produce the same difference of phase as the field-coils. On starting the motor the
shunt is closed around both coils. This is shown in Fig. 51, in which the armature coils are F G. To
indicate their different electrical character, there are shown in circuit with them, respectively, the
resistance R' and the self-induction coil S'. The two armature coils are in series with the field-coils
and the same disposition of the shunt or short-circuit D is used. It is of advantage in the operation
of motors of this kind to construct or wind the armature in such manner that when short-circuited
on the start it will have a tendency to reach a higher speed than that which synchronizes with the
generator. For example, a given motor having, say, eight poles should run, with the armature coil
short-circuited, at two thousand revolutions per minute to bring it up to synchronism. It will
generally happen, however, that this speed is not reached, owing to the fact that the armature and
field currents do not properly correspond, so that when the current is passed through the armature
(the motor not being quite up to synchronism) there is a liability that it will not "hold on," as it is
termed. It is preferable, therefore, to so wind or construct the motor that on the start, when the
armature coils are short-circuited, the motor will tend to reach a speed higher than the synchronous
—as for instance, double the latter. In such case the difficulty above alluded to is not felt, for the
motor will always hold up to synchronism if the synchronous speed—in the case supposed of two
thousand revolutions—is reached or passed. This may be accomplished in various ways; but for all
practical purposes the following will suffice: On the armature are wound two sets of coils. At the
start only one of these is short-circuited, thereby producing a number of poles on the armature,
which will tend to run the speed up above the synchronous limit. When such limit is reached or
passed, the current is directed through the other coil, which, by increasing the number of armature
poles, tends to maintain synchronism.

Fig. 53.
In Fig. 52, such a disposition is shown. The motor having, say, eight poles contains two field-
circuits A and B, of different self-induction. The armature has two coils F and G. The former is
closed upon itself, the latter connected with the field and line through contact-rings a b, brushes c
d, and a switch E. On the start the coil F alone is active and the motor tends to run at a speed above
the synchronous; but when the coil G is connected to the circuit the number of armature poles is
increased, while the motor is made a true synchronous motor. This disposition has the advantage
that the closed armature-circuit imparts to the motor torque when the speed falls off, but at the
same time the conditions are such that the motor comes out of synchronism more readily. To
increase the tendency to synchronism, two circuits may be used on the armature, one of which is
short-circuited on the start and both connected with the external circuit after the synchronous
speed is reached or passed. This disposition is shown in Fig. 53. There are three contact-rings a b e
and three brushes c d f, which connect the armature circuits with the external circuit. On starting,
the switch H is turned to complete the connection between one binding-post P and the field-coils.
This short-circuits one of the armature-coils, as G. The other coil F is out of circuit and open. When
the motor is up to speed, the switch H is turned back, so that the connection from binding-post P to
the field coils is through the coil G, and switch K is closed, thereby including coil F in multiple arc
with the field coils. Both armature coils are thus active.
From the above-described instances it is evident that many other dispositions for carrying out
the invention are possible.

CHAPTER XII.
"Magnetic Lag" Motor.

The following description deals with another form of motor, namely, depending on "magnetic
lag" or hysteresis, its peculiarity being that in it the attractive effects or phases while lagging
behind the phases of current which produce them, are manifested simultaneously and not
successively. The phenomenon utilized thus at an early stage by Mr. Tesla, was not generally
believed in by scientific men, and Prof. Ayrton was probably first to advocate it or to elucidate the
reason of its supposed existence.
Fig. 54 is a side view of the motor, in elevation. Fig. 55 is a part-sectional view at right angles
to Fig. 54. Fig. 56 is an end view in elevation and part section of a modification, and Fig. 57 is a
similar view of another modification.
In Figs. 54 and 55, A designates a base or stand, and B B the supporting-frame of the motor.
Bolted to the supporting-frame are two magnetic cores or pole-pieces C C', of iron or soft steel.
These may be subdivided or laminated, in which case hard iron or steel plates or bars should be
used, or they should be wound with closed coils. D is a circular disc armature, built up of sections
or plates of iron and mounted in the frame between the pole-pieces C C', curved to conform to the
circular shape thereof. This disc may be wound with a number of closed coils E. F F are the main
energizing coils, supported by the supporting-frame, so as to include within their magnetizing
influence both the pole-pieces C C' and the armature D. The pole-pieces C C' project out beyond the
coils F F on opposite sides, as indicated in the drawings. If an alternating current be passed through
the coils F F, rotation of the armature will be produced, and this rotation is explained by the
following apparent action, or mode of operation: An impulse of current in the coils F F establishes
two polarities in the motor. The protruding end of pole-piece C, for instance, will be of one sign,
and the corresponding end of pole-piece C' will be of the opposite sign. The armature also exhibits
two poles at right angles to the coils F F, like poles to those in the pole-pieces being on the same
side of the coils. While the current is flowing there is no appreciable tendency to rotation
developed; but after each current impulse ceases or begins to fall, the magnetism in the armature
and in the ends of the pole-pieces C C' lags or continues to manifest itself, which produces a
rotation of the armature by the repellent force between the more closely approximating points of
maximum magnetic effect. This effect is continued by the reversal of current, the polarities of field
and armature being simply reversed. One or both of the elements—the armature or field—may be
wound with closed induced coils to intensify this effect. Although in the illustrations but one of the
fields is shown, each element of the motor really constitutes a field, wound with the closed coils,
the currents being induced mainly in those convolutions or coils which are parallel to the coils F F.

Fig. 54. Fig. 55.

A modified form of this motor is shown in Fig. 56. In this form G is one of two standards that
support the bearings for the armature-shaft. H H are uprights or sides of a frame, preferably
magnetic, the ends C C' of which are bent in the manner indicated, to conform to the shape of the
armature D and form field-magnet poles. The construction of the armature may be the same as in
the previous figure, or it may be simply a magnetic disc or cylinder, as shown, and a coil or coils F
F are secured in position to surround both the armature and the poles C C'. The armature is
detachable from its shaft, the latter being passed through the armature after it has been inserted in
position. The operation of this form of motor is the same in principle as that previously described
and needs no further explanation.

Fig. 56.
Fig. 57.

One of the most important features in alternating current motors is, however, that they should
be adapted to and capable of running efficiently on the alternating circuits in present use, in which
almost without exception the generators yield a very high number of alternations. Such a motor, of
the type under consideration, Mr. Tesla has designed by a development of the principle of the
motor shown in Fig. 56, making a multipolar motor, which is illustrated in Fig. 57. In the
construction of this motor he employs an annular magnetic frame J, with inwardly-extending ribs
or projections K, the ends of which all bend or turn in one direction and are generally shaped to
conform to the curved surface of the armature. Coils F F are wound from one part K to the one next
adjacent, the ends or loops of each coil or group of wires being carried over toward the shaft, so as
to form U-shaped groups of convolutions at each end of the armature. The pole-pieces C C', being
substantially concentric with the armature, form ledges, along which the coils are laid and should
project to some extent beyond the the coils, as shown. The cylindrical or drum armature D is of the
same construction as in the other motors described, and is mounted to rotate within the annular
frame J and between the U-shaped ends or bends of the coils F. The coils F are connected in
multiple or in series with a source of alternating currents, and are so wound that with a current or
current impulse of given direction they will make the alternate pole-pieces C of one polarity and
the other pole-pieces C' of the opposite polarity. The principle of the operation of this motor is the
same as the other above described, for, considering any two pole-pieces C C', a current impulse
passing in the coil which bridges them or is wound over both tends to establish polarities in their
ends of opposite sign and to set up in the armature core between them a polarity of the same sign
as that of the nearest pole-piece C. Upon the fall or cessation of the current impulse that established
these polarities the magnetism which lags behind the current phase, and which continues to
manifest itself in the polar projections C C' and the armature, produces by repulsion a rotation of the
armature. The effect is continued by each reversal of the current. What occurs in the case of one
pair of pole-pieces occurs simultaneously in all, so that the tendency to rotation of the armature is
measured by the sum of all the forces exerted by the pole-pieces, as above described. In this motor
also the magnetic lag or effect is intensified by winding one or both cores with closed induced
coils. The armature core is shown as thus wound. When closed coils are used, the cores should be
laminated.
It is evident that a pulsatory as well as an alternating current might be used to drive or operate
the motors above described.
It will be understood that the degree of subdivision, the mass of the iron in the cores, their size
and the number of alternations in the current employed to run the motor, must be taken into
consideration in order to properly construct this motor. In other words, in all such motors the
proper relations between the number of alternations and the mass, size, or quality of the iron must
be preserved in order to secure the best results.

CHAPTER XIII.
Method of Obtaining Difference of Phase by
Magnetic Shielding.

In that class of motors in which two or more sets of energizing magnets are employed, and in
which by artificial means a certain interval of time is made to elapse between the respective
maximum or minimum periods or phases of their magnetic attraction or effect, the interval or
difference in phase between the two sets of magnets is limited in extent. It is desirable, however,
for the economical working of such motors that the strength or attraction of one set of magnets
should be maximum, at the time when that of the other set is minimum, and conversely; but these
conditions have not heretofore been realized except in cases where the two currents have been
obtained from independent sources in the same or different machines. Mr. Tesla has therefore
devised a motor embodying conditions that approach more nearly the theoretical requirements of
perfect working, or in other words, he produces artificially a difference of magnetic phase by
means of a current from a single primary source sufficient in extent to meet the requirements of
practical and economical working. He employs a motor with two sets of energizing or field
magnets, each wound with coils connected with a source of alternating or rapidly-varying currents,
but forming two separate paths or circuits. The magnets of one set are protected to a certain extent
from the energizing action of the current by means of a magnetic shield or screen interposed
between the magnet and its energizing coil. This shield is properly adapted to the conditions of
particular cases, so as to shield or protect the main core from magnetization until it has become
itself saturated and no longer capable of containing all the lines of force produced by the current. It
will be seen that by this means the energizing action begins in the protected set of magnets a
certain arbitrarily-determined period of time later than in the other, and that by this means alone or
in conjunction with other means or devices heretofore employed a practical difference of magnetic
phase may readily be secured.
Fig. 58 is a view of a motor, partly in section, with a diagram illustrating the invention. Fig. 59
is a similar view of a modification of the same.

Fig. 58. Fig. 59.

In Fig. 58, which exhibits the simplest form of the invention, A A is the field-magnet of a motor,
having, say, eight poles or inwardly-projecting cores B and C. The cores B form one set of magnets
and are energized by coils D. The cores C, forming the other set are energized by coils E, and the
coils are connected, preferably, in series with one another, in two derived or branched circuits, F G,
respectively, from a suitable source of current. Each coil E is surrounded by a magnetic shield H,
which is preferably composed of an annealed, insulated, or oxidized iron wire wrapped or wound
on the coils in the manner indicated so as to form a closed magnetic circuit around the coils and
between the same and the magnetic cores C. Between the pole pieces or cores B C is mounted the
armature K, which, as is usual in this type of machines, is wound with coils L closed upon
themselves. The operation resulting from this disposition is as follows: If a current impulse be
directed through the two circuits of the motor, it will quickly energize the cores B, but not so the
cores C, for the reason that in passing through the coils E there is encountered the influence of the
closed magnetic circuits formed by the shields H. The first effect is to retard effectively the current
impulse in circuit G, while at the same time the proportion of current which does pass does not
magnetize the cores C, which are shielded or screened by the shields H. As the increasing
electromotive force then urges more current through the coils E, the iron wire H becomes
magnetically saturated and incapable of carrying all the lines of force, and hence ceases to protect
the cores C, which becomes magnetized, developing their maximum effect after an interval of time
subsequent to the similar manifestation of strength in the other set of magnets, the extent of which
is arbitrarily determined by the thickness of the shield H, and other well-understood conditions.
From the above it will be seen that the apparatus or device acts in two ways. First, by retarding
the current, and, second, by retarding the magnetization of one set of the cores, from which its
effectiveness will readily appear.
Many modifications of the principle of this invention are possible. One useful and efficient
application of the invention is shown in Fig. 59. In this figure a motor is shown similar in all
respects to that above described, except that the iron wire H, which is wrapped around the coils E,
is in this case connected in series with the coils D. The iron-wire coils H, are connected and wound,
so as to have little or no self-induction, and being added to the resistance of the circuit F, the action

of the current in that circuit will be accelerated, while in the other circuit G it will be retarded. The
shield H may be made in many forms, as will be understood, and used in different ways, as appears
from the foregoing description.
As a modification of his type of motor with "shielded" fields, Mr. Tesla has constructed a
motor with a field-magnet having two sets of poles or inwardly-projecting cores and placed side by
side, so as practically to form two fields of force and alternately disposed—that is to say, with the
poles of one set or field opposite the spaces between the other. He then connects the free ends of
one set of poles by means of laminated iron bands or bridge-pieces of considerably smaller cross-
section than the cores themselves, whereby the cores will all form parts of complete magnetic
circuits. When the coils on each set of magnets are connected in multiple circuits or branches from
a source of alternating currents, electromotive forces are set up in or impressed upon each circuit
simultaneously; but the coils on the magnetically bridged or shunted cores will have, by reason of
the closed magnetic circuits, a high self-induction, which retards the current, permitting at the
beginning of each impulse but little current to pass. On the other hand, no such opposition being
encountered in the other set of coils, the current passes freely through them, magnetizing the poles
on which they are wound. As soon, however, as the laminated bridges become saturated and
incapable of carrying all the lines of force which the rising electromotive force, and consequently
increased current, produce, free poles are developed at the ends of the cores, which, acting in
conjunction with the others, produce rotation of the armature.
The construction in detail by which this invention is illustrated is shown in the accompanying
drawings.

Fig. 60. Fig. 61.

Fig. 60 is a view in side elevation of a motor embodying the principle. Fig. 61 is a vertical
cross-section of the motor. A is the frame of the motor, which should be built up of sheets of iron
punched out to the desired shape and bolted together with insulation between the sheets. When
complete, the frame makes a field-magnet with inwardly projecting pole-pieces B and C. To adapt
them to the requirements of this particular case these pole-pieces are out of line with one another,
those marked B surrounding one end of the armature and the others, as C, the opposite end, and
they are disposed alternately—that is to say, the pole-pieces of one set occur in line with the spaces
between those of the other sets.

The armature D is of cylindrical form, and is also laminated in the usual way and is wound
longitudinally with coils closed upon themselves. The pole-pieces C are connected or shunted by
bridge-pieces E. These may be made independently and attached to the pole-pieces, or they may be
parts of the forms or blanks stamped or punched out of sheet-iron. Their size or mass is determined
by various conditions, such as the strength of the current to be employed, the mass or size of the
cores to which they are applied, and other familiar conditions.
Coils F surround the pole-pieces B, and other coils G are wound on the pole-pieces C. These
coils are connected in series in two circuits, which are branches of a circuit from a generator of
alternating currents, and they may be so wound, or the respective circuits in which they are
included may be so arranged, that the circuit of coils G will have, independently of the particular
construction described, a higher self-induction than the other circuit or branch.
The function of the shunts or bridges E is that they shall form with the cores C a closed
magnetic circuit for a current up to a predetermined strength, so that when saturated by such
current and unable to carry more lines of force than such a current produces they will to no further
appreciable extent interfere with the development, by a stronger current, of free magnetic poles at
the ends of the cores C.
In such a motor the current is so retarded in the coils G, and the manifestation of the free
magnetism in the poles C is so delayed beyond the period of maximum magnetic effect in poles B,
that a strong torque is produced and the motor operates with approximately the power developed in
a motor of this kind energized by independently generated currents differing by a full quarter
phase.

CHAPTER XIV.
Type of Tesla Single-Phase Motor.

Up to this point, two principal types of Tesla motors have been described: First, those
containing two or more energizing circuits through which are caused to pass alternating currents
differing from one another in phase to an extent sufficient to produce a continuous progression or
shifting of the poles or points of greatest magnetic effect, in obedience to which the movable
element of the motor is maintained in rotation; second, those containing poles, or parts of different
magnetic susceptibility, which under the energizing influence of the same current or two currents
coinciding in phase will exhibit differences in their magnetic periods or phases. In the first class of
motors the torque is due to the magnetism established in different portions of the motor by currents
from the same or from independent sources, and exhibiting time differences in phase. In the
second class the torque results from the energizing effects of a current upon different parts of the
motor which differ in magnetic susceptibility—in other words, parts which respond in the same
relative degree to the action of a current, not simultaneously, but after different intervals of time.
In another Tesla motor, however, the torque, instead of being solely the result of a time
difference in the magnetic periods or phases of the poles or attractive parts to whatever cause due,
is produced by an angular displacement of the parts which, though movable with respect to one
another, are magnetized simultaneously, or approximately so, by the same currents. This principle
of operation has been embodied practically in a motor in which the necessary angular
displacement between the points of greatest magnetic attraction in the two elements of the motor—
the armature and field—is obtained by the direction of the lamination of the magnetic cores of the
elements.
Fig. 62 is a side view of such a motor with a portion of its armature core exposed. Fig. 63 is an
end or edge view of the same. Fig. 64 is a central cross-section of the same, the armature being
shown mainly in elevation.

Fig. 62. Fig. 63. Fig. 64.

Let A A designate two plates built up of thin sections or laminæ of soft iron insulated more or
less from one another and held together by bolts a and secured to a base B. The inner faces of these
plates contain recesses or grooves in which a coil or coils D are secured obliquely to the direction
of the laminations. Within the coils D is a disc E, preferably composed of a spirally-wound iron
wire or ribbon or a series of concentric rings and mounted on a shaft F, having bearings in the
plates A A. Such a device when acted upon by an alternating current is capable of rotation and
constitutes a motor, the operation of which may be explained in the following manner: A current or
current-impulse traversing the coils D tends to magnetize the cores A A and E, all of which are
within the influence of the field of the coils. The poles thus established would naturally lie in the
same line at right angles to the coils D, but in the plates A they are deflected by reason of the
direction of the laminations, and appear at or near the extremities of these plates. In the disc,
however, where these conditions are not present, the poles or points of greatest attraction are on a
line at right angles to the plane of the coils; hence there will be a torque established by this angular
displacement of the poles or magnetic lines, which starts the disc in rotation, the magnetic lines of
the armature and field tending toward a position of parallelism. This rotation is continued and
maintained by the reversals of the current in coils D D, which change alternately the polarity of the
field-cores A A. This rotary tendency or effect will be greatly increased by winding the disc with
conductors G, closed upon themselves and having a radial direction, whereby the magnetic
intensity of the poles of the disc will be greatly increased by the energizing effect of the currents
induced in the coils G by the alternating currents in coils D.
The cores of the disc and field may or may not be of different magnetic susceptibility—that is
to say, they may both be of the same kind of iron, so as to be magnetized at approximately the
same instant by the coils D; or one may be of soft iron and the other of hard, in order that a certain
time may elapse between the periods of their magnetization. In either case rotation will be
produced; but unless the disc is provided with the closed energizing coils it is desirable that the
above-described difference of magnetic susceptibility be utilized to assist in its rotation.
The cores of the field and armature may be made in various ways, as will be well understood,
it being only requisite that the laminations in each be in such direction as to secure the necessary
angular displacement of the points of greatest attraction. Moreover, since the disc may be
considered as made up of an infinite number of radial arms, it is obvious that what is true of a disc
holds for many other forms of armature.

CHAPTER XV.
Motors with Circuits of Different Resistance.

As has been pointed out elsewhere, the lag or retardation of the phases of an alternating current
is directly proportional to the self-induction and inversely proportional to the resistance of the
circuit through which the current flows. Hence, in order to secure the proper differences of phase
between the two motor-circuits, it is desirable to make the self-induction in one much higher and
the resistance much lower than the self-induction and resistance, respectively, in the other. At the
same time the magnetic quantities of the two poles or sets of poles which the two circuits produce
should be approximately equal. These requirements have led Mr. Tesla to the invention of a motor
having the following general characteristics: The coils which are included in that energizing circuit
which is to have the higher self-induction are made of coarse wire, or a conductor of relatively low
resistance, and with the greatest possible length or number of turns. In the other set of coils a
comparatively few turns of finer wire are used, or a wire of higher resistance. Furthermore, in
order to approximate the magnetic quantities of the poles excited by these coils, Mr. Tesla employs
in the self-induction circuit cores much longer than those in the other or resistance circuit.
Fig. 65 is a part sectional view of the motor at right angles to the shaft. Fig. 66 is a diagram of
the field circuits.
In Fig. 66, let A represent the coils in one motor circuit, and B those in the other. The circuit A
is to have the higher self-induction. There are, therefore, used a long length or a large number of
turns of coarse wire in forming the coils of this circuit. For the circuit B, a smaller conductor is
employed, or a conductor of a higher resistance than copper, such as German silver or iron, and the
coils are wound with fewer turns. In applying these coils to a motor, Mr. Tesla builds up a field-
magnet of plates C, of iron and steel, secured together in the usual manner by bolts D. Each plate is
formed with four (more or less) long cores E, around which is a space to receive the coil and an
equal number of short projections F to receive the coils of the resistance-circuit. The plates are
generally annular in shape, having an open space in the centre for receiving the armature G, which
Mr. Tesla prefers to wind with closed coils. An alternating current divided between the two circuits
is retarded as to its phases in the circuit A to a much greater extent than in the circuit B. By reason
of the relative sizes and disposition of the cores and coils the magnetic effect of the poles E and F
upon the armature closely approximate.

Fig. 65. Fig. 66.

An important result secured by the construction shown here is that these coils which are
designed to have the higher self-induction are almost completely surrounded by iron, and that the
retardation is thus very materially increased.

CHAPTER XVI.
Motor With Equal Magnetic Energies in Field
and Armature.

Let it be assumed that the energy as represented in the magnetism in the field of a given
rotating field motor is ninety and that of the armature ten. The sum of these quantities, which
represents the total energy expended in driving the motor, is one hundred; but, assuming that the
motor be so constructed that the energy in the field is represented by fifty, and that in the armature
by fifty, the sum is still one hundred; but while in the first instance the product is nine hundred, in
the second it is two thousand five hundred, and as the energy developed is in proportion to these
products it is clear that those motors are the most efficient—other things being equal—in which
the magnetic energies developed in the armature and field are equal. These results Mr. Tesla
obtains by using the same amount of copper or ampere turns in both elements when the cores of
both are equal, or approximately so, and the same current energizes both; or in cases where the
currents in one element are induced to those of the other he uses in the induced coils an excess of
copper over that in the primary element or conductor.

Fig. 67.
The conventional figure of a motor here introduced, Fig. 67, will give an idea of the solution
furnished by Mr. Tesla for the specific problem. Referring to the drawing, A is the field-magnet, B
the armature, C the field coils, and D the armature-coils of the motor.
Generally speaking, if the mass of the cores of armature and field be equal, the amount of
copper or ampere turns of the energizing coils on both should also be equal; but these conditions
will be modified in different forms of machine. It will be understood that these results are most

advantageous when existing under the conditions presented where the motor is running with its
normal load, a point to be well borne in mind.

CHAPTER XVII.
Motors With Coinciding Maxima of Magnetic
Effect in Armature and Field.

In this form of motor, Mr. Tesla's object is to design and build machines wherein the maxima
of the magnetic effects of the armature and field will more nearly coincide than in some of the
types previously under consideration. These types are: First, motors having two or more energizing
circuits of the same electrical character, and in the operation of which the currents used differ
primarily in phase; second, motors with a plurality of energizing circuits of different electrical
character, in or by means of which the difference of phase is produced artificially, and, third,
motors with a plurality of energizing circuits, the currents in one being induced from currents in
another. Considering the structural and operative conditions of any one of them—as, for example,
that first named—the armature which is mounted to rotate in obedience to the co-operative
influence or action of the energizing circuits has coils wound upon it which are closed upon
themselves and in which currents are induced by the energizing-currents with the object and result
of energizing the armature-core; but under any such conditions as must exist in these motors, it is
obvious that a certain time must elapse between the manifestations of an energizing current
impulse in the field coils, and the corresponding magnetic state or phase in the armature
established by the current induced thereby; consequently a given magnetic influence or effect in
the field which is the direct result of a primary current impulse will have become more or less
weakened or lost before the corresponding effect in the armature indirectly produced has reached
its maximum. This is a condition unfavorable to efficient working in certain cases—as, for
instance, when the progress of the resultant poles or points of maximum attraction is very great, or
when a very high number of alternations is employed—for it is apparent that a stronger tendency
to rotation will be maintained if the maximum magnetic attractions or conditions in both armature
and field coincide, the energy developed by a motor being measured by the product of the
magnetic quantities of the armature and field.
To secure this coincidence of maximum magnetic effects, Mr. Tesla has devised various means,
as explained below. Fig. 68 is a diagrammatic illustration of a Tesla motor system in which the
alternating currents proceed from independent sources and differ primarily in phase.

Fig. 68. Fig. 69.

A designates the field-magnet or magnetic frame of the motor; B B, oppositely located pole-
pieces adapted to receive the coils of one energizing circuit; and C C, similar pole-pieces for the
coils of the other energizing circuit. These circuits are designated, respectively, by D E, the
conductor D'' forming a common return to the generator G. Between these poles is mounted an
armature—for example, a ring or annular armature, wound with a series of coils F, forming a
closed circuit or circuits. The action or operation of a motor thus constructed is now well
understood. It will be observed, however, that the magnetism of poles B, for example, established
by a current impulse in the coils thereon, precedes the magnetic effect set up in the armature by the
induced current in coils F. Consequently the mutual attraction between the armature and field-poles
is considerably reduced. The same conditions will be found to exist if, instead of assuming the
poles B or C as acting independently, we regard the ideal resultant of both acting together, which is
the real condition. To remedy this, the motor field is constructed with secondary poles B' C', which
are situated between the others. These pole-pieces are wound with coils D' E', the former in
derivation to the coils D, the latter to coils E. The main or primary coils D and E are wound for a
different self-induction from that of the coils D' and E', the relations being so fixed that if the
currents in D and E differ, for example, by a quarter-phase, the currents in each secondary coil, as D'
E', will differ from those in its appropriate primary D or E by, say, forty-five degrees, or one-eighth
of a period.

Now, assuming that an impulse or alternation in circuit or branch E is just beginning, while in
the branch D it is just falling from maximum, the conditions are those of a quarter-phase
difference. The ideal resultant of the attractive forces of the two sets of poles B C therefore may be
considered as progressing from poles B to poles C, while the impulse in E is rising to maximum,
and that in D is falling to zero or minimum. The polarity set up in the armature, however, lags
behind the manifestations of field magnetism, and hence the maximum points of attraction in
armature and field, instead of coinciding, are angularly displaced. This effect is counteracted by
the supplemental poles B' C'. The magnetic phases of these poles succeed those of poles B C by the
same, or nearly the same, period of time as elapses between the effect of the poles B C and the
corresponding induced effect in the armature; hence the magnetic conditions of poles B' C' and of
the armature more nearly coincide and a better result is obtained. As poles B' C' act in conjunction
with the poles in the armature established by poles B C, so in turn poles C B act similarly with the
poles set up by B' C', respectively. Under such conditions the retardation of the magnetic effect of
the armature and that of the secondary poles will bring the maximum of the two more nearly into
coincidence and a correspondingly stronger torque or magnetic attraction secured.
In such a disposition as is shown in Fig. 68 it will be observed that as the adjacent pole-pieces
of either circuit are of like polarity they will have a certain weakening effect upon one another. Mr.
Tesla therefore prefers to remove the secondary poles from the direct influence of the others. This
may be done by constructing a motor with two independent sets of fields, and with either one or
two armatures electrically connected, or by using two armatures and one field. These
modifications are illustrated further on.

Fig. 70. Fig. 71.

Fig. 69 is a diagrammatic illustration of a motor and system in which the difference of phase is
artificially produced. There are two coils D D in one branch and two coils E E in another branch of
the main circuit from the generator G. These two circuits or branches are of different self-
induction, one, as D, being higher than the other. This is graphically indicated by making coils D
much larger than coils E. By reason of the difference in the electrical character of the two circuits,
the phases of current in one are retarded to a greater extent than the other. Let this difference be
thirty degrees. A motor thus constructed will rotate under the action of an alternating current; but
as happens in the case previously described the corresponding magnetic effects of the armature
and field do not coincide owing to the time that elapses between a given magnetic effect in the
armature and the condition of the field that produces it. The secondary or supplemental poles B' C'
are therefore availed of. There being thirty degrees difference of phase between the currents in
coils D E, the magnetic effect of poles B' C' should correspond to that produced by a current
differing from the current in coils D or E by fifteen degrees. This we can attain by winding each
supplemental pole B' C' with two coils H H'. The coils H are included in a derived circuit having the
same self-induction as circuit D, and coils H' in a circuit having the same self-induction as circuit E,
so that if these circuits differ by thirty degrees the magnetism of poles B' C' will correspond to that
produced by a current differing from that in either D or E by fifteen degrees. This is true in all other
cases. For example, if in Fig. 68 the coils D' E' be replaced by the coils H H' included in the derived
circuits, the magnetism of the poles B' C' will correspond in effect or phase, if it may be so termed,
to that produced by a current differing from that in either circuit D or E by forty-five degrees, or
one-eighth of a period.
This invention as applied to a derived circuit motor is illustrated in Figs. 70 and 71. The former
is an end view of the motor with the armature in section and a diagram of connections, and Fig. 71
a vertical section through the field. These figures are also drawn to show one of the dispositions of
two fields that may be adopted in carrying out the principle. The poles B B C C are in one field, the
remaining poles in the other. The former are wound with primary coils I J and secondary coils I' J',
the latter with coils K L. The primary coils I J are in derived circuits, between which, by reason of
their different self-induction, there is a difference of phase, say, of thirty degrees. The coils I' K are
in circuit with one another, as also are coils J' L, and there should be a difference of phase between
the currents in coils K and L and their corresponding primaries of, say, fifteen degrees. If the poles
B C are at right angles, the armature-coils should be connected directly across, or a single armature
core wound from end to end may be used; but if the poles B C be in line there should be an angular
displacement of the armature coils, as will be well understood.
The operation will be understood from the foregoing. The maximum magnetic condition of a
pair of poles, as B' B', coincides closely with the maximum effect in the armature, which lags
behind the corresponding condition in poles B B.

CHAPTER XVIII.
Motor Based on the Difference of Phase in the
Magnetization of the Inner and Outer Parts of an
Iron Core.

It is well known that if a magnetic core, even if laminated or subdivided, be wound with an
insulated coil and a current of electricity be directed through the coil, the magnetization of the
entire core does not immediately ensue, the magnetizing effect not being exhibited in all parts
simultaneously. This may be attributed to the fact that the action of the current is to energize first
those laminæ or parts of the core nearest the surface and adjacent to the exciting-coil, and from
thence the action progresses toward the interior. A certain interval of time therefore elapses
between the manifestation of magnetism in the external and the internal sections or layers of the
core. If the core be thin or of small mass, this effect may be inappreciable; but in the case of a
thick core, or even of a comparatively thin one, if the number of alternations or rate of change of
the current strength be very great, the time interval occurring between the manifestations of
magnetism in the interior of the core and in those parts adjacent to the coil is more marked. In the
construction of such apparatus as motors which are designed to be run by alternating or equivalent
currents—such as pulsating or undulating currents generally—Mr. Tesla found it desirable and
even necessary to give due consideration to this phenomenon and to make special provisions in
order to obviate its consequences. With the specific object of taking advantage of this action or
effect, and to render it more pronounced, he constructs a field magnet in which the parts of the
core or cores that exhibit at different intervals of time the magnetic effect imparted to them by
alternating or equivalent currents in an energizing coil or coils, are so placed with relation to a
rotating armature as to exert thereon their attractive effect successively in the order of their
magnetization. By this means he secures a result similar to that which he had previously attained in
other forms or types of motor in which by means of one or more alternating currents he has
produced the rotation or progression of the magnetic poles.
This new mode of operation will now be described. Fig. 72 is a side elevation of such motor.
Fig. 73 is a side elevation of a more practicable and efficient embodiment of the invention. Fig. 74
is a central vertical section of the same in the plane of the axis of rotation.

Figs. 72 and 73.
Referring to Fig. 72, let X represent a large iron core, which may be composed of a number of
sheets or laminæ of soft iron or steel. Surrounding this core is a coil Y, which is connected with a
source E of rapidly varying currents. Let us consider now the magnetic conditions existing in this
core at any point, as b, at or near the centre, and any other point, as a, nearer the surface. When a
current impulse is started in the magnetizing coil Y, the section or part at a, being close to the coil,
is immediately energized, while the section or part at b, which, to use a convenient expression, is
"protected" by the intervening sections or layers between a and b, does not at once exhibit its
magnetism. However, as the magnetization of a increases, b becomes also affected, reaching
finally its maximum strength some time later than a. Upon the weakening of the current the
magnetization of a first diminishes, while b still exhibits its maximum strength; but the continued
weakening of a is attended by a subsequent weakening of b. Assuming the current to be an
alternating one, a will now be reversed, while b still continues of the first imparted polarity. This
action continues the magnetic condition of b, following that of a in the manner above described. If
an armature—for instance, a simple disc F, mounted to rotate freely on an axis—be brought into
proximity to the core, a movement of rotation will be imparted to the disc, the direction depending
upon its position relatively to the core, the tendency being to turn the portion of the disc nearest to
the core from a to b, as indicated in Fig. 72.

Fig. 74.
This action or principle of operation has been embodied in a practicable form of motor, which
is illustrated in Fig. 73. Let A in that figure represent a circular frame of iron, from diametrically
opposite points of the interior of which the cores project. Each core is composed of three main
parts B, B and C, and they are similarly formed with a straight portion or body e, around which the
energizing coil is wound, a curved arm or extension c, and an inwardly projecting pole or end d.
Each core is made up of two parts B B, with their polar extensions reaching in one direction, and a
part C between the other two, and with its polar extension reaching in the opposite direction. In
order to lessen in the cores the circulation of currents induced therein, the several sections are
insulated from one another in the manner usually followed in such cases. These cores are wound
with coils D, which are connected in the same circuit, either in parallel or series, and supplied with
an alternating or a pulsating current, preferably the former, by a generator E, represented
diagrammatically. Between the cores or their polar extensions is mounted a cylindrical or similar
armature F, wound with magnetizing coils G, closed upon themselves.
The operation of this motor is as follows: When a current impulse or alternation is directed
through the coils D, the sections B B of the cores, being on the surface and in close proximity to the
coils, are immediately energized. The sections C, on the other hand, are protected from the
magnetizing influence of the coil by the interposed layers of iron B B. As the magnetism of B B
increases, however, the sections C are also energized; but they do not attain their maximum
strength until a certain time subsequent to the exhibition by the sections B B of their maximum.
Upon the weakening of the current the magnetic strength of B B first diminishes, while the sections
C have still their maximum strength; but as B B continue to weaken the interior sections are
similarly weakened. B B may then begin to exhibit an opposite polarity, which is followed later by
a similar change on C, and this action continues. B B and C may therefore be considered as separate
field-magnets, being extended so as to act on the armature in the most efficient positions, and the
effect is similar to that in the other forms of Tesla motor—viz., a rotation or progression of the

maximum points of the field of force. Any armature—such, for instance, as a disc—mounted in
this field would rotate from the pole first to exhibit its magnetism to that which exhibits it later.
It is evident that the principle here described may be carried out in conjunction with other
means for securing a more favorable or efficient action of the motor. For example, the polar
extensions of the sections C may be wound or surrounded by closed coils. The effect of these coils
will be to still more effectively retard the magnetization of the polar extensions of C.

CHAPTER XIX.
Another Type of Tesla Induction Motor.

It will have been gathered by all who are interested in the advance of the electrical arts, and
who follow carefully, step by step, the work of pioneers, that Mr. Tesla has been foremost to utilize
inductive effects in permanently closed circuits, in the operation of alternating motors. In this
chapter one simple type of such a motor is described and illustrated, which will serve as an
exemplification of the principle.
Let it be assumed that an ordinary alternating current generator is connected up in a circuit of
practically no self-induction, such, for example, as a circuit containing incandescent lamps only.
On the operation of the machine, alternating currents will be developed in the circuit, and the
phases of these currents will theoretically coincide with the phases of the impressed electromotive
force. Such currents may be regarded and designated as the "unretarded currents."
It will be understood, of course, that in practice there is always more or less self-induction in
the circuit, which modifies to a corresponding extent these conditions; but for convenience this
may be disregarded in the consideration of the principle of operation, since the same laws apply.
Assume next that a path of currents be formed across any two points of the above circuit,
consisting, for example, of the primary of an induction device. The phases of the currents passing
through the primary, owing to the self-induction of the same, will not coincide with the phases of
the impressed electromotive force, but will lag behind, such lag being directly proportional to the
self-induction and inversely proportional to the resistance of the said coil. The insertion of this coil
will also cause a lagging or retardation of the currents traversing and delivered by the generator
behind the impressed electromotive force, such lag being the mean or resultant of the lag of the
current through the primary alone and of the "unretarded current" in the entire working circuit.
Next consider the conditions imposed by the association in inductive relation with the primary
coil, of a secondary coil. The current generated in the secondary coil will react upon the primary
current, modifying the retardation of the same, according to the amount of self-induction and
resistance in the secondary circuit. If the secondary circuit has but little self-induction—as, for
instance, when it contains incandescent lamps only—it will increase the actual difference of phase
between its own and the primary current, first, by diminishing the lag between the primary current
and the impressed electromotive force, and, second, by its own lag or retardation behind the
impressed electromotive force. On the other hand, if the secondary circuit have a high self-
induction, its lag behind the current in the primary is directly increased, while it will be still further
increased if the primary have a very low self-induction. The better results are obtained when the
primary has a low self-induction.

Fig. 75. Fig. 76.

Fig. 75 is a diagram of a Tesla motor embodying this principle. Fig. 76 is a similar diagram of
a modification of the same. In Fig. 75 let A designate the field-magnet of a motor which, as in all
these motors, is built up of sections or plates. B C are polar projections upon which the coils are
wound. Upon one pair of these poles, as C, are wound primary coils D, which are directly
connected to the circuit of an alternating current generator G. On the same poles are also wound
secondary coils F, either side by side or over or under the primary coils, and these are connected
with other coils E, which surround the poles B B. The currents in both primary and secondary coils
in such a motor will be retarded or will lag behind the impressed electromotive force; but to secure
a proper difference in phase between the primary and secondary currents themselves, Mr. Tesla
increases the resistance of the circuit of the secondary and reduces as much as practicable its self-
induction. This is done by using for the secondary circuit, particularly in the coils E, wire of
comparatively small diameter and having but few turns around the cores; or by using some
conductor of higher specific resistance, such as German silver; or by introducing at some point in
the secondary circuit an artificial resistance R. Thus the self-induction of the secondary is kept
down and its resistance increased, with the result of decreasing the lag between the impressed
electro-motive force and the current in the primary coils and increasing the difference of phase
between the primary and secondary currents.
In the disposition shown in Fig. 76, the lag in the secondary is increased by increasing the self-
induction of that circuit, while the increasing tendency of the primary to lag is counteracted by
inserting therein a dead resistance. The primary coils D in this case have a low self-induction and
high resistance, while the coils E F, included in the secondary circuit, have a high self-induction
and low resistance. This may be done by the proper winding of the coils; or in the circuit including
the secondary coils E F, we may introduce a self-induction coil S, while in the primary circuit from
the generator G and including coils D, there may be inserted a dead resistance R. By this means the
difference of phase between the primary and secondary is increased. It is evident that both means

of increasing the difference of phase—namely, by the special winding as well as by the
supplemental or external inductive and dead resistance—may be employed conjointly.
In the operation of this motor the current impulses in the primary coils induce currents in the
secondary coils, and by the conjoint action of the two the points of greatest magnetic attraction are
shifted or rotated.
In practice it is found desirable to wind the armature with closed coils in which currents are
induced by the action thereon of the primaries.

CHAPTER XX.
Combinations of Synchronizing Motor and Torque
Motor.

In the preceding descriptions relative to synchronizing motors and methods of operating them,
reference has been made to the plan adopted by Mr. Tesla, which consists broadly in winding or
arranging the motor in such manner that by means of suitable switches it could be started as a
multiple-circuit motor, or one operating by a progression of its magnetic poles, and then, when up
to speed, or nearly so, converted into an ordinary synchronizing motor, or one in which the
magnetic poles were simply alternated. In some cases, as when a large motor is used and when the
number of alternations is very high, there is more or less difficulty in bringing the motor to speed
as a double or multiple-circuit motor, for the plan of construction which renders the motor best
adapted to run as a synchronizing motor impairs its efficiency as a torque or double-circuit motor
under the assumed conditions on the start. This will be readily understood, for in a large
synchronizing motor the length of the magnetic circuit of the polar projections, and their mass, are
so great that apparently considerable time is required for magnetization and demagnetization.
Hence with a current of a very high number of alternations the motor may not respond properly. To
avoid this objection and to start up a synchronizing motor in which these conditions obtain, Mr.
Tesla has combined two motors, one a synchronizing motor, the other a multiple-circuit or torque
motor, and by the latter he brings the first-named up to speed, and then either throws the whole
current into the synchronizing motor or operates jointly both of the motors.
This invention involves several novel and useful features. It will be observed, in the first place,
that both motors are run, without commutators of any kind, and, secondly, that the speed of the
torque motor may be higher than that of the synchronizing motor, as will be the case when it
contains a fewer number of poles or sets of poles, so that the motor will be more readily and easily
brought up to speed. Thirdly, the synchronizing motor may be constructed so as to have a much
more pronounced tendency to synchronism without lessening the facility with which it is started.
Fig. 77 is a part sectional view of the two motors; Fig. 78 an end view of the synchronizing
motor; Fig. 79 an end view and part section of the torque or double-circuit motor; Fig. 80 a
diagram of the circuit connections employed; and Figs. 81, 82, 83, 84 and 85 are diagrams of
modified dispositions of the two motors.

Fig. 77.
Inasmuch as neither motor is doing any work while the current is acting upon the other, the two
armatures are rigidly connected, both being mounted upon the same shaft A, the field-magnets B of
the synchronizing and C of the torque motor being secured to the same base D. The preferably
larger synchronizing motor has polar projections on its armature, which rotate in very close
proximity to the poles of the field, and in other respects it conforms to the conditions that are
necessary to secure synchronous action. The pole-pieces of the armature are, however, wound with
closed coils E, as this obviates the employment of sliding contacts. The smaller or torque motor, on
the other hand, has, preferably, a cylindrical armature F, without polar projections and wound with
closed coils G. The field-coils of the torque motor are connected up in two series H and I, and the
alternating current from the generator is directed through or divided between these two circuits in
any manner to produce a progression of the poles or points of maximum magnetic effect. This
result is secured by connecting the two motor-circuits in derivation with the circuit from the
generator, inserting in one motor circuit a dead resistance and in the other a self-induction coil, by
which means a difference in phase between the two divisions of the current is secured. If both
motors have the same number of field poles, the torque motor for a given number of alternations
will tend to run at double the speed of the other, for, assuming the connections to be such as to
give the best results, its poles are divided into two series and the number of poles is virtually
reduced one-half, which being acted upon by the same number of alternations tend to rotate the
armature at twice the speed. By this means the main armature is more easily brought to or above
the required speed. When the speed necessary for synchronism is imparted to the main motor, the
current is shifted from the torque motor into the other.

Fig. 78. Fig. 79.

A convenient arrangement for carrying out this invention is shown in Fig. 80, in which J J are
the field coils of the synchronizing, and H I the field coils of the torque motor. L L' are the
conductors of the main line. One end of, say, coils H is connected to wire L through a self-induction
coil M. One end of the other set of coils I is connected to the same wire through a dead resistance
N. The opposite ends of these two circuits are connected to the contact m of a switch, the handle or
lever of which is in connection with the line-wire L'. One end of the field circuit of the
synchronizing motor is connected to the wire L. The other terminates in the switch-contact n. From
the diagram it will be readily seen that if the lever P be turned upon contact m, the torque motor
will start by reason of the difference of phase between the currents in its two energizing circuits.
Then when the desired speed is attained, if the lever P be shifted upon contact n the entire current
will pass through the field coils of the synchronizing motor and the other will be doing no work.
The torque motor may be constructed and operated in various ways, many of which have
already been touched upon. It is not necessary that one motor be cut out of circuit while the other
is in, for both may be acted upon by current at the same time, and Mr. Tesla has devised various
dispositions or arrangements of the two motors for accomplishing this. Some of these
arrangements are illustrated in Figs. 81 to 85.

Fig. 80.
Referring to Fig. 81, let T designate the torque or multiple circuit motor and S the
synchronizing motor, L L' being the line-wires from a source of alternating current. The two circuits
of the torque motor of different degrees of self-induction, and designated by N M, are connected in
derivation to the wire L. They are then joined and connected to the energizing circuit of the
synchronizing motor, the opposite terminal of which is connected to wire L'. The two motors are
thus in series. To start them Mr. Tesla short-circuits the synchronizing motor by a switch P',
throwing the whole current through the torque motor. Then when the desired speed is reached the
switch P' is opened, so that the current passes through both motors. In such an arrangement as this
it is obviously desirable for economical and other reasons that a proper relation between the speeds
of the two motors should be observed.
In Fig. 82 another disposition is illustrated. S is the synchronizing motor and T the torque
motor, the circuits of both being in parallel. W is a circuit also in derivation to the motor circuits
and containing a switch P''. S' is a switch in the synchronizing motor circuit. On the start the switch
S' is opened, cutting out the motor S. Then P'' is opened, throwing the entire current through the
motor T, giving it a very strong torque. When the desired speed is reached, switch S' is closed and
the current divides between both motors. By means of switch P'' both motors may be cut out.

Figs. 81, 82, 83, 84 and 85.
In Fig. 83 the arrangement is substantially the same, except that a switch T' is placed in the
circuit which includes the two circuits of the torque motor. Fig. 84 shows the two motors in series,
with a shunt around both containing a switch S T. There is also a shunt around the synchronizing
motor S, with a switch P'. In Fig. 85 the same disposition is shown; but each motor is provided with
a shunt, in which are switches P' and T'', as shown.

CHAPTER XXI.
Motor with a Condenser in the Armature Circuit.

We now come to a new class of motors in which resort is had to condensers for the purpose of
developing the required difference of phase and neutralizing the effects of self-induction. Mr. Tesla
early began to apply the condenser to alternating apparatus, in just how many ways can only be
learned from a perusal of other portions of this volume, especially those dealing with his high
frequency work.
Certain laws govern the action or effects produced by a condenser when connected to an
electric circuit through which an alternating or in general an undulating current is made to pass.
Some of the most important of such effects are as follows: First, if the terminals or plates of a
condenser be connected with two points of a circuit, the potentials of which are made to rise and
fall in rapid succession, the condenser allows the passage, or more strictly speaking, the
transference of a current, although its plates or armatures may be so carefully insulated as to
prevent almost completely the passage of a current of unvarying strength or direction and of
moderate electromotive force. Second, if a circuit, the terminals of which are connected with the
plates of the condenser, possess a certain self-induction, the condenser will overcome or counteract
to a greater or less degree, dependent upon well-understood conditions, the effects of such self-
induction. Third, if two points of a closed or complete circuit through which a rapidly rising and
falling current flows be shunted or bridged by a condenser, a variation in the strength of the
currents in the branches and also a difference of phase of the currents therein is produced. These
effects Mr. Tesla has utilized and applied in a variety of ways in the construction and operation of
his motors, such as by producing a difference in phase in the two energizing circuits of an
alternating current motor by connecting the two circuits in derivation and connecting up a
condenser in series in one of the circuits. A further development, however, possesses certain novel
features of practical value and involves a knowledge of facts less generally understood. It
comprises the use of a condenser or condensers in connection with the induced or armature circuit
of a motor and certain details of the construction of such motors. In an alternating current motor of
the type particularly referred to above, or in any other which has an armature coil or circuit closed
upon itself, the latter represents not only an inductive resistance, but one which is periodically
varying in value, both of which facts complicate and render difficult the attainment of the
conditions best suited to the most efficient working conditions; in other words, they require, first,
that for a given inductive effect upon the armature there should be the greatest possible current
through the armature or induced coils, and, second, that there should always exist between the
currents in the energizing and the induced circuits a given relation of phase. Hence whatever tends
to decrease the self-induction and increase the current in the induced circuits will, other things
being equal, increase the output and efficiency of the motor, and the same will be true of causes
that operate to maintain the mutual attractive effect between the field magnets and armature at its
maximum. Mr. Tesla secures these results by connecting with the induced circuit or circuits a
condenser, in the manner described below, and he also, with this purpose in view, constructs the
motor in a special manner.

Fig. 86.

Fig. 88. Fig. 89.

Fig. 90.

Fig. 87.

Referring to the drawings, Fig. 86, is a view, mainly diagrammatic, of an alternating current
motor, in which the present principle is applied. Fig. 87 is a central section, in line with the shaft,
of a special form of armature core. Fig. 88 is a similar section of a modification of the same. Fig.
89 is one of the sections of the core detached. Fig. 90 is a diagram showing a modified disposition
of the armature or induced circuits.
The general plan of the invention is illustrated in Fig. 86. A A in this figure represent the the
frame and field magnets of an alternating current motor, the poles or projections of which are
wound with coils B and C, forming independent energizing circuits connected either to the same or
to independent sources of alternating currents, so that the currents flowing through the circuits,
respectively, will have a difference of phase. Within the influence of this field is an armature core
D, wound with coils E. In motors of this description heretofore these coils have been closed upon
themselves, or connected in a closed series; but in the present case each coil or the connected
series of coils terminates in the opposite plates of a condenser F. For this purpose the ends of the
series of coils are brought out through the shaft to collecting rings G, which are connected to the
condenser by contact brushes H and suitable conductors, the condenser being independent of the
machine. The armature coils are wound or connected in such manner that adjacent coils produce
opposite poles.
The action of this motor and the effect of the plan followed in its construction are as follows:
The motor being started in operation and the coils of the field magnets being traversed by
alternating currents, currents are induced in the armature coils by one set of field coils, as B, and
the poles thus established are acted upon by the other set, as C. The armature coils, however, have
necessarily a high self-induction, which opposes the flow of the currents thus set up. The
condenser F not only permits the passage or transference of these currents, but also counteracts the
effects of self-induction, and by a proper adjustment of the capacity of the condenser, the self-

induction of the coils, and the periods of the currents, the condenser may be made to overcome
entirely the effect of self-induction.
It is preferable on account of the undesirability of using sliding contacts of any kind, to
associate the condenser with the armature directly, or make it a part of the armature. In some cases
Mr. Tesla builds up the armature of annular plates K K, held by bolts L between heads M, which are
secured to the driving shaft, and in the hollow space thus formed he places a condenser F,
generally by winding the two insulated plates spirally around the shaft. In other cases he utilizes
the plates of the core itself as the plates of the condenser. For example, in Figs. 88 and 89, N is the
driving shaft, M M are the heads of the armature-core, and K K' the iron plates of which the core is
built up. These plates are insulated from the shaft and from one another, and are held together by
rods or bolts L. The bolts pass through a large hole in one plate and a small hole in the one next
adjacent, and so on, connecting electrically all of plates K, as one armature of a condenser, and all
of plates K' as the other.
To either of the condensers above described the armature coils may be connected, as explained
by reference to Fig. 86.
In motors in which the armature coils are closed upon themselves—as, for example, in any
form of alternating current motor in which one armature coil or set of coils is in the position of
maximum induction with respect to the field coils or poles, while the other is in the position of
minimum induction—the coils are best connected in one series, and two points of the circuit thus
formed are bridged by a condenser. This is illustrated in Fig. 90, in which E represents one set of
armature coils and E' the other. Their points of union are joined through a condenser F. It will be
observed that in this disposition the self-induction of the two branches E and E' varies with their
position relatively to the field magnet, and that each branch is alternately the predominating source
of the induced current. Hence the effect of the condenser F is twofold. First, it increases the current
in each of the branches alternately, and, secondly, it alters the phase of the currents in the branches,
this being the well-known effect which results from such a disposition of a condenser with a
circuit, as above described. This effect is favorable to the proper working of the motor, because it
increases the flow of current in the armature circuits due to a given inductive effect, and also
because it brings more nearly into coincidence the maximum magnetic effects of the coacting field
and armature poles.
It will be understood, of course, that the causes that contribute to the efficiency of condensers
when applied to such uses as the above must be given due consideration in determining the
practicability and efficiency of the motors. Chief among these is, as is well known, the periodicity
of the current, and hence the improvements described are more particularly adapted to systems in
which a very high rate of alternation or change is maintained.
Although this invention has been illustrated in connection with a special form of motor, it will
be understood that it is equally applicable to any other alternating current motor in which there is a
closed armature coil wherein the currents are induced by the action of the field, and the feature of
utilizing the plates or sections of a magnetic core for forming the condenser is applicable,
generally, to other kinds of alternating current apparatus.

CHAPTER XXII.
Motor with Condenser in one of the Field
Circuits.

If the field or energizing circuits of a rotary phase motor be both derived from the same source
of alternating currents and a condenser of proper capacity be included in one of the same,
approximately, the desired difference of phase may be obtained between the currents flowing
directly from the source and those flowing through the condenser; but the great size and expense
of condensers for this purpose that would meet the requirements of the ordinary systems of
comparatively low potential are particularly prohibitory to their employment.
Another, now well-known, method or plan of securing a difference of phase between the
energizing currents of motors of this kind is to induce by the currents in one circuit those in the
other circuit or circuits; but as no means had been proposed that would secure in this way between
the phases of the primary or inducing and the secondary or induced currents that difference—
theoretically ninety degrees—that is best adapted for practical and economical working, Mr. Tesla
devised a means which renders practicable both the above described plans or methods, and by
which he is enabled to obtain an economical and efficient alternating current motor. His invention
consists in placing a condenser in the secondary or induced circuit of the motor above described
and raising the potential of the secondary currents to such a degree that the capacity of the
condenser, which is in part dependent on the potential, need be quite small. The value of this
condenser is determined in a well-understood manner with reference to the self-induction and
other conditions of the circuit, so as to cause the currents which pass through it to differ from the
primary currents by a quarter phase.
Fig. 91 illustrates the invention as embodied in a motor in which the inductive relation of the
primary and secondary circuits is secured by winding them inside the motor partly upon the same
cores; but the invention applies, generally, to other forms of motor in which one of the energizing
currents is induced in any way from the other.
Let A B represent the poles of an alternating current motor, of which C is the armature wound
with coils D, closed upon themselves, as is now the general practice in motors of this kind. The
poles A, which alternate with poles B, are wound with coils of ordinary or coarse wire E in such
direction as to make them of alternate north and south polarity, as indicated in the diagram by the
characters N S. Over these coils, or in other inductive relation to the same, are wound long fine-
wire coils F F, and in the same direction throughout as the coils E. These coils are secondaries, in
which currents of very high potential are induced. All the coils E in one series are connected, and
all the secondaries F in another.

Fig. 91.
On the intermediate poles B are wound fine-wire energizing coils G, which are connected in
series with one another, and also with the series of secondary coils F, the direction of winding
being such that a current-impulse induced from the primary coils E imparts the same magnetism to
the poles B as that produced in poles A by the primary impulse. This condition is indicated by the
characters N' S'.
In the circuit formed by the two sets of coils F and G is introduced a condenser H; otherwise
this circuit is closed upon itself, while the free ends of the circuit of coils E are connected to a
source of alternating currents. As the condenser capacity which is needed in any particular motor
of this kind is dependent upon the rate of alternation or the potential, or both, its size or cost, as
before explained, may be brought within economical limits for use with the ordinary circuits if the
potential of the secondary circuit in the motor be sufficiently high. By giving to the condenser
proper values, any desired difference of phase between the primary and secondary energizing
circuits may be obtained.

CHAPTER XXIII.
Tesla Polyphase Transformer.

Applying the polyphase principle to the construction of transformers as well to the motors
already noticed, Mr. Tesla has invented some very interesting forms, which he considers free from
the defects of earlier and, at present, more familiar forms. In these transformers he provides a
series of inducing coils and corresponding induced coils, which are generally wound upon a core
closed upon itself, usually a ring of laminated iron.
The two sets of coils are wound side by side or superposed or otherwise placed in well-known
ways to bring them into the most effective relations to one another and to the core. The inducing or
primary coils wound on the core are divided into pairs or sets by the proper electrical connections,
so that while the coils of one pair or set co-operate in fixing the magnetic poles of the core at two
given diametrically opposite points, the coils of the other pair or set—assuming, for sake of
illustration, that there are but two—tend to fix the poles ninety degrees from such points. With this
induction device is used an alternating current generator with coils or sets of coils to correspond
with those of the converter, and the corresponding coils of the generator and converter are then
connected up in independent circuits. It results from this that the different electrical phases in the
generator are attended by corresponding magnetic changes in the converter; or, in other words, that
as the generator coils revolve, the points of greatest magnetic intensity in the converter will be
progressively shifted or whirled around.
Fig. 92 is a diagrammatic illustration of the converter and the electrical connections of the
same. Fig. 93 is a horizontal central cross-section of Fig. 92. Fig. 94 is a diagram of the circuits of
the entire system, the generator being shown in section.

Figs. 92 and 93.
Mr. Tesla uses a core, A, which is closed upon itself—that is to say, of an annular cylindrical or
equivalent form—and as the efficiency of the apparatus is largely increased by the subdivision of
this core, he makes it of thin strips, plates, or wires of soft iron electrically insulated as far as
practicable. Upon this core are wound, say, four coils, B B B' B', used as primary coils, and for which
long lengths of comparatively fine wire are employed. Over these coils are then wound shorter
coils of coarser wire, C C C' C', to constitute the induced or secondary coils. The construction of this
or any equivalent form of converter may be carried further, as above pointed out, by inclosing
these coils with iron—as, for example, by winding over the coils layers of insulated iron wire.

Fig. 94.
The device is provided with suitable binding posts, to which the ends of the coils are led. The
diametrically opposite coils B B and B' B' are connected, respectively, in series, and the four
terminals are connected to the binding posts. The induced coils are connected together in any
desired manner. For example, as shown in Fig. 94, C C may be connected in multiple arc when a
quantity current is desired—as for running a group of incandescent lamps—while C' C' may be
independently connected in series in a circuit including arc lamps or the like. The generator in this
system will be adapted to the converter in the manner illustrated. For example, in the present case
there are employed a pair of ordinary permanent or electro-magnets, E E, between which is
mounted a cylindrical armature on a shaft, F, and wound with two coils, G G'. The terminals of
these coils are connected, respectively, to four insulated contact or collecting rings, H H H' H', and
the four line circuit wires L connect the brushes K, bearing on these rings, to the converter in the
order shown. Noting the results of this combination, it will be observed that at a given point of
time the coil G is in its neutral position and is generating little or no current, while the other coil, G',
is in a position where it exerts its maximum effect. Assuming coil G to be connected in circuit with
coils B B of the converter, and coil G' with coils B' B', it is evident that the poles of the ring A will be
determined by coils B' B' alone; but as the armature of the generator revolves, coil G develops more
current and coil G' less, until G reaches its maximum and G' its neutral position. The obvious result
will be to shift the poles of the ring A through one-quarter of its periphery. The movement of the
coils through the next quarter of a turn—during which coil G' enters a field of opposite polarity and
generates a current of opposite direction and increasing strength, while coil G, in passing from its
maximum to its neutral position generates a current of decreasing strength and same direction as
before—causes a further shifting of the poles through the second quarter of the ring. The second
half-revolution will obviously be a repetition of the same action. By the shifting of the poles of the
ring A, a powerful dynamic inductive effect on the coils C C' is produced. Besides the currents
generated in the secondary coils by dynamo-magnetic induction, other currents will be set up in
the same coils in consequence of many variations in the intensity of the poles in the ring A. This

should be avoided by maintaining the intensity of the poles constant, to accomplish which care
should be taken in designing and proportioning the generator and in distributing the coils in the
ring A, and balancing their effect. When this is done, the currents are produced by dynamo-
magnetic induction only, the same result being obtained as though the poles were shifted by a
commutator with an infinite number of segments.
The modifications which are applicable to other forms of converter are in many respects
applicable to this, such as those pertaining more particularly to the form of the core, the relative
lengths and resistances of the primary and secondary coils, and the arrangements for running or
operating the same.

CHAPTER XXIV.
A Constant Current Transformer with Magnetic
Shield Between Coils of Primary and Secondary.

Mr. Tesla has applied his principle of magnetic shielding of parts to the construction also of
transformers, the shield being interposed between the primary and secondary coils. In transformers
of the ordinary type it will be found that the wave of electromotive force of the secondary very
nearly coincides with that of the primary, being, however, in opposite sign. At the same time the
currents, both primary and secondary, lag behind their respective electromotive forces; but as this
lag is practically or nearly the same in the case of each it follows that the maximum and minimum
of the primary and secondary currents will nearly coincide, but differ in sign or direction, provided
the secondary be not loaded or if it contain devices having the property of self-induction. On the
other hand, the lag of the primary behind the impressed electromotive force may be diminished by
loading the secondary with a non-inductive or dead resistance—such as incandescent lamps—
whereby the time interval between the maximum or minimum periods of the primary and
secondary currents is increased. This time interval, however, is limited, and the results obtained by
phase difference in the operation of such devices as the Tesla alternating current motors can only
be approximately realized by such means of producing or securing this difference, as above
indicated, for it is desirable in such cases that there should exist between the primary and
secondary currents, or those which, however produced, pass through the two circuits of the motor,
a difference of phase of ninety degrees; or, in other words, the current in one circuit should be a
maximum when that in the other circuit is a minimum. To attain to this condition more perfectly,
an increased retardation of the secondary current is secured in the following manner: Instead of
bringing the primary and secondary coils or circuits of a transformer into the closest possible
relations, as has hitherto been done, Mr. Tesla protects in a measure the secondary from the
inductive action or effect of the primary by surrounding either the primary or the secondary with a
comparatively thin magnetic shield or screen. Under these modified conditions, as long as the
primary current has a small value, the shield protects the secondary; but as soon as the primary
current has reached a certain strength, which is arbitrarily determined, the protecting magnetic
shield becomes saturated and the inductive action upon the secondary begins. It results, therefore,
that the secondary current begins to flow at a certain fraction of a period later than it would
without the interposed shield, and since this retardation may be obtained without necessarily
retarding the primary current also, an additional lag is secured, and the time interval between the
maximum or minimum periods of the primary and secondary currents is increased. Such a
transformer may, by properly proportioning its several elements and determining the proper
relations between the primary and secondary windings, the thickness of the magnetic shield, and
other conditions, be constructed to yield a constant current at all loads.

Fig. 95.
Fig. 95 is a cross-section of a transformer embodying this improvement. Fig. 96 is a similar
view of a modified form of transformer, showing diagrammatically the manner of using the same.
A A is the main core of the transformer, composed of a ring of soft annealed and insulated or
oxidized iron wire. Upon this core is wound the secondary circuit or coil B B. This latter is then
covered with a layer or layers of annealed and insulated iron wires C C, wound in a direction at
right angles to the secondary coil. Over the whole is then wound the primary coil or wire D D.
From the nature of this construction it will be obvious that as long as the shield formed by the
wires C is below magnetic saturation the secondary coil or circuit is effectually protected or
shielded from the inductive influence of the primary, although on open circuit it may exhibit some
electromotive force. When the strength of the primary reaches a certain value, the shield C,
becoming saturated, ceases to protect the secondary from inductive action, and current is in
consequence developed therein. For similar reasons, when the primary current weakens, the
weakening of the secondary is retarded to the same or approximately the same extent.

Fig. 96.
The specific construction of the transformer is largely immaterial. In Fig. 96, for example, the
core A is built up of thin insulated iron plates or discs. The primary circuit D is wound next the core
A. Over this is applied the shield C, which in this case is made up of thin strips or plates of iron
properly insulated and surrounding the primary, forming a closed magnetic circuit. The secondary
B is wound over the shield C. In Fig. 96, also, E is a source of alternating or rapidly changing
currents. The primary of the transformer is connected with the circuit of the generator. F is a two-
circuit alternating current motor, one of the circuits being connected with the main circuit from the
source E, and the other being supplied with currents from the secondary of the transformer.

PART II.

THE TESLA EFFECTS WITH HIGH
FREQUENCY
AND HIGH POTENTIAL CURRENTS.

CHAPTER XXV.
Introduction.—The Scope of the Tesla Lectures.

Before proceeding to study the three Tesla lectures here presented, the reader may find it of
some assistance to have his attention directed to the main points of interest and significance
therein. The first of these lectures was delivered in New York, at Columbia College, before the
American Institute of Electrical Engineers, May 20, 1891. The urgent desire expressed
immediately from all parts of Europe for an opportunity to witness the brilliant and unusual
experiments with which the lecture was accompanied, induced Mr. Tesla to go to England early in
1892, when he appeared before the Institution of Electrical Engineers, and a day later, by special
request, before the Royal Institution. His reception was of the most enthusiastic and flattering
nature on both occasions. He then went, by invitation, to France, and repeated his novel
demonstrations before the Société Internationale des Electriciens, and the Société Française de
Physique. Mr. Tesla returned to America in the fall of 1892, and in February, 1893, delivered his
third lecture before the Franklin Institute of Philadelphia, in fulfilment of a long standing promise
to Prof. Houston. The following week, at the request of President James I. Ayer, of the National
Electric Light Association, the same lecture was re-delivered in St. Louis. It had been intended to
limit the invitations to members, but the appeals from residents in the city were so numerous and
pressing that it became necessary to secure a very large hall. Hence it came about that the lecture
was listened to by an audience of over 5,000 people, and was in some parts of a more popular
nature than either of its predecessors. Despite this concession to the need of the hour and occasion,
Mr. Tesla did not hesitate to show many new and brilliant experiments, and to advance the frontier
of discovery far beyond any point he had theretofore marked publicly.
We may now proceed to a running review of the lectures themselves. The ground covered by
them is so vast that only the leading ideas and experiments can here be touched upon; besides, it is
preferable that the lectures should be carefully gone over for their own sake, it being more than
likely that each student will discover a new beauty or stimulus in them. Taking up the course of
reasoning followed by Mr. Tesla in his first lecture, it will be noted that he started out with the
recognition of the fact, which he has now experimentally demonstrated, that for the production of
light waves, primarily, electrostatic effects must be brought into play, and continued study has led
him to the opinion that all electrical and magnetic effects may be referred to electrostatic
molecular forces. This opinion finds a singular confirmation in one of the most striking
experiments which he describes, namely, the production of a veritable flame by the agitation of
electrostatically charged molecules. It is of the highest interest to observe that this result points out
a way of obtaining a flame which consumes no material and in which no chemical action whatever
takes place. It also throws a light on the nature of the ordinary flame, which Mr. Tesla believes to
be due to electrostatic molecular actions, which, if true, would lead directly to the idea that even
chemical affinities might be electrostatic in their nature and that, as has already been suggested,
molecular forces in general may be referable to one and the same cause. This singular
phenomenon accounts in a plausible manner for the unexplained fact that buildings are frequently
set on fire during thunder storms without having been at all struck by lightning. It may also explain
the total disappearance of ships at sea.

One of the striking proofs of the correctness of the ideas advanced by Mr. Tesla is the fact that,
notwithstanding the employment of the most powerful electromagnetic inductive effects, but
feeble luminosity is obtainable, and this only in close proximity to the source of disturbance;
whereas, when the electrostatic effects are intensified, the same initial energy suffices to excite
luminosity at considerable distances from the source. That there are only electrostatic effects active
seems to be clearly proved by Mr. Tesla's experiments with an induction coil operated with
alternating currents of very high frequency. He shows how tubes may be made to glow brilliantly
at considerable distances from any object when placed in a powerful, rapidly alternating,
electrostatic field, and he describes many interesting phenomena observed in such a field. His
experiments open up the possibility of lighting an apartment by simply creating in it such an
electrostatic field, and this, in a certain way, would appear to be the ideal method of lighting a
room, as it would allow the illuminating device to be freely moved about. The power with which
these exhausted tubes, devoid of any electrodes, light up is certainly remarkable.
That the principle propounded by Mr. Tesla is a broad one is evident from the many ways in
which it may be practically applied. We need only refer to the variety of the devices shown or
described, all of which are novel in character and will, without doubt, lead to further important
results at the hands of Mr. Tesla and other investigators. The experiment, for instance, of lighting
up a single filament or block of refractory material with a single wire, is in itself sufficient to give
Mr. Tesla's work the stamp of originality, and the numerous other experiments and effects which
may be varied at will, are equally new and interesting. Thus, the incandescent filament spinning in
an unexhausted globe, the well-known Crookes experiment on open circuit, and the many others
suggested, will not fail to interest the reader. Mr. Tesla has made an exhaustive study of the various
forms of the discharge presented by an induction coil when operated with these rapidly alternating
currents, starting from the thread-like discharge and passing through various stages to the true
electric flame.
A point of great importance in the introduction of high tension alternating current which Mr.
Tesla brings out is the necessity of carefully avoiding all gaseous matter in the high tension
apparatus. He shows that, at least with very rapidly alternating currents of high potential, the
discharge may work through almost any practicable thickness of the best insulators, if air is
present. In such cases the air included within the apparatus is violently agitated and by molecular
bombardment the parts may be so greatly heated as to cause a rupture of the insulation. The
practical outcome of this is, that, whereas with steady currents, any kind of insulation may be used,
with rapidly alternating currents oils will probably be the best to employ, a fact which has been
observed, but not until now satisfactorily explained. The recognition of the above fact is of special
importance in the construction of the costly commercial induction coils which are often rendered
useless in an unaccountable manner. The truth of these views of Mr. Tesla is made evident by the
interesting experiments illustrative of the behavior of the air between charged surfaces, the
luminous streams formed by the charged molecules appearing even when great thicknesses of the
best insulators are interposed between the charged surfaces. These luminous streams afford in
themselves a very interesting study for the experimenter. With these rapidly alternating currents
they become far more powerful and produce beautiful light effects when they issue from a wire,
pinwheel or other object attached to a terminal of the coil; and it is interesting to note that they
issue from a ball almost as freely as from a point, when the frequency is very high.
From these experiments we also obtain a better idea of the importance of taking into account
the capacity and self-induction in the apparatus employed and the possibilities offered by the use
of condensers in conjunction with alternate currents, the employment of currents of high

frequency, among other things, making it possible to reduce the condenser to practicable
dimensions. Another point of interest and practical bearing is the fact, proved by Mr. Tesla, that for
alternate currents, especially those of high frequency, insulators are required possessing a small
specific inductive capacity, which at the same time have a high insulating power.
Mr. Tesla also makes interesting and valuable suggestion in regard to the economical
utilization of iron in machines and transformers. He shows how, by maintaining by continuous
magnetization a flow of lines through the iron, the latter may be kept near its maximum
permeability and a higher output and economy may be secured in such apparatus. This principle
may prove of considerable commercial importance in the development of alternating systems. Mr.
Tesla's suggestion that the same result can be secured by heating the iron by hysteresis and eddy
currents, and increasing the permeability in this manner, while it may appear less practical,
nevertheless opens another direction for investigation and improvement.
The demonstration of the fact that with alternating currents of high frequency, sufficient energy
may be transmitted under practicable conditions through the glass of an incandescent lamp by
electrostatic or electromagnetic induction may lead to a departure in the construction of such
devices. Another important experimental result achieved is the operation of lamps, and even
motors, with the discharges of condensers, this method affording a means of converting direct or
alternating currents. In this connection Mr. Tesla advocates the perfecting of apparatus capable of
generating electricity of high tension from heat energy, believing this to be a better way of
obtaining electrical energy for practical purposes, particularly for the production of light.
While many were probably prepared to encounter curious phenomena of impedance in the use
of a condenser discharged disruptively, the experiments shown were extremely interesting on
account of their paradoxical character. The burning of an incandescent lamp at any candle power
when connected across a heavy metal bar, the existence of nodes on the bar and the possibility of
exploring the bar by means of an ordinary Cardew voltmeter, are all peculiar developments, but
perhaps the most interesting observation is the phenomenon of impedance observed in the lamp
with a straight filament, which remains dark while the bulb glows.
Mr. Tesla's manner of operating an induction coil by means of the disruptive discharge, and
thus obtaining enormous differences of potential from comparatively small and inexpensive coils,
will be appreciated by experimenters and will find valuable application in laboratories. Indeed, his
many suggestions and hints in regard to the construction and use of apparatus in these
investigations will be highly valued and will aid materially in future research.
The London lecture was delivered twice. In its first form, before the Institution of Electrical
Engineers, it was in some respects an amplification of several points not specially enlarged upon in
the New York lecture, but brought forward many additional discoveries and new investigations. Its
repetition, in another form, at the Royal Institution, was due to Prof. Dewar, who with Lord
Rayleigh, manifested a most lively interest in Mr. Tesla's work, and whose kindness illustrated
once more the strong English love of scientific truth and appreciation of its votaries. As an
indefatigable experimenter, Mr. Tesla was certainly nowhere more at home than in the haunts of
Faraday, and as the guest of Faraday's successor. This Royal Institution lecture summed up the
leading points of Mr. Tesla's work, in the high potential, high frequency field, and we may here
avail ourselves of so valuable a summarization, in a simple form, of a subject by no means easy of
comprehension until it has been thoroughly studied.

In these London lectures, among the many notable points made was first, the difficulty of
constructing the alternators to obtain the very high frequencies needed. To obtain the high
frequencies it was necessary to provide several hundred polar projections, which were necessarily
small and offered many drawbacks, and this the more as exceedingly high peripheral speeds had to
be resorted to. In some of the first machines both armature and field had polar projections. These
machines produced a curious noise, especially when the armature was started from the state of rest,
the field being charged. The most efficient machine was found to be one with a drum armature, the
iron body of which consisted of very thin wire annealed with special care. It was, of course,
desirable to avoid the employment of iron in the armature, and several machines of this kind, with
moving or stationary conductors were constructed, but the results obtained were not quite
satisfactory, on account of the great mechanical and other difficulties encountered.
The study of the properties of the high frequency currents obtained from these machines is
very interesting, as nearly every experiment discloses something new. Two coils traversed by such
a current attract or repel each other with a force which, owing to the imperfection of our sense of
touch, seems continuous. An interesting observation, already noted under another form, is that a
piece of iron, surrounded by a coil through which the current is passing appears to be continuously
magnetized. This apparent continuity might be ascribed to the deficiency of the sense of touch, but
there is evidence that in currents of such high frequencies one of the impulses preponderates over
the other.
As might be expected, conductors traversed by such currents are rapidly heated, owing to the
increase of the resistance, and the heating effects are relatively much greater in the iron. The
hysteresis losses in iron are so great that an iron core, even if finely subdivided, is heated in an
incredibly short time. To give an idea of this, an ordinary iron wire 1/16 inch in diameter inserted
within a coil having 250 turns, with a current estimated to be five amperes passing through the
coil, becomes within two seconds' time so hot as to scorch wood. Beyond a certain frequency, an
iron core, no matter how finely subdivided, exercises a dampening effect, and it was easy to find a
point at which the impedance of a coil was not affected by the presence of a core consisting of a
bundle of very thin well annealed and varnished iron wires.
Experiments with a telephone, a conductor in a strong magnetic field, or with a condenser or
arc, seem to afford certain proof that sounds far above the usually accepted limit of hearing would
be perceived if produced with sufficient power. The arc produced by these currents possesses
several interesting features. Usually it emits a note the pitch of which corresponds to twice the
frequency of the current, but if the frequency be sufficiently high it becomes noiseless, the limit of
audition being determined principally by the linear dimensions of the arc. A curious feature of the
arc is its persistency, which is due partly to the inability of the gaseous column to cool and increase
considerably in resistance, as is the case with low frequencies, and partly to the tendency of such a
high frequency machine to maintain a constant current.
In connection with these machines the condenser affords a particularly interesting study.
Striking effects are produced by proper adjustments of capacity and self-induction. It is easy to
raise the electromotive force of the machine to many times the original value by simply adjusting
the capacity of a condenser connected in the induced circuit. If the condenser be at some distance
from the machine, the difference of potential on the terminals of the latter may be only a small
fraction of that on the condenser.
But the most interesting experiences are gained when the tension of the currents from the
machine is raised by means of an induction coil. In consequence of the enormous rate of change

obtainable in the primary current, much higher potential differences are obtained than with coils
operated in the usual ways, and, owing to the high frequency, the secondary discharge possesses
many striking peculiarities. Both the electrodes behave generally alike, though it appears from
some observations that one current impulse preponderates over the other, as before mentioned.
The physiological effects of the high tension discharge are found to be so small that the shock
of the coil can be supported without any inconvenience, except perhaps a small burn produced by
the discharge upon approaching the hand to one of the terminals. The decidedly smaller
physiological effects of these currents are thought to be due either to a different distribution
through the body or to the tissues acting as condensers. But in the case of an induction coil with a
great many turns the harmlessness is principally due to the fact that but little energy is available in
the external circuit when the same is closed through the experimenter's body, on account of the
great impedance of the coil.
In varying the frequency and strength of the currents through the primary of the coil, the
character of the secondary discharge is greatly varied, and no less than five distinct forms are
observed:—A weak, sensitive thread discharge, a powerful flaming discharge, and three forms of
brush or streaming discharges. Each of these possesses certain noteworthy features, but the most
interesting to study are the latter.
Under certain conditions the streams, which are presumably due to the violent agitation of the
air molecules, issue freely from all points of the coil, even through a thick insulation. If there is the
smallest air space between the primary and secondary, they will form there and surely injure the
coil by slowly warming the insulation. As they form even with ordinary frequencies when the
potential is excessive, the air-space must be most carefully avoided. These high frequency
streamers differ in aspect and properties from those produced by a static machine. The wind
produced by them is small and should altogether cease if still considerably higher frequencies
could be obtained. A peculiarity is that they issue as freely from surfaces as from points. Owing to
this, a metallic vane, mounted in one of the terminals of the coil so as to rotate freely, and having
one of its sides covered with insulation, is spun rapidly around. Such a vane would not rotate with
a steady potential, but with a high frequency coil it will spin, even if it be entirely covered with
insulation, provided the insulation on one side be either thicker or of a higher specific inductive
capacity. A Crookes electric radiometer is also spun around when connected to one of the terminals
of the coil, but only at very high exhaustion or at ordinary pressures.
There is still another and more striking peculiarity of such a high frequency streamer, namely,
it is hot. The heat is easily perceptible with frequencies of about 10,000, even if the potential is not
excessively high. The heating effect is, of course, due to the molecular impacts and collisions.
Could the frequency and potential be pushed far enough, then a brush could be produced
resembling in every particular a flame and giving light and heat, yet without a chemical process
taking place.
The hot brush, when properly produced, resembles a jet of burning gas escaping under great
pressure, and it emits an extraordinary strong smell of ozone. The great ozonizing action is
ascribed to the fact that the agitation of the molecules of the air is more violent in such a brush
than in the ordinary streamer of a static machine. But the most powerful brush discharges were
produced by employing currents of much higher frequencies than it was possible to obtain by
means of the alternators. These currents were obtained by disruptively discharging a condenser and
setting up oscillations. In this manner currents of a frequency of several hundred thousand were
obtained.

Currents of this kind, Mr. Tesla pointed out, produce striking effects. At these frequencies, the
impedance of a copper bar is so great that a potential difference of several hundred volts can be
maintained between two points of a short and thick bar, and it is possible to keep an ordinary
incandescent lamp burning at full candle power by attaching the terminals of the lamp to two
points of the bar no more than a few inches apart. When the frequency is extremely high, nodes are
found to exist on such a bar, and it is easy to locate them by means of a lamp.
By converting the high tension discharges of a low frequency coil in this manner, it was found
practicable to keep a few lamps burning on the ordinary circuit in the laboratory, and by bringing
the undulation to a low pitch, it was possible to operate small motors.
This plan likewise allows of converting high tension discharges of one direction into low
tension unidirectional currents, by adjusting the circuit so that there are no oscillations. In passing
the oscillating discharges through the primary of a specially constructed coil, it is easy to obtain
enormous potential differences with only few turns of the secondary.
Great difficulties were at first experienced in producing a successful coil on this plan. It was
found necessary to keep all air, or gaseous matter in general, away from the charged surfaces, and
oil immersion was resorted to. The wires used were heavily covered with gutta-percha and wound
in oil, or the air was pumped out by means of a Sprengel pump. The general arrangement was the
following:—An ordinary induction coil, operated from a low frequency alternator, was used to
charge Leyden jars. The jars were made to discharge over a single or multiple gap through the
primary of the second coil. To insure the action of the gap, the arc was blown out by a magnet or
air blast. To adjust the potential in the secondary a small oil condenser was used, or polished brass
spheres of different sizes were screwed on the terminals and their distance adjusted.
When the conditions were carefully determined to suit each experiment, magnificent effects
were obtained. Two wires, stretched through the room, each being connected to one of the
terminals of the coil, emitted streams so powerful that the light from them allowed distinguishing
the objects in the room; the wires became luminous even though covered with thick and most
excellent insulation. When two straight wires, or two concentric circles of wire, are connected to
the terminals, and set at the proper distance, a uniform luminous sheet is produced between them.
It was possible in this way to cover an area of more than one meter square completely with the
streams. By attaching to one terminal a large circle of wire and to the other terminal a small
sphere, the streams are focused upon the sphere, produce a strongly lighted spot upon the same,
and present the appearance of a luminous cone. A very thin wire glued upon a plate of hard rubber
of great thickness, on the opposite side of which is fastened a tinfoil coating, is rendered intensely
luminous when the coating is connected to the other terminal of the coil. Such an experiment can
be performed also with low frequency currents, but much less satisfactorily.
When the terminals of such a coil, even of a very small one, are separated by a rubber or glass
plate, the discharge spreads over the plate in the form of streams, threads or brilliant sparks, and
affords a magnificent display, which cannot be equaled by the largest coil operated in the usual
ways. By a simple adjustment it is possible to produce with the coil a succession of brilliant
sparks, exactly as with a Holtz machine.
Under certain conditions, when the frequency of the oscillation is very great, white, phantom-
like streams are seen to break forth from the terminals of the coil. The chief interesting feature
about them is, that they stream freely against the outstretched hand or other conducting object
without producing any sensation, and the hand may be approached very near to the terminal

without a spark being induced to jump. This is due presumably to the fact that a considerable
portion of the energy is carried away or dissipated in the streamers, and the difference of potential
between the terminal and the hand is diminished.
It is found in such experiments that the frequency of the vibration and the quickness of
succession of the sparks between the knobs affect to a marked degree the appearance of the
streams. When the frequency is very low, the air gives way in more or less the same manner as by
a steady difference of potential, and the streams consist of distinct threads, generally mingled with
thin sparks, which probably correspond to the successive discharges occurring between the knobs.
But when the frequency is very high, and the arc of the discharge produces a sound which is loud
and smooth (which indicates both that oscillation takes place and that the sparks succeed each
other with great rapidity), then the luminous streams formed are perfectly uniform. They are
generally of a purplish hue, but when the molecular vibration is increased by raising the potential,
they assume a white color.
The luminous intensity of the streams increases rapidly when the potential is increased; and
with frequencies of only a few hundred thousand, could the coil be made to withstand a
sufficiently high potential difference, there is no doubt that the space around a wire could be made
to emit a strong light, merely by the agitation of the molecules of the air at ordinary pressure.
Such discharges of very high frequency which render luminous the air at ordinary pressure we
have very likely occasion to witness in the aurora borealis. From many of these experiments it
seems reasonable to infer that sudden cosmic disturbances, such as eruptions on the sun, set the
electrostatic charge of the earth in an extremely rapid vibration, and produce the glow by the
violent agitation of the air in the upper and even in the lower strata. It is thought that if the
frequency were low, or even more so if the charge were not at all vibrating, the lower dense strata
would break down as in a lightning discharge. Indications of such breaking down have been
repeatedly observed, but they can be attributed to the fundamental disturbances, which are few in
number, for the superimposed vibration would be so rapid as not to allow a disruptive break.
The study of these discharge phenomena has led Mr. Tesla to the recognition of some
important facts. It was found, as already stated, that gaseous matter must be most carefully
excluded from any dielectric which is subjected to great, rapidly changing electrostatic stresses.
Since it is difficult to exclude the gas perfectly when solid insulators are used, it is necessary to
resort to liquid dielectrics. When a solid dielectric is used, it matters little how thick and how good
it is; if air be present, streamers form, which gradually heat the dielectric and impair its insulating
power, and the discharge finally breaks through. Under ordinary conditions the best insulators are
those which possess the highest specific inductive capacity, but such insulators are not the best to
employ when working with these high frequency currents, for in most cases the higher specific
inductive capacity is rather a disadvantage. The prime quality of the insulating medium for these
currents is continuity. For this reason principally it is necessary to employ liquid insulators, such as
oils. If two metal plates, connected to the terminals of the coil, are immersed in oil and set a
distance apart, the coil may be kept working for any length of time without a break occurring, or
without the oil being warmed, but if air bubbles are introduced, they become luminous; the air
molecules, by their impact against the oil, heat it, and after some time cause the insulation to give
way. If, instead of the oil, a solid plate of the best dielectric, even several times thicker than the oil
intervening between the metal plates, is inserted between the latter, the air having free access to the
charged surfaces, the dielectric invariably is warmed and breaks down.

The employment of oil is advisable or necessary even with low frequencies, if the potentials
are such that streamers form, but only in such cases, as is evident from the theory of the action. If
the potentials are so low that streamers do not form, then it is even disadvantageous to employ oil,
for it may, principally by confining the heat, be the cause of the breaking down of the insulation.
The exclusion of gaseous matter is not only desirable on account of the safety of the apparatus,
but also on account of economy, especially in a condenser, in which considerable waste of power
may occur merely owing to the presence of air, if the electric density on the charged surfaces is
great.
In the course of these investigations a phenomenon of special scientific interest was observed.
It may be ranked among the brush phenomena, in fact it is a kind of brush which forms at, or near,
a single terminal in high vacuum. In a bulb with a conducting electrode, even if the latter be of
aluminum, the brush has only a very short existence, but it can be preserved for a considerable
length of time in a bulb devoid of any conducting electrode. To observe the phenomenon it is
found best to employ a large spherical bulb having in its centre a small bulb supported on a tube
sealed to the neck of the former. The large bulb being exhausted to a high degree, and the inside of
the small bulb being connected to one of the terminals of the coil, under certain conditions there
appears a misty haze around the small bulb, which, after passing through some stages, assumes the
form of a brush, generally at right angles to the tube supporting the small bulb. When the brush
assumes this form it may be brought to a state of extreme sensitiveness to electrostatic and
magnetic influence. The bulb hanging straight down, and all objects being remote from it, the
approach of the observer within a few paces will cause the brush to fly to the opposite side, and if
he walks around the bulb it will always keep on the opposite side. It may begin to spin around the
terminal long before it reaches that sensitive stage. When it begins to turn around, principally, but
also before, it is affected by a magnet, and at a certain stage it is susceptible to magnetic influence
to an astonishing degree. A small permanent magnet, with its poles at a distance of no more than
two centimetres will affect it visibly at a distance of two metres, slowing down or accelerating the
rotation according to how it is held relatively to the brush.
When the bulb hangs with the globe down, the rotation is always clockwise. In the southern
hemisphere it would occur in the opposite direction, and on the (magnetic) equator the brush
should not turn at all. The rotation may be reversed by a magnet kept at some distance. The brush
rotates best, seemingly, when it is at right angles to the lines of force of the earth. It very likely
rotates, when at its maximum speed, in synchronism with the alternations, say, 10,000 times a
second. The rotation can be slowed down or accelerated by the approach or recession of the
observer, or any conducting body, but it cannot be reversed by putting the bulb in any position.
Very curious experiments may be performed with the brush when in its most sensitive state. For
instance, the brush resting in one position, the experimenter may, by selecting a proper position,
approach the hand at a certain considerable distance to the bulb, and he may cause the brush to
pass off by merely stiffening the muscles of the arm, the mere change of configuration of the arm
and the consequent imperceptible displacement being sufficient to disturb the delicate balance.
When it begins to rotate slowly, and the hands are held at a proper distance, it is impossible to
make even the slightest motion without producing a visible effect upon the brush. A metal plate
connected to the other terminal of the coil affects it at a great distance, slowing down the rotation
often to one turn a second.
Mr. Tesla hopes that this phenomenon will prove a valuable aid in the investigation of the
nature of the forces acting in an electrostatic or magnetic field. If there is any motion which is

measurable going on in the space, such a brush would be apt to reveal it. It is, so to speak, a beam
of light, frictionless, devoid of inertia. On account of its marvellous sensitiveness to electrostatic or
magnetic disturbances it may be the means of sending signals through submarine cables with any
speed, and even of transmitting intelligence to a distance without wires.
In operating an induction coil with these rapidly alternating currents, it is astonishing to note,
for the first time, the great importance of the relation of capacity, self-induction, and frequency as
bearing upon the general result. The combined effect of these elements produces many curious
effects. For instance, two metal plates are connected to the terminals and set at a small distance, so
that an arc is formed between them. This arc prevents a strong current from flowing through the
coil. If the arc be interrupted by the interposition of a glass plate, the capacity of the condenser
obtained counteracts the self-induction, and a stronger current is made to pass. The effects of
capacity are the most striking, for in these experiments, since the self-induction and frequency
both are high, the critical capacity is very small, and need be but slightly varied to produce a very
considerable change. The experimenter brings his body in contact with the terminals of the
secondary of the coil, or attaches to one or both terminals insulated bodies of very small bulk, such
as exhausted bulbs, and he produces a considerable rise or fall of potential on the secondary, and
greatly affects the flow of the current through the primary coil.
In many of the phenomena observed, the presence of the air, or, generally speaking, of a
medium of a gaseous nature (using this term not to imply specific properties, but in
contradistinction to homogeneity or perfect continuity) plays an important part, as it allows energy
to be dissipated by molecular impact or bombardment. The action is thus explained:—When an
insulated body connected to a terminal of the coil is suddenly charged to high potential, it acts
inductively upon the surrounding air, or whatever gaseous medium there might be. The molecules
or atoms which are near it are, of course, more attracted, and move through a greater distance than
the further ones. When the nearest molecules strike the body they are repelled, and collisions occur
at all distances within the inductive distance. It is now clear that, if the potential be steady, but
little loss of energy can be caused in this way, for the molecules which are nearest to the body
having had an additional charge imparted to them by contact, are not attracted until they have
parted, if not with all, at least with most of the additional charge, which can be accomplished only
after a great many collisions. This is inferred from the fact that with a steady potential there is but
little loss in dry air. When the potential, instead of being steady, is alternating, the conditions are
entirely different. In this case a rhythmical bombardment occurs, no matter whether the molecules
after coming in contact with the body lose the imparted charge or not, and, what is more, if the
charge is not lost, the impacts are all the more violent. Still, if the frequency of the impulses be
very small, the loss caused by the impacts and collisions would not be serious unless the potential
was excessive. But when extremely high frequencies and more or less high potentials are used, the
loss may be very great. The total energy lost per unit of time is proportionate to the product of the
number of impacts per second, or the frequency and the energy lost in each impact. But the energy
of an impact must be proportionate to the square of the electric density of the body, on the
assumption that the charge imparted to the molecule is proportionate to that density. It is concluded
from this that the total energy lost must be proportionate to the product of the frequency and the
square of the electric density; but this law needs experimental confirmation. Assuming the
preceding considerations to be true, then, by rapidly alternating the potential of a body immersed
in an insulating gaseous medium, any amount of energy may be dissipated into space. Most of that
energy, then, is not dissipated in the form of long ether waves, propagated to considerable distance,
as is thought most generally, but is consumed in impact and collisional losses—that is, heat

vibrations—on the surface and in the vicinity of the body. To reduce the dissipation it is necessary
to work with a small electric density—the smaller, the higher the frequency.
The behavior of a gaseous medium to such rapid alternations of potential makes it appear
plausible that electrostatic disturbances of the earth, produced by cosmic events, may have great
influence upon the meteorological conditions. When such disturbances occur both the frequency of
the vibrations of the charge and the potential are in all probability excessive, and the energy
converted into heat may be considerable. Since the density must be unevenly distributed, either in
consequence of the irregularity of the earth's surface, or on account of the condition of the
atmosphere in various places, the effect produced would accordingly vary from place to place.
Considerable variations in the temperature and pressure of the atmosphere may in this manner be
caused at any point of the surface of the earth. The variations may be gradual or very sudden,
according to the nature of the original disturbance, and may produce rain and storms, or locally
modify the weather in any way.
From many experiences gathered in the course of these investigations it appears certain that in
lightning discharges the air is an element of importance. For instance, during a storm a stream may
form on a nail or pointed projection of a building. If lightning strikes somewhere in the
neighborhood, the harmless static discharge may, in consequence of the oscillations set up, assume
the character of a high-frequency streamer, and the nail or projection may be brought to a high
temperature by the violent impact of the air molecules. Thus, it is thought, a building may be set
on fire without the lightning striking it. In like manner small metallic objects may be fused and
volatilized—as frequently occurs in lightning discharges—merely because they are surrounded by
air. Were they immersed in a practically continuous medium, such as oil, they would probably be
safe, as the energy would have to spend itself elsewhere.
An instructive experience having a bearing on this subject is the following:—A glass tube of
an inch or so in diameter and several inches long is taken, and a platinum wire sealed into it, the
wire running through the center of the tube from end to end. The tube is exhausted to a moderate
degree. If a steady current is passed through the wire it is heated uniformly in all parts and the gas
in the tube is of no consequence. But if high frequency discharges are directed through the wire, it
is heated more on the ends than in the middle portion, and if the frequency, or rate of charge, is
high enough, the wire might as well be cut in the middle as not, for most of the heating on the ends
is due to the rarefied gas. Here the gas might only act as a conductor of no impedance, diverting
the current from the wire as the impedance of the latter is enormously increased, and merely
heating the ends of the wire by reason of their resistance to the passage of the discharge. But it is
not at all necessary that the gas in the tube should be conducting; it might be at an extremely low
pressure, still the ends of the wire would be heated; however, as is ascertained by experience, only
the two ends would in such case not be electrically connected through the gaseous medium. Now,
what with these frequencies and potentials occurs in an exhausted tube, occurs in the lightning
discharge at ordinary pressure.
From the facility with which any amount of energy may be carried off through a gas, Mr. Tesla
infers that the best way to render harmless a lightning discharge is to afford it in some way a
passage through a volume of gas.
The recognition of some of the above facts has a bearing upon far-reaching scientific
investigations in which extremely high frequencies and potentials are used. In such cases the air is
an important factor to be considered. So, for instance, if two wires are attached to the terminals of
the coil, and the streamers issue from them, there is dissipation of energy in the form of heat and

light, and the wires behave like a condenser of larger capacity. If the wires be immersed in oil, the
dissipation of energy is prevented, or at least reduced, and the apparent capacity is diminished. The
action of the air would seem to make it very difficult to tell, from the measured or computed
capacity of a condenser in which the air is acted upon, its actual capacity or vibration period,
especially if the condenser is of very small surface and is charged to a very high potential. As
many important results are dependant upon the correctness of the estimation of the vibration
period, this subject demands the most careful scrutiny of investigators.
In Leyden jars the loss due to the presence of air is comparatively small, principally on account
of the great surface of the coatings and the small external action, but if there are streamers on the
top, the loss may be considerable, and the period of vibration is affected. In a resonator, the density
is small, but the frequency is extreme, and may introduce a considerable error. It appears certain, at
any rate, that the periods of vibration of a charged body in a gaseous and in a continuous medium,
such as oil, are different, on account of the action of the former, as explained.
Another fact recognized, which is of some consequence, is, that in similar investigations the
general considerations of static screening are not applicable when a gaseous medium is present.
This is evident from the following experiment:—A short and wide glass tube is taken and covered
with a substantial coating of bronze powder, barely allowing the light to shine a little through. The
tube is highly exhausted and suspended on a metallic clasp from the end of a wire. When the wire
is connected with one of the terminals of the coil, the gas inside of the tube is lighted in spite of the
metal coating. Here the metal evidently does not screen the gas inside as it ought to, even if it be
very thin and poorly conducting. Yet, in a condition of rest the metal coating, however thin,
screens the inside perfectly.
One of the most interesting results arrived at in pursuing these experiments, is the
demonstration of the fact that a gaseous medium, upon which vibration is impressed by rapid
changes of electrostatic potential, is rigid. In illustration of this result an experiment made by Mr.
Tesla may by cited:—A glass tube about one inch in diameter and three feet long, with outside
condenser coatings on the ends, was exhausted to a certain point, when, the tube being suspended
freely from a wire connecting the upper coating to one of the terminals of the coil, the discharge
appeared in the form of a luminous thread passing through the axis of the tube. Usually the thread
was sharply defined in the upper part of the tube and lost itself in the lower part. When a magnet or
the finger was quickly passed near the upper part of the luminous thread, it was brought out of
position by magnetic or electrostatic influence, and a transversal vibration like that of a suspended
cord, with one or more distinct nodes, was set up, which lasted for a few minutes and gradually
died out. By suspending from the lower condenser coating metal plates of different sizes, the speed
of the vibration was varied. This vibration would seem to show beyond doubt that the thread
possessed rigidity, at least to transversal displacements.
Many experiments were tried to demonstrate this property in air at ordinary pressure. Though
no positive evidence has been obtained, it is thought, nevertheless, that a high frequency brush or
streamer, if the frequency could be pushed far enough, would be decidedly rigid. A small sphere
might then be moved within it quite freely, but if thrown against it the sphere would rebound. An
ordinary flame cannot possess rigidity to a marked degree because the vibration is directionless;
but an electric arc, it is believed, must possess that property more or less. A luminous band excited
in a bulb by repeated discharges of a Leyden jar must also possess rigidity, and if deformed and
suddenly released should vibrate.

From like considerations other conclusions of interest are reached. The most probable medium
filling the space is one consisting of independent carriers immersed in an insulating fluid. If
through this medium enormous electrostatic stresses are assumed to act, which vary rapidly in
intensity, it would allow the motion of a body through it, yet it would be rigid and elastic, although
the fluid itself might be devoid of these properties. Furthermore, on the assumption that the
independent carriers are of any configuration such that the fluid resistance to motion in one
direction is greater than in another, a stress of that nature would cause the carriers to arrange
themselves in groups, since they would turn to each other their sides of the greatest electric
density, in which position the fluid resistance to approach would be smaller than to receding. If in
a medium of the above characteristics a brush would be formed by a steady potential, an exchange
of the carriers would go on continually, and there would be less carriers per unit of volume in the
brush than in the space at some distance from the electrode, this corresponding to rarefaction. If
the potential were rapidly changing, the result would be very different; the higher the frequency of
the pulses, the slower would be the exchange of the carriers; finally, the motion of translation
through measurable space would cease, and, with a sufficiently high frequency and intensity of the
stress, the carriers would be drawn towards the electrode, and compression would result.
An interesting feature of these high frequency currents is that they allow of operating all kinds
of devices by connecting the device with only one leading wire to the electric source. In fact, under
certain conditions it may be more economical to supply the electrical energy with one lead than
with two.
An experiment of special interest shown by Mr. Tesla, is the running, by the use of only one
insulated line, of a motor operating on the principle of the rotating magnetic field enunciated by
Mr. Tesla. A simple form of such a motor is obtained by winding upon a laminated iron core a
primary and close to it a secondary coil, closing the ends of the latter and placing a freely movable
metal disc within the influence of the moving field. The secondary coil may, however, be omitted.
When one of the ends of the primary coil of the motor is connected to one of the terminals of the
high frequency coil and the other end to an insulated metal plate, which, it should be stated, is not
absolutely necessary for the success of the experiment, the disc is set in rotation.
Experiments of this kind seem to bring it within possibility to operate a motor at any point of
the earth's surface from a central source, without any connection to the same except through the
earth. If, by means of powerful machinery, rapid variations of the earth's potential were produced,
a grounded wire reaching up to some height would be traversed by a current which could be
increased by connecting the free end of the wire to a body of some size. The current might be
converted to low tension and used to operate a motor or other device. The experiment, which
would be one of great scientific interest, would probably best succeed on a ship at sea. In this
manner, even if it were not possible to operate machinery, intelligence might be transmitted quite
certainly.
In the course of this experimental study special attention was devoted to the heating effects
produced by these currents, which are not only striking, but open up the possibility of producing a
more efficient illuminant. It is sufficient to attach to the coil terminal a thin wire or filament, to
have the temperature of the latter perceptibly raised. If the wire or filament be enclosed in a bulb,
the heating effect is increased by preventing the circulation of the air. If the air in the bulb be
strongly compressed, the displacements are smaller, the impacts less violent, and the heating effect
is diminished. On the contrary, if the air in the bulb be exhausted, an inclosed lamp filament is
brought to incandescence, and any amount of light may thus be produced.

The heating of the inclosed lamp filament depends on so many things of a different nature, that
it is difficult to give a generally applicable rule under which the maximum heating occurs. As
regards the size of the bulb, it is ascertained that at ordinary or only slightly differing atmospheric
pressures, when air is a good insulator, the filament is heated more in a small bulb, because of the
better confinement of heat in this case. At lower pressures, when air becomes conducting, the
heating effect is greater in a large bulb, but at excessively high degrees of exhaustion there seems
to be, beyond a certain and rather small size of the vessel, no perceptible difference in the heating.
The shape of the vessel is also of some importance, and it has been found of advantage for
reasons of economy to employ a spherical bulb with the electrode mounted in its centre, where the
rebounding molecules collide.
It is desirable on account of economy that all the energy supplied to the bulb from the source
should reach without loss the body to be heated. The loss in conveying the energy from the source
to the body may be reduced by employing thin wires heavily coated with insulation, and by the use
of electrostatic screens. It is to be remarked, that the screen cannot be connected to the ground as
under ordinary conditions.
In the bulb itself a large portion of the energy supplied may be lost by molecular bombardment
against the wire connecting the body to be heated with the source. Considerable improvement was
effected by covering the glass stem containing the wire with a closely fitting conducting tube. This
tube is made to project a little above the glass, and prevents the cracking of the latter near the
heated body. The effectiveness of the conducting tube is limited to very high degrees of
exhaustion. It diminishes the energy lost in bombardment for two reasons; first, the charge given
up by the atoms spreads over a greater area, and hence the electric density at any point is small,
and the atoms are repelled with less energy than if they would strike against a good insulator;
secondly, as the tube is electrified by the atoms which first come in contact with it, the progress of
the following atoms against the tube is more or less checked by the repulsion which the electrified
tube must exert upon the similarly electrified atoms. This, it is thought, explains why the discharge
through a bulb is established with much greater facility when an insulator, than when a conductor,
is present.
During the investigations a great many bulbs of different construction, with electrodes of
different material, were experimented upon, and a number of observations of interest were made.
Mr. Tesla has found that the deterioration of the electrode is the less, the higher the frequency. This
was to be expected, as then the heating is effected by many small impacts, instead by fewer and
more violent ones, which quickly shatter the structure. The deterioration is also smaller when the
vibration is harmonic. Thus an electrode, maintained at a certain degree of heat, lasts much longer
with currents obtained from an alternator, than with those obtained by means of a disruptive
discharge. One of the most durable electrodes was obtained from strongly compressed
carborundum, which is a kind of carbon recently produced by Mr. E. G. Acheson, of Monongahela
City, Pa. From experience, it is inferred, that to be most durable, the electrode should be in the
form of a sphere with a highly polished surface.
In some bulbs refractory bodies were mounted in a carbon cup and put under the molecular
impact. It was observed in such experiments that the carbon cup was heated at first, until a higher
temperature was reached; then most of the bombardment was directed against the refractory body,
and the carbon was relieved. In general, when different bodies were mounted in the bulb, the
hardest fusible would be relieved, and would remain at a considerably lower temperature. This was

necessitated by the fact that most of the energy supplied would find its way through the body
which was more easily fused or "evaporated."
Curiously enough it appeared in some of the experiments made, that a body was fused in a
bulb under the molecular impact by evolution of less light than when fused by the application of
heat in ordinary ways. This may be ascribed to a loosening of the structure of the body under the
violent impacts and changing stresses.
Some experiments seem to indicate that under certain conditions a body, conducting or
nonconducting, may, when bombarded, emit light, which to all appearances is due to
phosphorescence, but may in reality be caused by the incandescence of an infinitesimal layer, the
mean temperature of the body being comparatively small. Such might be the case if each single
rhythmical impact were capable of instantaneously exciting the retina, and the rhythm were just
high enough to cause a continuous impression in the eye. According to this view, a coil operated
by disruptive discharge would be eminently adapted to produce such a result, and it is found by
experience that its power of exciting phosphorescence is extraordinarily great. It is capable of
exciting phosphorescence at comparatively low degrees of exhaustion, and also projects shadows
at pressures far greater than those at which the mean free path is comparable to the dimensions of
the vessel. The latter observation is of some importance, inasmuch as it may modify the generally
accepted views in regard to the "radiant state" phenomena.
A thought which early and naturally suggested itself to Mr. Tesla, was to utilize the great
inductive effects of high frequency currents to produce light in a sealed glass vessel without the
use of leading in wires. Accordingly, many bulbs were constructed in which the energy necessary
to maintain a button or filament at high incandescence, was supplied through the glass by either
electrostatic or electrodynamic induction. It was easy to regulate the intensity of the light emitted
by means of an externally applied condenser coating connected to an insulated plate, or simply by
means of a plate attached to the bulb which at the same time performed the function of a shade.
A subject of experiment, which has been exhaustively treated in England by Prof. J. J.
Thomson, has been followed up independently by Mr. Tesla from the beginning of this study,
namely, to excite by electrodynamic induction a luminous band in a closed tube or bulb. In
observing the behavior of gases, and the luminous phenomena obtained, the importance of the
electrostatic effects was noted and it appeared desirable to produce enormous potential differences,
alternating with extreme rapidity. Experiments in this direction led to some of the most interesting
results arrived at in the course of these investigations. It was found that by rapid alternations of a
high electrostatic potential, exhausted tubes could be lighted at considerable distances from a
conductor connected to a properly constructed coil, and that it was practicable to establish with the
coil an alternating electrostatic field, acting through the whole room and lighting a tube wherever
it was placed within the four walls. Phosphorescent bulbs may be excited in such a field, and it is
easy to regulate the effect by connecting to the bulb a small insulated metal plate. It was likewise
possible to maintain a filament or button mounted in a tube at bright incandescence, and, in one
experiment, a mica vane was spun by the incandescence of a platinum wire.
Coming now to the lecture delivered in Philadelphia and St. Louis, it may be remarked that to
the superficial reader, Mr. Tesla's introduction, dealing with the importance of the eye, might
appear as a digression, but the thoughtful reader will find therein much food for meditation and
speculation. Throughout his discourse one can trace Mr. Tesla's effort to present in a popular way
thoughts and views on the electrical phenomena which have in recent years captivated the
scientific world, but of which the general public has even yet merely received an inkling. Mr. Tesla

also dwells rather extensively on his well-known method of high-frequency conversion; and the
large amount of detail information will be gratefully received by students and experimenters in this
virgin field. The employment of apt analogies in explaining the fundamental principles involved
makes it easy for all to gain a clear idea of their nature. Again, the ease with which, thanks to Mr.
Tesla's efforts, these high-frequency currents may now be obtained from circuits carrying almost
any kind of current, cannot fail to result in an extensive broadening of this field of research, which
offers so many possibilities. Mr. Tesla, true philosopher as he is, does not hesitate to point out
defects in some of his methods, and indicates the lines which to him seem the most promising.
Particular stress is laid by him upon the employment of a medium in which the discharge
electrodes should be immersed in order that this method of conversion may be brought to the
highest perfection. He has evidently taken pains to give as much useful information as possible to
those who wish to follow in his path, as he shows in detail the circuit arrangements to be adopted
in all ordinary cases met with in practice, and although some of these methods were described by
him two years before, the additional information is still timely and welcome.
In his experiments he dwells first on some phenomena produced by electrostatic force, which
he considers in the light of modern theories to be the most important force in nature for us to
investigate. At the very outset he shows a strikingly novel experiment illustrating the effect of a
rapidly varying electrostatic force in a gaseous medium, by touching with one hand one of the
terminals of a 200,000 volt transformer and bringing the other hand to the opposite terminal. The
powerful streamers which issued from his hand and astonished his audiences formed a capital
illustration of some of the views advanced, and afforded Mr. Tesla an opportunity of pointing out
the true reasons why, with these currents, such an amount of energy can be passed through the
body with impunity. He then showed by experiment the difference between a steady and a rapidly
varying force upon the dielectric. This difference is most strikingly illustrated in the experiment in
which a bulb attached to the end of a wire in connection with one of the terminals of the
transformer is ruptured, although all extraneous bodies are remote from the bulb. He next
illustrates how mechanical motions are produced by a varying electrostatic force acting through a
gaseous medium. The importance of the action of the air is particularly illustrated by an interesting
experiment.
Taking up another class of phenomena, namely, those of dynamic electricity, Mr. Tesla
produced in a number of experiments a variety of effects by the employment of only a single wire
with the evident intent of impressing upon his audience the idea that electric vibration or current
can be transmitted with ease, without any return circuit; also how currents so transmitted can be
converted and used for many practical purposes. A number of experiments are then shown,
illustrating the effects of frequency, self-induction and capacity; then a number of ways of
operating motive and other devices by the use of a single lead. A number of novel impedance
phenomena are also shown which cannot fail to arouse interest.
Mr. Tesla next dwelt upon a subject which he thinks of great importance, that is, electrical
resonance, which he explained in a popular way. He expressed his firm conviction that by
observing proper conditions, intelligence, and possibly even power, can be transmitted through the
medium or through the earth; and he considers this problem worthy of serious and immediate
consideration.
Coming now to the light phenomena in particular, he illustrated the four distinct kinds of these
phenomena in an original way, which to many must have been a revelation. Mr. Tesla attributes
these light effects to molecular or atomic impacts produced by a varying electrostatic stress in a

gaseous medium. He illustrated in a series of novel experiments the effect of the gas surrounding
the conductor and shows beyond a doubt that with high frequency and high potential currents, the
surrounding gas is of paramount importance in the heating of the conductor. He attributes the
heating partially to a conduction current and partially to bombardment, and demonstrates that in
many cases the heating may be practically due to the bombardment alone. He pointed out also that
the skin effect is largely modified by the presence of the gas or of an atomic medium in general.
He showed also some interesting experiments in which the effect of convection is illustrated.
Probably one of the most curious experiments in this connection is that in which a thin platinum
wire stretched along the axis of an exhausted tube is brought to incandescence at certain points
corresponding to the position of the striæ, while at others it remains dark. This experiment throws
an interesting light upon the nature of the striæ and may lead to important revelations.
Mr. Tesla also demonstrated the dissipation of energy through an atomic medium and dwelt
upon the behavior of vacuous space in conveying heat, and in this connection showed the curious
behavior of an electrode stream, from which he concludes that the molecules of a gas probably
cannot be acted upon directly at measurable distances.
Mr. Tesla summarized the chief results arrived at in pursuing his investigations in a manner
which will serve as a valuable guide to all who may engage in this work. Perhaps most interest
will centre on his general statements regarding the phenomena of phosphorescence, the most
important fact revealed in this direction being that when exciting a phosphorescent bulb a certain
definite potential gives the most economical result.
The lectures will now be presented in the order of their date of delivery.

CHAPTER XXVI.
Experiments With Alternate Currents of Very
High Frequency and Their Application to Methods
of Artificial Illumination.
[1]

There is no subject more captivating, more worthy of study, than nature. To understand this
great mechanism, to discover the forces which are active, and the laws which govern them, is the
highest aim of the intellect of man.
Nature has stored up in the universe infinite energy. The eternal recipient and transmitter of
this infinite energy is the ether. The recognition of the existence of ether, and of the functions it
performs, is one of the most important results of modern scientific research. The mere abandoning
of the idea of action at a distance, the assumption of a medium pervading all space and connecting
all gross matter, has freed the minds of thinkers of an ever present doubt, and, by opening a new
horizon—new and unforeseen possibilities—has given fresh interest to phenomena with which we
are familiar of old. It has been a great step towards the understanding of the forces of nature and
their multifold manifestations to our senses. It has been for the enlightened student of physics what
the understanding of the mechanism of the firearm or of the steam engine is for the barbarian.
Phenomena upon which we used to look as wonders baffling explanation, we now see in a
different light. The spark of an induction coil, the glow of an incandescent lamp, the
manifestations of the mechanical forces of currents and magnets are no longer beyond our grasp;
instead of the incomprehensible, as before, their observation suggests now in our minds a simple
mechanism, and although as to its precise nature all is still conjecture, yet we know that the truth
cannot be much longer hidden, and instinctively we feel that the understanding is dawning upon
us. We still admire these beautiful phenomena, these strange forces, but we are helpless no longer;
we can in a certain measure explain them, account for them, and we are hopeful of finally
succeeding in unraveling the mystery which surrounds them.
In how far we can understand the world around us is the ultimate thought of every student of
nature. The coarseness of our senses prevents us from recognizing the ulterior construction of
matter, and astronomy, this grandest and most positive of natural sciences, can only teach us
something that happens, as it were, in our immediate neighborhood: of the remoter portions of the
boundless universe, with its numberless stars and suns, we know nothing. But far beyond the limit
of perception of our senses the spirit still can guide us, and so we may hope that even these
unknown worlds—infinitely small and great—may in a measure become known to us. Still, even if
this knowledge should reach us, the searching mind will find a barrier, perhaps forever
unsurpassable, to the true recognition of that which seems to be, the mere appearance of which is
the only and slender basis of all our philosophy.
Of all the forms of nature's immeasurable, all-pervading energy, which ever and ever changing
and moving, like a soul animates the inert universe, electricity and magnetism are perhaps the most
fascinating. The effects of gravitation, of heat and light we observe daily, and soon we get
accustomed to them, and soon they lose for us the character of the marvelous and wonderful; but
electricity and magnetism, with their singular relationship, with their seemingly dual character,
unique among the forces in nature, with their phenomena of attractions, repulsions and rotations,

strange manifestations of mysterious agents, stimulate and excite the mind to thought and research.
What is electricity, and what is magnetism? These questions have been asked again and again. The
most able intellects have ceaselessly wrestled with the problem; still the question has not as yet
been fully answered. But while we cannot even to-day state what these singular forces are, we
have made good headway towards the solution of the problem. We are now confident that electric
and magnetic phenomena are attributable to ether, and we are perhaps justified in saying that the
effects of static electricity are effects of ether under strain, and those of dynamic electricity and
electro-magnetism effects of ether in motion. But this still leaves the question, as to what
electricity and magnetism are, unanswered.
First, we naturally inquire, What is electricity, and is there such a thing as electricity? In
interpreting electric phenomena, we may speak of electricity or of an electric condition, state or
effect. If we speak of electric effects we must distinguish two such effects, opposite in character
and neutralizing each other, as observation shows that two such opposite effects exist. This is
unavoidable, for in a medium of the properties of ether, we cannot possibly exert a strain, or
produce a displacement or motion of any kind, without causing in the surrounding medium an
equivalent and opposite effect. But if we speak of electricity, meaning a thing, we must, I think,
abandon the idea of two electricities, as the existence of two such things is highly improbable. For
how can we imagine that there should be two things, equivalent in amount, alike in their
properties, but of opposite character, both clinging to matter, both attracting and completely
neutralizing each other? Such an assumption, though suggested by many phenomena, though most
convenient for explaining them, has little to commend it. If there is such a thing as electricity, there
can be only one such thing, and excess and want of that one thing, possibly; but more probably its
condition determines the positive and negative character. The old theory of Franklin, though
falling short in some respects, is, from a certain point of view, after all, the most plausible one.
Still, in spite of this, the theory of the two electricities is generally accepted, as it apparently
explains electric phenomena in a more satisfactory manner. But a theory which better explains the
facts is not necessarily true. Ingenious minds will invent theories to suit observation, and almost
every independent thinker has his own views on the subject.
It is not with the object of advancing an opinion, but with the desire of acquainting you better
with some of the results, which I will describe, to show you the reasoning I have followed, the
departures I have made—that I venture to express, in a few words, the views and convictions
which have led me to these results.
I adhere to the idea that there is a thing which we have been in the habit of calling electricity.
The question is, What is that thing? or, What, of all things, the existence of which we know, have
we the best reason to call electricity? We know that it acts like an incompressible fluid; that there
must be a constant quantity of it in nature; that it can be neither produced nor destroyed; and, what
is more important, the electro-magnetic theory of light and all facts observed teach us that electric
and ether phenomena are identical. The idea at once suggests itself, therefore, that electricity might
be called ether. In fact, this view has in a certain sense been advanced by Dr. Lodge. His
interesting work has been read by everyone and many have been convinced by his arguments. His
great ability and the interesting nature of the subject, keep the reader spellbound; but when the
impressions fade, one realizes that he has to deal only with ingenious explanations. I must confess,
that I cannot believe in two electricities, much less in a doubly-constituted ether. The puzzling
behavior of the ether as a solid to waves of light and heat, and as a fluid to the motion of bodies
through it, is certainly explained in the most natural and satisfactory manner by assuming it to be
in motion, as Sir William Thomson has suggested; but regardless of this, there is nothing which

would enable us to conclude with certainty that, while a fluid is not capable of transmitting
transverse vibrations of a few hundred or thousand per second, it might not be capable of
transmitting such vibrations when they range into hundreds of million millions per second. Nor
can anyone prove that there are transverse ether waves emitted from an alternate current machine,
giving a small number of alternations per second; to such slow disturbances, the ether, if at rest,
may behave as a true fluid.
Returning to the subject, and bearing in mind that the existence of two electricities is, to say
the least, highly improbable, we must remember, that we have no evidence of electricity, nor can
we hope to get it, unless gross matter is present. Electricity, therefore, cannot be called ether in the
broad sense of the term; but nothing would seem to stand in the way of calling electricity ether
associated with matter, or bound ether; or, in other words, that the so-called static charge of the
molecule is ether associated in some way with the molecule. Looking at it in that light, we would
be justified in saying, that electricity is concerned in all molecular actions.
Now, precisely what the ether surrounding the molecules is, wherein it differs from ether in
general, can only be conjectured. It cannot differ in density, ether being incompressible: it must,
therefore, be under some strain or in motion, and the latter is the most probable. To understand its
functions, it would be necessary to have an exact idea of the physical construction of matter, of
which, of course, we can only form a mental picture.
But of all the views on nature, the one which assumes one matter and one force, and a perfect
uniformity throughout, is the most scientific and most likely to be true. An infinitesimal world,
with the molecules and their atoms spinning and moving in orbits, in much the same manner as
celestial bodies, carrying with them and probably spinning with them ether, or in other words,
carrying with them static charges, seems to my mind the most probable view, and one which, in a
plausible manner, accounts for most of the phenomena observed. The spinning of the molecules
and their ether sets up the ether tensions or electrostatic strains; the equalization of ether tensions
sets up ether motions or electric currents, and the orbital movements produce the effects of electro
and permanent magnetism.
About fifteen years ago, Prof. Rowland demonstrated a most interesting and important fact,
namely, that a static charge carried around produces the effects of an electric current. Leaving out
of consideration the precise nature of the mechanism, which produces the attraction and repulsion
of currents, and conceiving the electrostatically charged molecules in motion, this experimental
fact gives us a fair idea of magnetism. We can conceive lines or tubes of force which physically
exist, being formed of rows of directed moving molecules; we can see that these lines must be
closed, that they must tend to shorten and expand, etc. It likewise explains in a reasonable way, the
most puzzling phenomenon of all, permanent magnetism, and, in general, has all the beauties of
the Ampere theory without possessing the vital defect of the same, namely, the assumption of
molecular currents. Without enlarging further upon the subject, I would say, that I look upon all
electrostatic, current and magnetic phenomena as being due to electrostatic molecular forces.
The preceding remarks I have deemed necessary to a full understanding of the subject as it
presents itself to my mind.
Of all these phenomena the most important to study are the current phenomena, on account of
the already extensive and ever-growing use of currents for industrial purposes. It is now a century
since the first practical source of current was produced, and, ever since, the phenomena which
accompany the flow of currents have been diligently studied, and through the untiring efforts of

scientific men the simple laws which govern them have been discovered. But these laws are found
to hold good only when the currents are of a steady character. When the currents are rapidly
varying in strength, quite different phenomena, often unexpected, present themselves, and quite
different laws hold good, which even now have not been determined as fully as is desirable,
though through the work, principally, of English scientists, enough knowledge has been gained on
the subject to enable us to treat simple cases which now present themselves in daily practice.
The phenomena which are peculiar to the changing character of the currents are greatly exalted
when the rate of change is increased, hence the study of these currents is considerably facilitated
by the employment of properly constructed apparatus. It was with this and other objects in view
that I constructed alternate current machines capable of giving more than two million reversals of
current per minute, and to this circumstance it is principally due, that I am able to bring to your
attention some of the results thus far reached, which I hope will prove to be a step in advance on
account of their direct bearing upon one of the most important problems, namely, the production of
a practical and efficient source of light.
The study of such rapidly alternating currents is very interesting. Nearly every experiment
discloses something new. Many results may, of course, be predicted, but many more are
unforeseen. The experimenter makes many interesting observations. For instance, we take a piece
of iron and hold it against a magnet. Starting from low alternations and running up higher and
higher we feel the impulses succeed each other faster and faster, get weaker and weaker, and
finally disappear. We then observe a continuous pull; the pull, of course, is not continuous; it only
appears so to us; our sense of touch is imperfect.
We may next establish an arc between the electrodes and observe, as the alternations rise, that
the note which accompanies alternating arcs gets shriller and shriller, gradually weakens, and
finally ceases. The air vibrations, of course, continue, but they are too weak to be perceived; our
sense of hearing fails us.
We observe the small physiological effects, the rapid heating of the iron cores and conductors,
curious inductive effects, interesting condenser phenomena, and still more interesting light
phenomena with a high tension induction coil. All these experiments and observations would be of
the greatest interest to the student, but their description would lead me too far from the principal
subject. Partly for this reason, and partly on account of their vastly greater importance, I will
confine myself to the description of the light effects produced by these currents.
In the experiments to this end a high tension induction coil or equivalent apparatus for
converting currents of comparatively low into currents of high tension is used.
If you will be sufficiently interested in the results I shall describe as to enter into an
experimental study of this subject; if you will be convinced of the truth of the arguments I shall
advance—your aim will be to produce high frequencies and high potentials; in other words,
powerful electrostatic effects. You will then encounter many difficulties, which, if completely
overcome, would allow us to produce truly wonderful results.
First will be met the difficulty of obtaining the required frequencies by means of mechanical
apparatus, and, if they be obtained otherwise, obstacles of a different nature will present
themselves. Next it will be found difficult to provide the requisite insulation without considerably
increasing the size of the apparatus, for the potentials required are high, and, owing to the rapidity
of the alternations, the insulation presents peculiar difficulties. So, for instance, when a gas is
present, the discharge may work, by the molecular bombardment of the gas and consequent

heating, through as much as an inch of the best solid insulating material, such as glass, hard
rubber, porcelain, sealing wax, etc.; in fact, through any known insulating substance. The chief
requisite in the insulation of the apparatus is, therefore, the exclusion of any gaseous matter.
In general my experience tends to show that bodies which possess the highest specific
inductive capacity, such as glass, afford a rather inferior insulation to others, which, while they are
good insulators, have a much smaller specific inductive capacity, such as oils, for instance, the
dielectric losses being no doubt greater in the former. The difficulty of insulating, of course, only
exists when the potentials are excessively high, for with potentials such as a few thousand volts
there is no particular difficulty encountered in conveying currents from a machine giving, say,
20,000 alternations per second, to quite a distance. This number of alternations, however, is by far
too small for many purposes, though quite sufficient for some practical applications. This
difficulty of insulating is fortunately not a vital drawback; it affects mostly the size of the
apparatus, for, when excessively high potentials would be used, the light-giving devices would be
located not far from the apparatus, and often they would be quite close to it. As the air-
bombardment of the insulated wire is dependent on condenser action, the loss may be reduced to a
trifle by using excessively thin wires heavily insulated.
Another difficulty will be encountered in the capacity and self-induction necessarily possessed
by the coil. If the coil be large, that is, if it contain a great length of wire, it will be generally
unsuited for excessively high frequencies; if it be small, it may be well adapted for such
frequencies, but the potential might then not be as high as desired. A good insulator, and preferably
one possessing a small specific inductive capacity, would afford a two-fold advantage. First, it
would enable us to construct a very small coil capable of withstanding enormous differences of
potential; and secondly, such a small coil, by reason of its smaller capacity and self-induction,
would be capable of a quicker and more vigorous vibration. The problem then of constructing a
coil or induction apparatus of any kind possessing the requisite qualities I regard as one of no
small importance, and it has occupied me for a considerable time.
The investigator who desires to repeat the experiments which I will describe, with an alternate
current machine, capable of supplying currents of the desired frequency, and an induction coil, will
do well to take the primary coil out and mount the secondary in such a manner as to be able to look
through the tube upon which the secondary is wound. He will then be able to observe the streams
which pass from the primary to the insulating tube, and from their intensity he will know how far
he can strain the coil. Without this precaution he is sure to injure the insulation. This arrangement
permits, however, an easy exchange of the primaries, which is desirable in these experiments.
The selection of the type of machine best suited for the purpose must be left to the judgment of
the experimenter. There are here illustrated three distinct types of machines, which, besides others,
I have used in my experiments.
Fig. 97 represents the machine used in my experiments before this Institute. The field magnet
consists of a ring of wrought iron with 384 pole projections. The armature comprises a steel disc to
which is fastened a thin, carefully welded rim of wrought iron. Upon the rim are wound several
layers of fine, well annealed iron wire, which, when wound, is passed through shellac. The
armature wires are wound around brass pins, wrapped with silk thread. The diameter of the
armature wire in this type of machine should not be more than 1/6 of the thickness of the pole
projections, else the local action will be considerable.

Fig. 97.
Fig. 98 represents a larger machine of a different type. The field magnet of this machine
consists of two like parts which either enclose an exciting coil, or else are independently wound.
Each part has 480 pole projections, the projections of one facing those of the other. The armature
consists of a wheel of hard bronze, carrying the conductors which revolve between the projections
of the field magnet. To wind the armature conductors, I have found it most convenient to proceed
in the following manner. I construct a ring of hard bronze of the required size. This ring and the
rim of the wheel are provided with the proper number of pins, and both fastened upon a plate. The
armature conductors being wound, the pins are cut off and the ends of the conductors fastened by
two rings which screw to the bronze ring and the rim of the wheel, respectively. The whole may
then be taken off and forms a solid structure. The conductors in such a type of machine should
consist of sheet copper, the thickness of which, of course, depends on the thickness of the pole
projections; or else twisted thin wires should be employed.
Fig. 99 is a smaller machine, in many respects similar to the former, only here the armature
conductors and the exciting coil are kept stationary, while only a block of wrought iron is
revolved.

Fig. 98.
It would be uselessly lengthening this description were I to dwell more on the details of
construction of these machines. Besides, they have been described somewhat more elaborately in
The Electrical Engineer, of March 18, 1891. I deem it well, however, to call the attention of the
investigator to two things, the importance of which, though self evident, he is nevertheless apt to
underestimate; namely, to the local action in the conductors which must be carefully avoided, and
to the clearance, which must be small. I may add, that since it is desirable to use very high
peripheral speeds, the armature should be of very large diameter in order to avoid impracticable
belt speeds. Of the several types of these machines which have been constructed by me, I have
found that the type illustrated in Fig. 97 caused me the least trouble in construction, as well as in
maintenance, and on the whole, it has been a good experimental machine.
In operating an induction coil with very rapidly alternating currents, among the first luminous
phenomena noticed are naturally those presented by the high-tension discharge. As the number of
alternations per second is increased, or as—the number being high—the current through the
primary is varied, the discharge gradually changes in appearance. It would be difficult to describe
the minor changes which occur, and the conditions which bring them about, but one may note five
distinct forms of the discharge.

Fig. 99.
First, one may observe a weak, sensitive discharge in the form of a thin, feeble-colored thread.
(Fig. 100a.) It always occurs when, the number of alternations per second being high, the current
through the primary is very small. In spite of the excessively small current, the rate of change is
great, and the difference of potential at the terminals of the secondary is therefore considerable, so
that the arc is established at great distances; but the quantity of "electricity" set in motion is
insignificant, barely sufficient to maintain a thin, threadlike arc. It is excessively sensitive and may
be made so to such a degree that the mere act of breathing near the coil will affect it, and unless it
is perfectly well protected from currents of air, it wriggles around constantly. Nevertheless, it is in
this form excessively persistent, and when the terminals are approached to, say, one-third of the
striking distance, it can be blown out only with difficulty. This exceptional persistency, when short,
is largely due to the arc being excessively thin; presenting, therefore, a very small surface to the
blast. Its great sensitiveness, when very long, is probably due to the motion of the particles of dust
suspended in the air.

Fig. 100a. Fig. 100b.

When the current through the primary is increased, the discharge gets broader and stronger,
and the effect of the capacity of the coil becomes visible until, finally, under proper conditions, a
white flaming arc, Fig. 100B, often as thick as one's finger, and striking across the whole coil, is
produced. It develops remarkable heat, and may be further characterized by the absence of the high
note which accompanies the less powerful discharges. To take a shock from the coil under these
conditions would not be advisable, although under different conditions, the potential being much
higher, a shock from the coil may be taken with impunity. To produce this kind of discharge the
number of alternations per second must not be too great for the coil used; and, generally speaking,
certain relations between capacity, self-induction and frequency must be observed.
The importance of these elements in an alternate current circuit is now well-known, and under
ordinary conditions, the general rules are applicable. But in an induction coil exceptional
conditions prevail. First, the self-induction is of little importance before the arc is established,
when it asserts itself, but perhaps never as prominently as in ordinary alternate current circuits,
because the capacity is distributed all along the coil, and by reason of the fact that the coil usually
discharges through very great resistances; hence the currents are exceptionally small. Secondly, the
capacity goes on increasing continually as the potential rises, in consequence of absorption which
takes place to a considerable extent. Owing to this there exists no critical relationship between
these quantities, and ordinary rules would not seem to be applicable. As the potential is increased
either in consequence of the increased frequency or of the increased current through the primary,
the amount of the energy stored becomes greater and greater, and the capacity gains more and
more in importance. Up to a certain point the capacity is beneficial, but after that it begins to be an
enormous drawback. It follows from this that each coil gives the best result with a given frequency
and primary current. A very large coil, when operated with currents of very high frequency, may
not give as much as 1/8 inch spark. By adding capacity to the terminals, the condition may be
improved, but what the coil really wants is a lower frequency.
When the flaming discharge occurs, the conditions are evidently such that the greatest current
is made to flow through the circuit. These conditions may be attained by varying the frequency
within wide limits, but the highest frequency at which the flaming arc can still be produced,
determines, for a given primary current, the maximum striking distance of the coil. In the flaming
discharge the eclat effect of the capacity is not perceptible; the rate at which the energy is being
stored then just equals the rate at which it can be disposed of through the circuit. This kind of
discharge is the severest test for a coil; the break, when it occurs, is of the nature of that in an
overcharged Leyden jar. To give a rough approximation I would state that, with an ordinary coil of,
say 10,000 ohms resistance, the most powerful arc would be produced with about 12,000
alternations per second.
When the frequency is increased beyond that rate, the potential, of course, rises, but the
striking distance may, nevertheless, diminish, paradoxical as it may seem. As the potential rises the
coil attains more and more the properties of a static machine until, finally, one may observe the
beautiful phenomenon of the streaming discharge, Fig. 101, which may be produced across the
whole length of the coil. At that stage streams begin to issue freely from all points and projections.
These streams will also be seen to pass in abundance in the space between the primary and the
insulating tube. When the potential is excessively high they will always appear, even if the
frequency be low, and even if the primary be surrounded by as much as an inch of wax, hard
rubber, glass, or any other insulating substance. This limits greatly the output of the coil, but I will
later show how I have been able to overcome to a considerable extent this disadvantage in the
ordinary coil.

Besides the potential, the intensity of the streams depends on the frequency; but if the coil be
very large they show themselves, no matter how low the frequencies used. For instance, in a very
large coil of a resistance of 67,000 ohms, constructed by me some time ago, they appear with as
low as 100 alternations per second and less, the insulation of the secondary being 3/4 inch of
ebonite. When very intense they produce a noise similar to that produced by the charging of a
Holtz machine, but much more powerful, and they emit a strong smell of ozone. The lower the
frequency, the more apt they are to suddenly injure the coil. With excessively high frequencies
they may pass freely without producing any other effect than to heat the insulation slowly and
uniformly.

Fig. 101. Fig. 102.

The existence of these streams shows the importance of constructing an expensive coil so as to
permit of one's seeing through the tube surrounding the primary, and the latter should be easily
exchangeable; or else the space between the primary and secondary should be completely filled up
with insulating material so as to exclude all air. The non-observance of this simple rule in the
construction of commercial coils is responsible for the destruction of many an expensive coil.
At the stage when the streaming discharge occurs, or with somewhat higher frequencies, one
may, by approaching the terminals quite nearly, and regulating properly the effect of capacity,
produce a veritable spray of small silver-white sparks, or a bunch of excessively thin silvery
threads (Fig. 102) amidst a powerful brush—each spark or thread possibly corresponding to one
alternation. This, when produced under proper conditions, is probably the most beautiful
discharge, and when an air blast is directed against it, it presents a singular appearance. The spray
of sparks, when received through the body, causes some inconvenience, whereas, when the
discharge simply streams, nothing at all is likely to be felt if large conducting objects are held in
the hands to protect them from receiving small burns.
If the frequency is still more increased, then the coil refuses to give any spark unless at
comparatively small distances, and the fifth typical form of discharge may be observed (Fig. 103).
The tendency to stream out and dissipate is then so great that when the brush is produced at one
terminal no sparking occurs, even if, as I have repeatedly tried, the hand, or any conducting object,
is held within the stream; and, what is more singular, the luminous stream is not at all easily
deflected by the approach of a conducting body.

Fig. 103. Fig. 104.

At this stage the streams seemingly pass with the greatest freedom through considerable
thicknesses of insulators, and it is particularly interesting to study their behavior. For this purpose
it is convenient to connect to the terminals of the coil two metallic spheres which may be placed at
any desired distance, Fig. 104. Spheres are preferable to plates, as the discharge can be better
observed. By inserting dielectric bodies between the spheres, beautiful discharge phenomena may
be observed. If the spheres be quite close and a spark be playing between them, by interposing a
thin plate of ebonite between the spheres the spark instantly ceases and the discharge spreads into
an intensely luminous circle several inches in diameter, provided the spheres are sufficiently large.
The passage of the streams heats, and, after a while, softens, the rubber so much that two plates
may be made to stick together in this manner. If the spheres are so far apart that no spark occurs,
even if they are far beyond the striking distance, by inserting a thick plate of glass the discharge is
instantly induced to pass from the spheres to the glass in the form of luminous streams. It appears
almost as though these streams pass through the dielectric. In reality this is not the case, as the
streams are due to the molecules of the air which are violently agitated in the space between the
oppositely charged surfaces of the spheres. When no dielectric other than air is present, the
bombardment goes on, but is too weak to be visible; by inserting a dielectric the inductive effect is
much increased, and besides, the projected air molecules find an obstacle and the bombardment
becomes so intense that the streams become luminous. If by any mechanical means we could
effect such a violent agitation of the molecules we could produce the same phenomenon. A jet of
air escaping through a small hole under enormous pressure and striking against an insulating
substance, such as glass, may be luminous in the dark, and it might be possible to produce a
phosphorescence of the glass or other insulators in this manner.
The greater the specific inductive capacity of the interposed dielectric, the more powerful the
effect produced. Owing to this, the streams show themselves with excessively high potentials even
if the glass be as much as one and one-half to two inches thick. But besides the heating due to
bombardment, some heating goes on undoubtedly in the dielectric, being apparently greater in
glass than in ebonite. I attribute this to the greater specific inductive capacity of the glass, in
consequence of which, with the same potential difference, a greater amount of energy is taken up
in it than in rubber. It is like connecting to a battery a copper and a brass wire of the same
dimensions. The copper wire, though a more perfect conductor, would heat more by reason of its

taking more current. Thus what is otherwise considered a virtue of the glass is here a defect. Glass
usually gives way much quicker than ebonite; when it is heated to a certain degree, the discharge
suddenly breaks through at one point, assuming then the ordinary form of an arc.
The heating effect produced by molecular bombardment of the dielectric would, of course,
diminish as the pressure of the air is increased, and at enormous pressure it would be negligible,
unless the frequency would increase correspondingly.
It will be often observed in these experiments that when the spheres are beyond the striking
distance, the approach of a glass plate, for instance, may induce the spark to jump between the
spheres. This occurs when the capacity of the spheres is somewhat below the critical value which
gives the greatest difference of potential at the terminals of the coil. By approaching a dielectric,
the specific inductive capacity of the space between the spheres is increased, producing the same
effect as if the capacity of the spheres were increased. The potential at the terminals may then rise
so high that the air space is cracked. The experiment is best performed with dense glass or mica.
Another interesting observation is that a plate of insulating material, when the discharge is
passing through it, is strongly attracted by either of the spheres, that is by the nearer one, this being
obviously due to the smaller mechanical effect of the bombardment on that side, and perhaps also
to the greater electrification.
From the behavior of the dielectrics in these experiments, we may conclude that the best
insulator for these rapidly alternating currents would be the one possessing the smallest specific
inductive capacity and at the same time one capable of withstanding the greatest differences of
potential; and thus two diametrically opposite ways of securing the required insulation are
indicated, namely, to use either a perfect vacuum or a gas under great pressure; but the former
would be preferable. Unfortunately neither of these two ways is easily carried out in practice.
It is especially interesting to note the behavior of an excessively high vacuum in these
experiments. If a test tube, provided with external electrodes and exhausted to the highest possible
degree, be connected to the terminals of the coil, Fig. 105, the electrodes of the tube are instantly
brought to a high temperature and the glass at each end of the tube is rendered intensely
phosphorescent, but the middle appears comparatively dark, and for a while remains cool.
When the frequency is so high that the discharge shown in Fig. 103 is observed, considerable
dissipation no doubt occurs in the coil. Nevertheless the coil may be worked for a long time, as the
heating is gradual.
In spite of the fact that the difference of potential may be enormous, little is felt when the
discharge is passed through the body, provided the hands are armed. This is to some extent due to
the higher frequency, but principally to the fact that less energy is available externally, when the
difference of potential reaches an enormous value, owing to the circumstance that, with the rise of
potential, the energy absorbed in the coil increases as the square of the potential. Up to a certain
point the energy available externally increases with the rise of potential, then it begins to fall off
rapidly. Thus, with the ordinary high tension induction coil, the curious paradox exists, that, while
with a given current through the primary the shock might be fatal, with many times that current it
might be perfectly harmless, even if the frequency be the same. With high frequencies and
excessively high potentials when the terminals are not connected to bodies of some size,
practically all the energy supplied to the primary is taken up by the coil. There is no breaking
through, no local injury, but all the material, insulating and conducting, is uniformly heated.

Fig. 105. Fig. 106.

To avoid misunderstanding in regard to the physiological effect of alternating currents of very
high frequency, I think it necessary to state that, while it is an undeniable fact that they are
incomparably less dangerous than currents of low frequencies, it should not be thought that they
are altogether harmless. What has just been said refers only to currents from an ordinary high
tension induction coil, which currents are necessarily very small; if received directly from a
machine or from a secondary of low resistance, they produce more or less powerful effects, and
may cause serious injury, especially when used in conjunction with condensers.
The streaming discharge of a high tension induction coil differs in many respects from that of a
powerful static machine. In color it has neither the violet of the positive, nor the brightness of the
negative, static discharge, but lies somewhere between, being, of course, alternatively positive and
negative. But since the streaming is more powerful when the point or terminal is electrified
positively, than when electrified negatively, it follows that the point of the brush is more like the
positive, and the root more like the negative, static discharge. In the dark, when the brush is very
powerful, the root may appear almost white. The wind produced by the escaping streams, though it
may be very strong—often indeed to such a degree that it may be felt quite a distance from the coil
—is, nevertheless, considering the quantity of the discharge, smaller than that produced by the
positive brush of a static machine, and it affects the flame much less powerfully. From the nature
of the phenomenon we can conclude that the higher the frequency, the smaller must, of course, be
the wind produced by the streams, and with sufficiently high frequencies no wind at all would be
produced at the ordinary atmospheric pressures. With frequencies obtainable by means of a
machine, the mechanical effect is sufficiently great to revolve, with considerable speed, large pin-
wheels, which in the dark present a beautiful appearance owing to the abundance of the streams
(Fig. 106).

Fig. 107. Fig. 108.

In general, most of the experiments usually performed with a static machine can be performed
with an induction coil when operated with very rapidly alternating currents. The effects produced,
however, are much more striking, being of incomparably greater power. When a small length of
ordinary cotton covered wire, Fig. 107, is attached to one terminal of the coil, the streams issuing
from all points of the wire may be so intense as to produce a considerable light effect. When the
potentials and frequencies are very high, a wire insulated with gutta percha or rubber and attached
to one of the terminals, appears to be covered with a luminous film. A very thin bare wire when
attached to a terminal emits powerful streams and vibrates continually to and fro or spins in a
circle, producing a singular effect (Fig. 108). Some of these experiments have been described by
me in The Electrical World, of February 21, 1891.
Another peculiarity of the rapidly alternating discharge of the induction coil is its radically
different behavior with respect to points and rounded surfaces.
If a thick wire, provided with a ball at one end and with a point at the other, be attached to the
positive terminal of a static machine, practically all the charge will be lost through the point, on
account of the enormously greater tension, dependent on the radius of curvature. But if such a wire
is attached to one of the terminals of the induction coil, it will be observed that with very high
frequencies streams issue from the ball almost as copiously as from the point (Fig. 109).
It is hardly conceivable that we could produce such a condition to an equal degree in a static
machine, for the simple reason, that the tension increases as the square of the density, which in
turn is proportional to the radius of curvature; hence, with a steady potential an enormous charge
would be required to make streams issue from a polished ball while it is connected with a point.
But with an induction coil the discharge of which alternates with great rapidity it is different. Here
we have to deal with two distinct tendencies. First, there is the tendency to escape which exists in a
condition of rest, and which depends on the radius of curvature; second, there is the tendency to
dissipate into the surrounding air by condenser action, which depends on the surface. When one of
these tendencies is a maximum, the other is at a minimum. At the point the luminous stream is
principally due to the air molecules coming bodily in contact with the point; they are attracted and
repelled, charged and discharged, and, their atomic charges being thus disturbed, vibrate and emit

light waves. At the ball, on the contrary, there is no doubt that the effect is to a great extent
produced inductively, the air molecules not necessarily coming in contact with the ball, though
they undoubtedly do so. To convince ourselves of this we only need to exalt the condenser action,
for instance, by enveloping the ball, at some distance, by a better conductor than the surrounding
medium, the conductor being, of course, insulated; or else by surrounding it with a better dielectric
and approaching an insulated conductor; in both cases the streams will break forth more copiously.
Also, the larger the ball with a given frequency, or the higher the frequency, the more will the ball
have the advantage over the point. But, since a certain intensity of action is required to render the
streams visible, it is obvious that in the experiment described the ball should not be taken too
large.
In consequence of this two-fold tendency, it is possible to produce by means of points, effects
identical to those produced by capacity. Thus, for instance, by attaching to one terminal of the coil
a small length of soiled wire, presenting many points and offering great facility to escape, the
potential of the coil may be raised to the same value as by attaching to the terminal a polished ball
of a surface many times greater than that of the wire.

Fig. 109. Fig. 110.

An interesting experiment, showing the effect of the points, may be performed in the following
manner: Attach to one of the terminals of the coil a cotton covered wire about two feet in length,
and adjust the conditions so that streams issue from the wire. In this experiment the primary coil
should be preferably placed so that it extends only about half way into the secondary coil. Now
touch the free terminal of the secondary with a conducting object held in the hand, or else connect
it to an insulated body of some size. In this manner the potential on the wire may be enormously
raised. The effect of this will be either to increase, or to diminish, the streams. If they increase, the
wire is too short; if they diminish, it is too long. By adjusting the length of the wire, a point is
found where the touching of the other terminal does not at all affect the streams. In this case the
rise of potential is exactly counteracted by the drop through the coil. It will be observed that small
lengths of wire produce considerable difference in the magnitude and luminosity of the streams.
The primary coil is placed sidewise for two reasons: First, to increase the potential at the wire; and,
second, to increase the drop through the coil. The sensitiveness is thus augmented.
There is still another and far more striking peculiarity of the brush discharge produced by very
rapidly alternating currents. To observe this it is best to replace the usual terminals of the coil by

two metal columns insulated with a good thickness of ebonite. It is also well to close all fissures
and cracks with wax so that the brushes cannot form anywhere except at the tops of the columns. If
the conditions are carefully adjusted—which, of course, must be left to the skill of the
experimenter—so that the potential rises to an enormous value, one may produce two powerful
brushes several inches long, nearly white at their roots, which in the dark bear a striking
resemblance to two flames of a gas escaping under pressure (Fig. 110). But they do not only
resemble, they are veritable flames, for they are hot. Certainly they are not as hot as a gas burner,
but they would be so if the frequency and the potential would be sufficiently high. Produced with,
say, twenty thousand alternations per second, the heat is easily perceptible even if the potential is
not excessively high. The heat developed is, of course, due to the impact of the air molecules
against the terminals and against each other. As, at the ordinary pressures, the mean free path is
excessively small, it is possible that in spite of the enormous initial speed imparted to each
molecule upon coming in contact with the terminal, its progress—by collision with other
molecules—is retarded to such an extent, that it does not get away far from the terminal, but may
strike the same many times in succession. The higher the frequency, the less the molecule is able to
get away, and this the more so, as for a given effect the potential required is smaller; and a
frequency is conceivable—perhaps even obtainable—at which practically the same molecules
would strike the terminal. Under such conditions the exchange of the molecules would be very
slow, and the heat produced at, and very near, the terminal would be excessive. But if the
frequency would go on increasing constantly, the heat produced would begin to diminish for
obvious reasons. In the positive brush of a static machine the exchange of the molecules is very
rapid, the stream is constantly of one direction, and there are fewer collisions; hence the heating
effect must be very small. Anything that impairs the facility of exchange tends to increase the local
heat produced. Thus, if a bulb be held over the terminal of the coil so as to enclose the brush, the
air contained in the bulb is very quickly brought to a high temperature. If a glass tube be held over
the brush so as to allow the draught to carry the brush upwards, scorching hot air escapes at the top
of the tube. Anything held within the brush is, of course, rapidly heated, and the possibility of
using such heating effects for some purpose or other suggests itself.
When contemplating this singular phenomenon of the hot brush, we cannot help being
convinced that a similar process must take place in the ordinary flame, and it seems strange that
after all these centuries past of familiarity with the flame, now, in this era of electric lighting and
heating, we are finally led to recognize, that since time immemorial we have, after all, always had
"electric light and heat" at our disposal. It is also of no little interest to contemplate, that we have a
possible way of producing—by other than chemical means—a veritable flame, which would give
light and heat without any material being consumed, without any chemical process taking place,
and to accomplish this, we only need to perfect methods of producing enormous frequencies and
potentials. I have no doubt that if the potential could be made to alternate with sufficient rapidity
and power, the brush formed at the end of a wire would lose its electrical characteristics and would
become flamelike. The flame must be due to electrostatic molecular action.
This phenomenon now explains in a manner which can hardly be doubted the frequent
accidents occurring in storms. It is well known that objects are often set on fire without the
lightning striking them. We shall presently see how this can happen. On a nail in a roof, for
instance, or on a projection of any kind, more or less conducting, or rendered so by dampness, a
powerful brush may appear. If the lightning strikes somewhere in the neighborhood the enormous
potential may be made to alternate or fluctuate perhaps many million times a second. The air
molecules are violently attracted and repelled, and by their impact produce such a powerful
heating effect that a fire is started. It is conceivable that a ship at sea may, in this manner, catch fire

at many points at once. When we consider, that even with the comparatively low frequencies
obtained from a dynamo machine, and with potentials of no more than one or two hundred
thousand volts, the heating effects are considerable, we may imagine how much more powerful
they must be with frequencies and potentials many times greater; and the above explanation seems,
to say the least, very probable. Similar explanations may have been suggested, but I am not aware
that, up to the present, the heating effects of a brush produced by a rapidly alternating potential
have been experimentally demonstrated, at least not to such a remarkable degree.

Fig. 111.
By preventing completely the exchange of the air molecules, the local heating effect may be so
exalted as to bring a body to incandescence. Thus, for instance, if a small button, or preferably a
very thin wire or filament be enclosed in an unexhausted globe and connected with the terminal of
the coil, it may be rendered incandescent. The phenomenon is made much more interesting by the
rapid spinning round in a circle of the top of the filament, thus presenting the appearance of a
luminous funnel, Fig. 111, which widens when the potential is increased. When the potential is
small the end of the filament may perform irregular motions, suddenly changing from one to the
other, or it may describe an ellipse; but when the potential is very high it always spins in a circle;
and so does generally a thin straight wire attached freely to the terminal of the coil. These motions
are, of course, due to the impact of the molecules, and the irregularity in the distribution of the
potential, owing to the roughness and dissymmetry of the wire or filament. With a perfectly
symmetrical and polished wire such motions would probably not occur. That the motion is not
likely to be due to others causes is evident from the fact that it is not of a definite direction, and
that in a very highly exhausted globe it ceases altogether. The possibility of bringing a body to
incandescence in an exhausted globe, or even when not at all enclosed, would seem to afford a
possible way of obtaining light effects, which, in perfecting methods of producing rapidly
alternating potentials, might be rendered available for useful purposes.

Fig. 112a.
In employing a commercial coil, the production of very powerful brush effects is attended with
considerable difficulties, for when these high frequencies and enormous potentials are used, the
best insulation is apt to give way. Usually the coil is insulated well enough to stand the strain from
convolution to convolution, since two double silk covered paraffined wires will withstand a
pressure of several thousand volts; the difficulty lies principally in preventing the breaking through
from the secondary to the primary, which is greatly facilitated by the streams issuing from the
latter. In the coil, of course, the strain is greatest from section to section, but usually in a larger coil
there are so many sections that the danger of a sudden giving way is not very great. No difficulty
will generally be encountered in that direction, and besides, the liability of injuring the coil
internally is very much reduced by the fact that the effect most likely to be produced is simply a
gradual heating, which, when far enough advanced, could not fail to be observed. The principal
necessity is then to prevent the streams between the primary and the tube, not only on account of
the heating and possible injury, but also because the streams may diminish very considerably the
potential difference available at the terminals. A few hints as to how this may be accomplished will
probably be found useful in most of these experiments with the ordinary induction coil.

Fig. 112b.
One of the ways is to wind a short primary, Fig. 112a, so that the difference of potential is not
at that length great enough to cause the breaking forth of the streams through the insulating tube.
The length of the primary should be determined by experiment. Both the ends of the coil should be
brought out on one end through a plug of insulating material fitting in the tube as illustrated. In
such a disposition one terminal of the secondary is attached to a body, the surface of which is
determined with the greatest care so as to produce the greatest rise in the potential. At the other
terminal a powerful brush appears, which may be experimented upon.
The above plan necessitates the employment of a primary of comparatively small size, and it is
apt to heat when powerful effects are desirable for a certain length of time. In such a case it is
better to employ a larger coil, Fig. 112b, and introduce it from one side of the tube, until the
streams begin to appear. In this case the nearest terminal of the secondary may be connected to the
primary or to the ground, which is practically the same thing, if the primary is connected directly
to the machine. In the case of ground connections it is well to determine experimentally the
frequency which is best suited under the conditions of the test. Another way of obviating the
streams, more or less, is to make the primary in sections and supply it from separate, well insulated
sources.
In many of these experiments, when powerful effects are wanted for a short time, it is
advantageous to use iron cores with the primaries. In such case a very large primary coil may be
wound and placed side by side with the secondary, and, the nearest terminal of the latter being
connected to the primary, a laminated iron core is introduced through the primary into the
secondary as far as the streams will permit. Under these conditions an excessively powerful brush,
several inches long, which may be appropriately called "St. Elmo's hot fire," may be caused to
appear at the other terminal of the secondary, producing striking effects. It is a most powerful
ozonizer, so powerful indeed, that only a few minutes are sufficient to fill the whole room with the
smell of ozone, and it undoubtedly possesses the quality of exciting chemical affinities.
For the production of ozone, alternating currents of very high frequency are eminently suited,
not only on account of the advantages they offer in the way of conversion but also because of the

fact, that the ozonizing action of a discharge is dependent on the frequency as well as on the
potential, this being undoubtedly confirmed by observation.
In these experiments if an iron core is used it should be carefully watched, as it is apt to get
excessively hot in an incredibly short time. To give an idea of the rapidity of the heating, I will
state, that by passing a powerful current through a coil with many turns, the inserting within the
same of a thin iron wire for no more than one second's time is sufficient to heat the wire to
something like 100° C.
But this rapid heating need not discourage us in the use of iron cores in connection with
rapidly alternating currents. I have for a long time been convinced that in the industrial distribution
by means of transformers, some such plan as the following might be practicable. We may use a
comparatively small iron core, subdivided, or perhaps not even subdivided. We may surround this
core with a considerable thickness of material which is fire-proof and conducts the heat poorly,
and on top of that we may place the primary and secondary windings. By using either higher
frequencies or greater magnetizing forces, we may by hysteresis and eddy currents heat the iron
core so far as to bring it nearly to its maximum permeability, which, as Hopkinson has shown, may
be as much as sixteen times greater than that at ordinary temperatures. If the iron core were
perfectly enclosed, it would not be deteriorated by the heat, and, if the enclosure of fire-proof
material would be sufficiently thick, only a limited amount of energy could be radiated in spite of
the high temperature. Transformers have been constructed by me on that plan, but for lack of time,
no thorough tests have as yet been made.
Another way of adapting the iron core to rapid alternations, or, generally speaking, reducing
the frictional losses, is to produce by continuous magnetization a flow of something like seven
thousand or eight thousand lines per square centimetre through the core, and then work with weak
magnetizing forces and preferably high frequencies around the point of greatest permeability. A
higher efficiency of conversion and greater output are obtainable in this manner. I have also
employed this principle in connection with machines in which there is no reversal of polarity. In
these types of machines, as long as there are only few pole projections, there is no great gain, as
the maxima and minima of magnetization are far from the point of maximum permeability; but
when the number of the pole projections is very great, the required rate of change may be obtained,
without the magnetization varying so far as to depart greatly from the point of maximum
permeability, and the gain is considerable.
The above described arrangements refer only to the use of commercial coils as ordinarily
constructed. If it is desired to construct a coil for the express purpose of performing with it such
experiments as I have described, or, generally, rendering it capable of withstanding the greatest
possible difference of potential, then a construction as indicated in Fig. 113 will be found of
advantage. The coil in this case is formed of two independent parts which are wound oppositely,
the connection between both being made near the primary. The potential in the middle being zero,
there is not much tendency to jump to the primary and not much insulation is required. In some
cases the middle point may, however, be connected to the primary or to the ground. In such a coil
the places of greatest difference of potential are far apart and the coil is capable of withstanding an
enormous strain. The two parts may be movable so as to allow a slight adjustment of the capacity
effect.
As to the manner of insulating the coil, it will be found convenient to proceed in the following
way: First, the wire should be boiled in paraffine until all the air is out; then the coil is wound by
running the wire through melted paraffine, merely for the purpose of fixing the wire. The coil is

then taken off from the spool, immersed in a cylindrical vessel filled with pure melted wax and
boiled for a long time until the bubbles cease to appear. The whole is then left to cool down
thoroughly, and then the mass is taken out of the vessel and turned up in a lathe. A coil made in
this manner and with care is capable of withstanding enormous potential differences.

Fig. 113.
It may be found convenient to immerse the coil in paraffine oil or some other kind of oil; it is a
most effective way of insulating, principally on account of the perfect exclusion of air, but it may
be found that, after all, a vessel filled with oil is not a very convenient thing to handle in a
laboratory.
If an ordinary coil can be dismounted, the primary may be taken out of the tube and the latter
plugged up at one end, filled with oil, and the primary reinserted. This affords an excellent
insulation and prevents the formation of the streams.
Of all the experiments which may be performed with rapidly alternating currents the most
interesting are those which concern the production of a practical illuminant. It cannot be denied
that the present methods, though they were brilliant advances, are very wasteful. Some better
methods must be invented, some more perfect apparatus devised. Modern research has opened new
possibilities for the production of an efficient source of light, and the attention of all has been
turned in the direction indicated by able pioneers. Many have been carried away by the enthusiasm
and passion to discover, but in their zeal to reach results, some have been misled. Starting with the
idea of producing electro-magnetic waves, they turned their attention, perhaps, too much to the
study of electro-magnetic effects, and neglected the study of electrostatic phenomena. Naturally,
nearly every investigator availed himself of an apparatus similar to that used in earlier
experiments. But in those forms of apparatus, while the electro-magnetic inductive effects are
enormous, the electrostatic effects are excessively small.

In the Hertz experiments, for instance, a high tension induction coil is short circuited by an arc,
the resistance of which is very small, the smaller, the more capacity is attached to the terminals;
and the difference of potential at these is enormously diminished. On the other hand, when the
discharge is not passing between the terminals, the static effects may be considerable, but only
qualitatively so, not quantitatively, since their rise and fall is very sudden, and since their
frequency is small. In neither case, therefore, are powerful electrostatic effects perceivable. Similar
conditions exist when, as in some interesting experiments of Dr. Lodge, Leyden jars are discharged
disruptively. It has been thought—and I believe asserted—that in such cases most of the energy is
radiated into space. In the light of the experiments which I have described above, it will now not
be thought so. I feel safe in asserting that in such cases most of the energy is partly taken up and
converted into heat in the arc of the discharge and in the conducting and insulating material of the
jar, some energy being, of course, given off by electrification of the air; but the amount of the
directly radiated energy is very small.
When a high tension induction coil, operated by currents alternating only 20,000 times a
second, has its terminals closed through even a very small jar, practically all the energy passes
through the dielectric of the jar, which is heated, and the electrostatic effects manifest themselves
outwardly only to a very weak degree. Now the external circuit of a Leyden jar, that is, the arc and
the connections of the coatings, may be looked upon as a circuit generating alternating currents of
excessively high frequency and fairly high potential, which is closed through the coatings and the
dielectric between them, and from the above it is evident that the external electrostatic effects must
be very small, even if a recoil circuit be used. These conditions make it appear that with the
apparatus usually at hand, the observation of powerful electrostatic effects was impossible, and
what experience has been gained in that direction is only due to the great ability of the
investigators.
But powerful electrostatic effects are a sine qua non of light production on the lines indicated
by theory. Electro-magnetic effects are primarily unavailable, for the reason that to produce the
required effects we would have to pass current impulses through a conductor, which, long before
the required frequency of the impulses could be reached, would cease to transmit them. On the
other hand, electro-magnetic waves many times longer than those of light, and producible by
sudden discharge of a condenser, could not be utilized, it would seem, except we avail ourselves of
their effect upon conductors as in the present methods, which are wasteful. We could not affect by
means of such waves the static molecular or atomic charges of a gas, cause them to vibrate and to
emit light. Long transverse waves cannot, apparently, produce such effects, since excessively small
electro-magnetic disturbances may pass readily through miles of air. Such dark waves, unless they
are of the length of true light waves, cannot, it would seem, excite luminous radiation in a Geissler
tube, and the luminous effects, which are producible by induction in a tube devoid of electrodes, I
am inclined to consider as being of an electrostatic nature.
To produce such luminous effects, straight electrostatic thrusts are required; these, whatever be
their frequency, may disturb the molecular charges and produce light. Since current impulses of the
required frequency cannot pass through a conductor of measurable dimensions, we must work with
a gas, and then the production of powerful electrostatic effects becomes an imperative necessity.
It has occurred to me, however, that electrostatic effects are in many ways available for the
production of light. For instance, we may place a body of some refractory material in a closed, and
preferably more or less exhausted, globe, connect it to a source of high, rapidly alternating
potential, causing the molecules of the gas to strike it many times a second at enormous speeds,

and in this manner, with trillions of invisible hammers, pound it until it gets incandescent; or we
may place a body in a very highly exhausted globe, in a non-striking vacuum, and, by employing
very high frequencies and potentials, transfer sufficient energy from it to other bodies in the
vicinity, or in general to the surroundings, to maintain it at any degree of incandescence; or we
may, by means of such rapidly alternating high potentials, disturb the ether carried by the
molecules of a gas or their static charges, causing them to vibrate and to emit light.
But, electrostatic effects being dependent upon the potential and frequency, to produce the
most powerful action it is desirable to increase both as far as practicable. It may be possible to
obtain quite fair results by keeping either of these factors small, provided the other is sufficiently
great; but we are limited in both directions. My experience demonstrates that we cannot go below
a certain frequency, for, first, the potential then becomes so great that it is dangerous; and,
secondly, the light production is less efficient.
I have found that, by using the ordinary low frequencies, the physiological effect of the current
required to maintain at a certain degree of brightness a tube four feet long, provided at the ends
with outside and inside condenser coatings, is so powerful that, I think, it might produce serious
injury to those not accustomed to such shocks; whereas, with twenty thousand alternations per
second, the tube may be maintained at the same degree of brightness without any effect being felt.
This is due principally to the fact that a much smaller potential is required to produce the same
light effect, and also to the higher efficiency in the light production. It is evident that the efficiency
in such cases is the greater, the higher the frequency, for the quicker the process of charging and
discharging the molecules, the less energy will be lost in the form of dark radiation. But,
unfortunately, we cannot go beyond a certain frequency on account of the difficulty of producing
and conveying the effects.
I have stated above that a body inclosed in an unexhausted bulb may be intensely heated by
simply connecting it with a source of rapidly alternating potential. The heating in such a case is, in
all probability, due mostly to the bombardment of the molecules of the gas contained in the bulb.
When the bulb is exhausted, the heating of the body is much more rapid, and there is no difficulty
whatever in bringing a wire or filament to any degree of incandescence by simply connecting it to
one terminal of a coil of the proper dimensions. Thus, if the well-known apparatus of Prof.
Crookes, consisting of a bent platinum wire with vanes mounted over it (Fig. 114), be connected to
one terminal of the coil—either one or both ends of the platinum wire being connected—the wire
is rendered almost instantly incandescent, and the mica vanes are rotated as though a current from
a battery were used. A thin carbon filament, or, preferably, a button of some refractory material
(Fig. 115), even if it be a comparatively poor conductor, inclosed in an exhausted globe, may be
rendered highly incandescent; and in this manner a simple lamp capable of giving any desired
candle power is provided.
The success of lamps of this kind would depend largely on the selection of the light-giving
bodies contained within the bulb. Since, under the conditions described, refractory bodies—which
are very poor conductors and capable of withstanding for a long time excessively high degrees of
temperature—may be used, such illuminating devices may be rendered successful.

Fig. 114. Fig. 115.

It might be thought at first that if the bulb, containing the filament or button of refractory
material, be perfectly well exhausted—that is, as far as it can be done by the use of the best
apparatus—the heating would be much less intense, and that in a perfect vacuum it could not occur
at all. This is not confirmed by my experience; quite the contrary, the better the vacuum the more
easily the bodies are brought to incandescence. This result is interesting for many reasons.
At the outset of this work the idea presented itself to me, whether two bodies of refractory
material enclosed in a bulb exhausted to such a degree that the discharge of a large induction coil,
operated in the usual manner, cannot pass through, could be rendered incandescent by mere
condenser action. Obviously, to reach this result enormous potential differences and very high
frequencies are required, as is evident from a simple calculation.
But such a lamp would possess a vast advantage over an ordinary incandescent lamp in regard
to efficiency. It is well-known that the efficiency of a lamp is to some extent a function of the
degree of incandescence, and that, could we but work a filament at many times higher degrees of
incandescence, the efficiency would be much greater. In an ordinary lamp this is impracticable on
account of the destruction of the filament, and it has been determined by experience how far it is
advisable to push the incandescence. It is impossible to tell how much higher efficiency could be
obtained if the filament could withstand indefinitely, as the investigation to this end obviously
cannot be carried beyond a certain stage; but there are reasons for believing that it would be very
considerably higher. An improvement might be made in the ordinary lamp by employing a short
and thick carbon; but then the leading-in wires would have to be thick, and, besides, there are
many other considerations which render such a modification entirely impracticable. But in a lamp
as above described, the leading in wires may be very small, the incandescent refractory material
may be in the shape of blocks offering a very small radiating surface, so that less energy would be
required to keep them at the desired incandescence; and in addition to this, the refractory material
need not be carbon, but may be manufactured from mixtures of oxides, for instance, with carbon or
other material, or may be selected from bodies which are practically non-conductors, and capable
of withstanding enormous degrees of temperature.
All this would point to the possibility of obtaining a much higher efficiency with such a lamp
than is obtainable in ordinary lamps. In my experience it has been demonstrated that the blocks are
brought to high degrees of incandescence with much lower potentials than those determined by

calculation, and the blocks may be set at greater distances from each other. We may freely assume,
and it is probable, that the molecular bombardment is an important element in the heating, even if
the globe be exhausted with the utmost care, as I have done; for although the number of the
molecules is, comparatively speaking, insignificant, yet on account of the mean free path being
very great, there are fewer collisions, and the molecules may reach much higher speeds, so that the
heating effect due to this cause may be considerable, as in the Crookes experiments with radiant
matter.
But it is likewise possible that we have to deal here with an increased facility of losing the
charge in very high vacuum, when the potential is rapidly alternating, in which case most of the
heating would be directly due to the surging of the charges in the heated bodies. Or else the
observed fact may be largely attributable to the effect of the points which I have mentioned above,
in consequence of which the blocks or filaments contained in the vacuum are equivalent to
condensers of many times greater surface than that calculated from their geometrical dimensions.
Scientific men still differ in opinion as to whether a charge should, or should not, be lost in a
perfect vacuum, or in other words, whether ether is, or is not, a conductor. If the former were the
case, then a thin filament enclosed in a perfectly exhausted globe, and connected to a source of
enormous, steady potential, would be brought to incandescence.

Fig. 116. Fig. 117.

Various forms of lamps on the above described principle, with the refractory bodies in the form
of filaments, Fig. 116, or blocks, Fig. 117, have been constructed and operated by me, and
investigations are being carried on in this line. There is no difficulty in reaching such high degrees
of incandescence that ordinary carbon is to all appearance melted and volatilized. If the vacuum
could be made absolutely perfect, such a lamp, although inoperative with apparatus ordinarily
used, would, if operated with currents of the required character, afford an illuminant which would
never be destroyed, and which would be far more efficient than an ordinary incandescent lamp.

This perfection can, of course, never be reached, and a very slow destruction and gradual
diminution in size always occurs, as in incandescent filaments; but there is no possibility of a
sudden and premature disabling which occurs in the latter by the breaking of the filament,
especially when the incandescent bodies are in the shape of blocks.
With these rapidly alternating potentials there is, however, no necessity of enclosing two
blocks in a globe, but a single block, as in Fig. 115, or filament, Fig. 118, may be used. The
potential in this case must of course be higher, but is easily obtainable, and besides it is not
necessarily dangerous.

Fig. 118.
The facility with which the button or filament in such a lamp is brought to incandescence,
other things being equal, depends on the size of the globe. If a perfect vacuum could be obtained,
the size of the globe would not be of importance, for then the heating would be wholly due to the
surging of the charges, and all the energy would be given off to the surroundings by radiation. But
this can never occur in practice. There is always some gas left in the globe, and although the
exhaustion may be carried to the highest degree, still the space inside of the bulb must be
considered as conducting when such high potentials are used, and I assume that, in estimating the
energy that may be given off from the filament to the surroundings, we may consider the inside
surface of the bulb as one coating of a condenser, the air and other objects surrounding the bulb
forming the other coating. When the alternations are very low there is no doubt that a considerable
portion of the energy is given off by the electrification of the surrounding air.
In order to study this subject better, I carried on some experiments with excessively high
potentials and low frequencies. I then observed that when the hand is approached to the bulb,—the
filament being connected with one terminal of the coil,—a powerful vibration is felt, being due to
the attraction and repulsion of the molecules of the air which are electrified by induction through
the glass. In some cases when the action is very intense I have been able to hear a sound, which
must be due to the same cause.

Fig. 119. Fig. 120.

When the alternations are low, one is apt to get an excessively powerful shock from the bulb.
In general, when one attaches bulbs or objects of some size to the terminals of the coil, one should
look out for the rise of potential, for it may happen that by merely connecting a bulb or plate to the
terminal, the potential may rise to many times its original value. When lamps are attached to the
terminals, as illustrated in Fig. 119, then the capacity of the bulbs should be such as to give the
maximum rise of potential under the existing conditions. In this manner one may obtain the
required potential with fewer turns of wire.
The life of such lamps as described above depends, of course, largely on the degree of
exhaustion, but to some extent also on the shape of the block of refractory material. Theoretically
it would seem that a small sphere of carbon enclosed in a sphere of glass would not suffer
deterioration from molecular bombardment, for, the matter in the globe being radiant, the
molecules would move in straight lines, and would seldom strike the sphere obliquely. An
interesting thought in connection with such a lamp is, that in it "electricity" and electrical energy
apparently must move in the same lines.

Fig. 121a. Fig. 121b.

The use of alternating currents of very high frequency makes it possible to transfer, by
electrostatic or electromagnetic induction through the glass of a lamp, sufficient energy to keep a
filament at incandescence and so do away with the leading-in wires. Such lamps have been
proposed, but for want of proper apparatus they have not been successfully operated. Many forms
of lamps on this principle with continuous and broken filaments have been constructed by me and
experimented upon. When using a secondary enclosed within the lamp, a condenser is
advantageously combined with the secondary. When the transference is effected by electrostatic
induction, the potentials used are, of course, very high with frequencies obtainable from a
machine. For instance, with a condenser surface of forty square centimetres, which is not
impracticably large, and with glass of good quality 1 mm. thick, using currents alternating twenty
thousand times a second, the potential required is approximately 9,000 volts. This may seem large,
but since each lamp may be included in the secondary of a transformer of very small dimensions, it
would not be inconvenient, and, moreover, it would not produce fatal injury. The transformers
would all be preferably in series. The regulation would offer no difficulties, as with currents of
such frequencies it is very easy to maintain a constant current.
In the accompanying engravings some of the types of lamps of this kind are shown. Fig. 120 is
such a lamp with a broken filament, and Figs. 121 A and 121 B one with a single outside and inside
coating and a single filament. I have also made lamps with two outside and inside coatings and a
continuous loop connecting the latter. Such lamps have been operated by me with current impulses
of the enormous frequencies obtainable by the disruptive discharge of condensers.

The disruptive discharge of a condenser is especially suited for operating such lamps—with no
outward electrical connections—by means of electromagnetic induction, the electromagnetic
inductive effects being excessively high; and I have been able to produce the desired
incandescence with only a few short turns of wire. Incandescence may also be produced in this
manner in a simple closed filament.
Leaving now out of consideration the practicability of such lamps, I would only say that they
possess a beautiful and desirable feature, namely, that they can be rendered, at will, more or less
brilliant simply by altering the relative position of the outside and inside condenser coatings, or
inducing and induced circuits.
When a lamp is lighted by connecting it to one terminal only of the source, this may be
facilitated by providing the globe with an outside condenser coating, which serves at the same time
as a reflector, and connecting this to an insulated body of some size. Lamps of this kind are
illustrated in Fig. 122 and Fig. 123. Fig. 124 shows the plan of connection. The brilliancy of the
lamp may, in this case, be regulated within wide limits by varying the size of the insulated metal
plate to which the coating is connected.
It is likewise practicable to light with one leading wire lamps such as illustrated in Fig. 116 and
Fig. 117, by connecting one terminal of the lamp to one terminal of the source, and the other to an
insulated body of the required size. In all cases the insulated body serves to give off the energy
into the surrounding space, and is equivalent to a return wire. Obviously, in the two last-named
cases, instead of connecting the wires to an insulated body, connections may be made to the
ground.
The experiments which will prove most suggestive and of most interest to the investigator are
probably those performed with exhausted tubes. As might be anticipated, a source of such rapidly
alternating potentials is capable of exciting the tubes at a considerable distance, and the light
effects produced are remarkable.

Fig. 122. Fig. 123.

During my investigations in this line I endeavored to excite tubes, devoid of any electrodes, by
electromagnetic induction, making the tube the secondary of the induction device, and passing
through the primary the discharges of a Leyden jar. These tubes were made of many shapes, and I
was able to obtain luminous effects which I then thought were due wholly to electromagnetic
induction. But on carefully investigating the phenomena I found that the effects produced were
more of an electrostatic nature. It may be attributed to this circumstance that this mode of exciting
tubes is very wasteful, namely, the primary circuit being closed, the potential, and consequently
the electrostatic inductive effect, is much diminished.
When an induction coil, operated as above described, is used, there is no doubt that the tubes
are excited by electrostatic induction, and that electromagnetic induction has little, if anything, to
do with the phenomena.

Fig. 124.
This is evident from many experiments. For instance, if a tube be taken in one hand, the
observer being near the coil, it is brilliantly lighted and remains so no matter in what position it is
held relatively to the observer's body. Were the action electromagnetic, the tube could not be
lighted when the observer's body is interposed between it and the coil, or at least its luminosity
should be considerably diminished. When the tube is held exactly over the centre of the coil—the
latter being wound in sections and the primary placed symmetrically to the secondary—it may
remain completely dark, whereas it is rendered intensely luminous by moving it slightly to the
right or left from the centre of the coil. It does not light because in the middle both halves of the
coil neutralize each other, and the electric potential is zero. If the action were electromagnetic, the
tube should light best in the plane through the centre of the coil, since the electromagnetic effect
there should be a maximum. When an arc is established between the terminals, the tubes and
lamps in the vicinity of the coil go out, but light up again when the arc is broken, on account of the
rise of potential. Yet the electromagnetic effect should be practically the same in both cases.
By placing a tube at some distance from the coil, and nearer to one terminal—preferably at a
point on the axis of the coil—one may light it by touching the remote terminal with an insulated
body of some size or with the hand, thereby raising the potential at that terminal nearer to the tube.
If the tube is shifted nearer to the coil so that it is lighted by the action of the nearer terminal, it
may be made to go out by holding, on an insulated support, the end of a wire connected to the
remote terminal, in the vicinity of the nearer terminal, by this means counteracting the action of
the latter upon the tube. These effects are evidently electrostatic. Likewise, when a tube is placed
at a considerable distance from the coil, the observer may, standing upon an insulated support
between coil and tube, light the latter by approaching the hand to it; or he may even render it
luminous by simply stepping between it and the coil. This would be impossible with electro-
magnetic induction, for the body of the observer would act as a screen.
When the coil is energized by excessively weak currents, the experimenter may, by touching
one terminal of the coil with the tube, extinguish the latter, and may again light it by bringing it out
of contact with the terminal and allowing a small arc to form. This is clearly due to the respective
lowering and raising of the potential at that terminal. In the above experiment, when the tube is
lighted through a small arc, it may go out when the arc is broken, because the electrostatic
inductive effect alone is too weak, though the potential may be much higher; but when the arc is
established, the electrification of the end of the tube is much greater, and it consequently lights.

If a tube is lighted by holding it near to the coil, and in the hand which is remote, by grasping
the tube anywhere with the other hand, the part between the hands is rendered dark, and the
singular effect of wiping out the light of the tube may be produced by passing the hand quickly
along the tube and at the same time withdrawing it gently from the coil, judging properly the
distance so that the tube remains dark afterwards.
If the primary coil is placed sidewise, as in Fig. 112 B for instance, and an exhausted tube be
introduced from the other side in the hollow space, the tube is lighted most intensely because of
the increased condenser action, and in this position the striæ are most sharply defined. In all these
experiments described, and in many others, the action is clearly electrostatic.
The effects of screening also indicate the electrostatic nature of the phenomena and show
something of the nature of electrification through the air. For instance, if a tube is placed in the
direction of the axis of the coil, and an insulated metal plate be interposed, the tube will generally
increase in brilliancy, or if it be too far from the coil to light, it may even be rendered luminous by
interposing an insulated metal plate. The magnitude of the effects depends to some extent on the
size of the plate. But if the metal plate be connected by a wire to the ground, its interposition will
always make the tube go out even if it be very near the coil. In general, the interposition of a body
between the coil and tube, increases or diminishes the brilliancy of the tube, or its facility to light
up, according to whether it increases or diminishes the electrification. When experimenting with
an insulated plate, the plate should not be taken too large, else it will generally produce a
weakening effect by reason of its great facility for giving off energy to the surroundings.
If a tube be lighted at some distance from the coil, and a plate of hard rubber or other
insulating substance be interposed, the tube may be made to go out. The interposition of the
dielectric in this case only slightly increases the inductive effect, but diminishes considerably the
electrification through the air.
In all cases, then, when we excite luminosity in exhausted tubes by means of such a coil, the
effect is due to the rapidly alternating electrostatic potential; and, furthermore, it must be attributed
to the harmonic alternation produced directly by the machine, and not to any superimposed
vibration which might be thought to exist. Such superimposed vibrations are impossible when we
work with an alternate current machine. If a spring be gradually tightened and released, it does not
perform independent vibrations; for this a sudden release is necessary. So with the alternate
currents from a dynamo machine; the medium is harmonically strained and released, this giving
rise to only one kind of waves; a sudden contact or break, or a sudden giving way of the dielectric,
as in the disruptive discharge of a Leyden jar, are essential for the production of superimposed
waves.
In all the last described experiments, tubes devoid of any electrodes may be used, and there is
no difficulty in producing by their means sufficient light to read by. The light effect is, however,
considerably increased by the use of phosphorescent bodies such as yttria, uranium glass, etc. A
difficulty will be found when the phosphorescent material is used, for with these powerful effects,
it is carried gradually away, and it is preferable to use material in the form of a solid.
Instead of depending on induction at a distance to light the tube, the same may be provided
with an external—and, if desired, also with an internal—condenser coating, and it may then be
suspended anywhere in the room from a conductor connected to one terminal of the coil, and in
this manner a soft illumination may be provided.

Fig. 125.
The ideal way of lighting a hall or room would, however, be to produce such a condition in it
that an illuminating device could be moved and put anywhere, and that it is lighted, no matter
where it is put and without being electrically connected to anything. I have been able to produce
such a condition by creating in the room a powerful, rapidly alternating electrostatic field. For this
purpose I suspend a sheet of metal a distance from the ceiling on insulating cords and connect it to
one terminal of the induction coil, the other terminal being preferably connected to the ground. Or
else I suspend two sheets as illustrated in Fig. 125, each sheet being connected with one of the
terminals of the coil, and their size being carefully determined. An exhausted tube may then be
carried in the hand anywhere between the sheets or placed anywhere, even a certain distance
beyond them; it remains always luminous.
In such an electrostatic field interesting phenomena may be observed, especially if the
alternations are kept low and the potentials excessively high. In addition to the luminous
phenomena mentioned, one may observe that any insulated conductor gives sparks when the hand
or another object is approached to it, and the sparks may often be powerful. When a large
conducting object is fastened on an insulating support, and the hand approached to it, a vibration,
due to the rythmical motion of the air molecules is felt, and luminous streams may be perceived
when the hand is held near a pointed projection. When a telephone receiver is made to touch with
one or both of its terminals an insulated conductor of some size, the telephone emits a loud sound;

it also emits a sound when a length of wire is attached to one or both terminals, and with very
powerful fields a sound may be perceived even without any wire.
How far this principle is capable of practical application, the future will tell. It might be
thought that electrostatic effects are unsuited for such action at a distance. Electromagnetic
inductive effects, if available for the production of light, might be thought better suited. It is true
the electrostatic effects diminish nearly with the cube of the distance from the coil, whereas the
electromagnetic inductive effects diminish simply with the distance. But when we establish an
electrostatic field of force, the condition is very different, for then, instead of the differential effect
of both the terminals, we get their conjoint effect. Besides, I would call attention to the effect, that
in an alternating electrostatic field, a conductor, such as an exhausted tube, for instance, tends to
take up most of the energy, whereas in an electromagnetic alternating field the conductor tends to
take up the least energy, the waves being reflected with but little loss. This is one reason why it is
difficult to excite an exhausted tube, at a distance, by electromagnetic induction. I have wound
coils of very large diameter and of many turns of wire, and connected a Geissler tube to the ends
of the coil with the object of exciting the tube at a distance; but even with the powerful inductive
effects producible by Leyden jar discharges, the tube could not be excited unless at a very small
distance, although some judgment was used as to the dimensions of the coil. I have also found that
even the most powerful Leyden jar discharges are capable of exciting only feeble luminous effects
in a closed exhausted tube, and even these effects upon thorough examination I have been forced
to consider of an electrostatic nature.
How then can we hope to produce the required effects at a distance by means of
electromagnetic action, when even in the closest proximity to the source of disturbance, under the
most advantageous conditions, we can excite but faint luminosity? It is true that when acting at a
distance we have the resonance to help us out. We can connect an exhausted tube, or whatever the
illuminating device may be, with an insulated system of the proper capacity, and so it may be
possible to increase the effect qualitatively, and only qualitatively, for we would not get more
energy through the device. So we may, by resonance effect, obtain the required electromotive force
in an exhausted tube, and excite faint luminous effects, but we cannot get enough energy to render
the light practically available, and a simple calculation, based on experimental results, shows that
even if all the energy which a tube would receive at a certain distance from the source should be
wholly converted into light, it would hardly satisfy the practical requirements. Hence the necessity
of directing, by means of a conducting circuit, the energy to the place of transformation. But in so
doing we cannot very sensibly depart from present methods, and all we could do would be to
improve the apparatus.
From these considerations it would seem that if this ideal way of lighting is to be rendered
practicable it will be only by the use of electrostatic effects. In such a case the most powerful
electrostatic inductive effects are needed; the apparatus employed must, therefore, be capable of
producing high electrostatic potentials changing in value with extreme rapidity. High frequencies
are especially wanted, for practical considerations make it desirable to keep down the potential. By
the employment of machines, or, generally speaking, of any mechanical apparatus, but low
frequencies can be reached; recourse must, therefore, be had to some other means. The discharge
of a condenser affords us a means of obtaining frequencies by far higher than are obtainable
mechanically, and I have accordingly employed condensers in the experiments to the above end.
When the terminals of a high tension induction coil, Fig. 126, are connected to a Leyden jar,
and the latter is discharging disruptively into a circuit, we may look upon the arc playing between

the knobs as being a source of alternating, or generally speaking, undulating currents, and then we
have to deal with the familiar system of a generator of such currents, a circuit connected to it, and
a condenser bridging the circuit. The condenser in such case is a veritable transformer, and since
the frequency is excessive, almost any ratio in the strength of the currents in both the branches
may be obtained. In reality the analogy is not quite complete, for in the disruptive discharge we
have most generally a fundamental instantaneous variation of comparatively low frequency, and a
superimposed harmonic vibration, and the laws governing the flow of currents are not the same for
both.
In converting in this manner, the ratio of conversion should not be too great, for the loss in the
arc between the knobs increases with the square of the current, and if the jar be discharged through
very thick and short conductors, with the view of obtaining a very rapid oscillation, a very
considerable portion of the energy stored is lost. On the other hand, too small ratios are not
practicable for many obvious reasons.
As the converted currents flow in a practically closed circuit, the electrostatic effects are
necessarily small, and I therefore convert them into currents or effects of the required character. I
have effected such conversions in several ways. The preferred plan of connections is illustrated in
Fig. 127. The manner of operating renders it easy to obtain by means of a small and inexpensive
apparatus enormous differences of potential which have been usually obtained by means of large
and expensive coils. For this it is only necessary to take an ordinary small coil, adjust to it a
condenser and discharging circuit, forming the primary of an auxiliary small coil, and convert
upward. As the inductive effect of the primary currents is excessively great, the second coil need
have comparatively but very few turns. By properly adjusting the elements, remarkable results
may be secured.
In endeavoring to obtain the required electrostatic effects in this manner, I have, as might be
expected, encountered many difficulties which I have been gradually overcoming, but I am not as
yet prepared to dwell upon my experiences in this direction.
I believe that the disruptive discharge of a condenser will play an important part in the future,
for it offers vast possibilities, not only in the way of producing light in a more efficient manner and
in the line indicated by theory, but also in many other respects.

Fig. 126.
For years the efforts of inventors have been directed towards obtaining electrical energy from
heat by means of the thermopile. It might seem invidious to remark that but few know what is the
real trouble with the thermopile. It is not the inefficiency or small output—though these are great
drawbacks—but the fact that the thermopile has its phylloxera, that is, that by constant use it is

deteriorated, which has thus far prevented its introduction on an industrial scale. Now that all
modern research seems to point with certainty to the use of electricity of excessively high tension,
the question must present itself to many whether it is not possible to obtain in a practicable manner
this form of energy from heat. We have been used to look upon an electrostatic machine as a
plaything, and somehow we couple with it the idea of the inefficient and impractical. But now we
must think differently, for now we know that everywhere we have to deal with the same forces,
and that it is a mere question of inventing proper methods or apparatus for rendering them
available.
In the present systems of electrical distribution, the employment of the iron with its wonderful
magnetic properties allows us to reduce considerably the size of the apparatus; but, in spite of this,
it is still very cumbersome. The more we progress in the study of electric and magnetic
phenomena, the more we become convinced that the present methods will be short-lived. For the
production of light, at least, such heavy machinery would seem to be unnecessary. The energy
required is very small, and if light can be obtained as efficiently as, theoretically, it appears
possible, the apparatus need have but a very small output. There being a strong probability that the
illuminating methods of the future will involve the use of very high potentials, it seems very
desirable to perfect a contrivance capable of converting the energy of heat into energy of the
requisite form. Nothing to speak of has been done towards this end, for the thought that electricity
of some 50,000 or 100,000 volts pressure or more, even if obtained, would be unavailable for
practical purposes, has deterred inventors from working in this direction.

Fig. 127.
In Fig. 126 a plan of connections is shown for converting currents of high, into currents of low,
tension by means of the disruptive discharge of a condenser. This plan has been used by me
frequently for operating a few incandescent lamps required in the laboratory. Some difficulties
have been encountered in the arc of the discharge which I have been able to overcome to a great
extent; besides this, and the adjustment necessary for the proper working, no other difficulties have
been met with, and it was easy to operate ordinary lamps, and even motors, in this manner. The
line being connected to the ground, all the wires could be handled with perfect impunity, no matter
how high the potential at the terminals of the condenser. In these experiments a high tension
induction coil, operated from a battery or from an alternate current machine, was employed to
charge the condenser; but the induction coil might be replaced by an apparatus of a different kind,
capable of giving electricity of such high tension. In this manner, direct or alternating currents may
be converted, and in both cases the current-impulses may be of any desired frequency. When the
currents charging the condenser are of the same direction, and it is desired that the converted
currents should also be of one direction, the resistance of the discharging circuit should, of course,
be so chosen that there are no oscillations.

Fig. 128.
In operating devices on the above plan I have observed curious phenomena of impedance
which are of interest. For instance if a thick copper bar be bent, as indicated in Fig. 128, and
shunted by ordinary incandescent lamps, then, by passing the discharge between the knobs, the
lamps may be brought to incandescence although they are short-circuited. When a large induction
coil is employed it is easy to obtain nodes on the bar, which are rendered evident by the different
degree of brilliancy of the lamps, as shown roughly in Fig. 128. The nodes are never clearly
defined, but they are simply maxima and minima of potentials along the bar. This is probably due
to the irregularity of the arc between the knobs. In general when the above-described plan of
conversion from high to low tension is used, the behavior of the disruptive discharge may be
closely studied. The nodes may also be investigated by means of an ordinary Cardew voltmeter
which should be well insulated. Geissler tubes may also be lighted across the points of the bent
bar; in this case, of course, it is better to employ smaller capacities. I have found it practicable to
light up in this manner a lamp, and even a Geissler tube, shunted by a short, heavy block of metal,
and this result seems at first very curious. In fact, the thicker the copper bar in Fig. 128, the better
it is for the success of the experiments, as they appear more striking. When lamps with long
slender filaments are used it will be often noted that the filaments are from time to time violently
vibrated, the vibration being smallest at the nodal points. This vibration seems to be due to an
electrostatic action between the filament and the glass of the bulb.

Fig. 129.
In some of the above experiments it is preferable to use special lamps having a straight
filament as shown in Fig. 129. When such a lamp is used a still more curious phenomenon than
those described may be observed. The lamp may be placed across the copper bar and lighted, and
by using somewhat larger capacities, or, in other words, smaller frequencies or smaller impulsive
impedances, the filament may be brought to any desired degree of incandescence. But when the
impedance is increased, a point is reached when comparatively little current passes through the
carbon, and most of it through the rarefied gas; or perhaps it may be more correct to state that the
current divides nearly evenly through both, in spite of the enormous difference in the resistance,
and this would be true unless the gas and the filament behave differently. It is then noted that the
whole bulb is brilliantly illuminated, and the ends of the leading-in wires become incandescent and
often throw off sparks in consequence of the violent bombardment, but the carbon filament
remains dark. This is illustrated in Fig. 129. Instead of the filament a single wire extending through
the whole bulb may be used, and in this case the phenomenon would seem to be still more
interesting.
From the above experiment it will be evident, that when ordinary lamps are operated by the
converted currents, those should be preferably taken in which the platinum wires are far apart, and
the frequencies used should not be too great, else the discharge will occur at the ends of the
filament or in the base of the lamp between the leading-in wires, and the lamp might then be
damaged.
In presenting to you these results of my investigation on the subject under consideration, I have
paid only a passing notice to facts upon which I could have dwelt at length, and among many
observations I have selected only those which I thought most likely to interest you. The field is
wide and completely unexplored, and at every step a new truth is gleaned, a novel fact observed.
How far the results here borne out are capable of practical applications will be decided in the
future. As regards the production of light, some results already reached are encouraging and make

me confident in asserting that the practical solution of the problem lies in the direction I have
endeavored to indicate. Still, whatever may be the immediate outcome of these experiments I am
hopeful that they will only prove a step in further development towards the ideal and final
perfection. The possibilities which are opened by modern research are so vast that even the most
reserved must feel sanguine of the future. Eminent scientists consider the problem of utilizing one
kind of radiation without the others a rational one. In an apparatus designed for the production of
light by conversion from any form of energy into that of light, such a result can never be reached,
for no matter what the process of producing the required vibrations, be it electrical, chemical or
any other, it will not be possible to obtain the higher light vibrations without going through the
lower heat vibrations. It is the problem of imparting to a body a certain velocity without passing
through all lower velocities. But there is a possibility of obtaining energy not only in the form of
light, but motive power, and energy of any other form, in some more direct way from the medium.
The time will be when this will be accomplished, and the time has come when one may utter such
words before an enlightened audience without being considered a visionary. We are whirling
through endless space with an inconceivable speed, all around us everything is spinning,
everything is moving, everywhere is energy. There must be some way of availing ourselves of this
energy more directly. Then, with the light obtained from the medium, with the power derived from
it, with every form of energy obtained without effort, from the store forever inexhaustible,
humanity will advance with giant strides. The mere contemplation of these magnificent
possibilities expands our minds, strengthens our hopes and fills our hearts with supreme delight.

CHAPTER XXVII.
Experiments with Alternate Currents of High
Potential and High Frequency.[2]

I cannot find words to express how deeply I feel the honor of addressing some of the foremost
thinkers of the present time, and so many able scientific men, engineers and electricians, of the
country greatest in scientific achievements.
The results which I have the honor to present before such a gathering I cannot call my own.
There are among you not a few who can lay better claim than myself on any feature of merit which
this work may contain. I need not mention many names which are world-known—names of those
among you who are recognized as the leaders in this enchanting science; but one, at least, I must
mention—a name which could not be omitted in a demonstration of this kind. It is a name
associated with the most beautiful invention ever made: it is Crookes!
When I was at college, a good while ago, I read, in a translation (for then I was not familiar
with your magnificent language), the description of his experiments on radiant matter. I read it
only once in my life—that time—yet every detail about that charming work I can remember to this
day. Few are the books, let me say, which can make such an impression upon the mind of a
student.
But if, on the present occasion, I mention this name as one of many your Institution can boast
of, it is because I have more than one reason to do so. For what I have to tell you and to show you
this evening concerns, in a large measure, that same vague world which Professor Crookes has so
ably explored; and, more than this, when I trace back the mental process which led me to these
advances—which even by myself cannot be considered trifling, since they are so appreciated by
you—I believe that their real origin, that which started me to work in this direction, and brought
me to them, after a long period of constant thought, was that fascinating little book which I read
many years ago.
And now that I have made a feeble effort to express my homage and acknowledge my
indebtedness to him and others among you, I will make a second effort, which I hope you will not
find so feeble as the first, to entertain you.
Give me leave to introduce the subject in a few words.
A short time ago I had the honor to bring before our American Institute of Electrical Engineers
some results then arrived at by me in a novel line of work. I need not assure you that the many
evidences which I have received that English scientific men and engineers were interested in this
work have been for me a great reward and encouragement. I will not dwell upon the experiments
already described, except with the view of completing, or more clearly expressing, some ideas
advanced by me before, and also with the view of rendering the study here presented self-
contained, and my remarks on the subject of this evening's lecture consistent.
This investigation, then, it goes without saying, deals with alternating currents, and to be more
precise, with alternating currents of high potential and high frequency. Just in how much a very

high frequency is essential for the production of the results presented is a question which, even
with my present experience, would embarrass me to answer. Some of the experiments may be
performed with low frequencies; but very high frequencies are desirable, not only on account of
the many effects secured by their use, but also as a convenient means of obtaining, in the induction
apparatus employed, the high potentials, which in their turn are necessary to the demonstration of
most of the experiments here contemplated.
Of the various branches of electrical investigation, perhaps the most interesting and the most
immediately promising is that dealing with alternating currents. The progress in this branch of
applied science has been so great in recent years that it justifies the most sanguine hopes. Hardly
have we become familiar with one fact, when novel experiences are met and new avenues of
research are opened. Even at this hour possibilities not dreamed of before are, by the use of these
currents, partly realized. As in nature all is ebb and tide, all is wave motion, so it seems that in all
branches of industry alternating currents—electric wave motion—will have the sway.
One reason, perhaps, why this branch of science is being so rapidly developed is to be found in
the interest which is attached to its experimental study. We wind a simple ring of iron with coils;
we establish the connections to the generator, and with wonder and delight we note the effects of
strange forces which we bring into play, which allow us to transform, to transmit and direct energy
at will. We arrange the circuits properly, and we see the mass of iron and wires behave as though it
were endowed with life, spinning a heavy armature, through invisible connections, with great
speed and power—with the energy possibly conveyed from a great distance. We observe how the
energy of an alternating current traversing the wire manifests itself—not so much in the wire as in
the surrounding space—in the most surprising manner, taking the forms of heat, light, mechanical
energy, and, most surprising of all, even chemical affinity. All these observations fascinate us, and
fill us with an intense desire to know more about the nature of these phenomena. Each day we go
to our work in the hope of discovering,—in the hope that some one, no matter who, may find a
solution of one of the pending great problems,—and each succeeding day we return to our task
with renewed ardor; and even if we are unsuccessful, our work has not been in vain, for in these
strivings, in these efforts, we have found hours of untold pleasure, and we have directed our
energies to the benefit of mankind.
We may take—at random, if you choose—any of the many experiments which may be
performed with alternating currents; a few of which only, and by no means the most striking, form
the subject of this evening's demonstration; they are all equally interesting, equally inciting to
thought.
Here is a simple glass tube from which the air has been partially exhausted. I take hold of it; I
bring my body in contact with a wire conveying alternating currents of high potential, and the tube
in my hand is brilliantly lighted. In whatever position I may put it, wherever I move it in space, as
far as I can reach, its soft, pleasing light persists with undiminished brightness.
Here is an exhausted bulb suspended from a single wire. Standing on an insulated support, I
grasp it, and a platinum button mounted in it is brought to vivid incandescence.
Here, attached to a leading wire, is another bulb, which, as I touch its metallic socket, is filled
with magnificent colors of phosphorescent light.
Here still another, which by my fingers' touch casts a shadow—the Crookes shadow—of the
stem inside of it.

Here, again, insulated as I stand on this platform, I bring my body in contact with one of the
terminals of the secondary of this induction coil—with the end of a wire many miles long—and
you see streams of light break forth from its distant end, which is set in violent vibration.
Here, once more, I attach these two plates of wire gauze to the terminals of the coil; I set them
a distance apart, and I set the coil to work. You may see a small spark pass between the plates. I
insert a thick plate of one of the best dielectrics between them, and instead of rendering altogether
impossible, as we are used to expect, I aid the passage of the discharge, which, as I insert the plate,
merely changes in appearance and assumes the form of luminous streams.
Is there, I ask, can there be, a more interesting study than that of alternating currents?
In all these investigations, in all these experiments, which are so very, very interesting, for
many years past—ever since the greatest experimenter who lectured in this hall discovered its
principle—we have had a steady companion, an appliance familiar to every one, a plaything once,
a thing of momentous importance now—the induction coil. There is no dearer appliance to the
electrician. From the ablest among you, I dare say, down to the inexperienced student, to your
lecturer, we all have passed many delightful hours in experimenting with the induction coil. We
have watched its play, and thought and pondered over the beautiful phenomena which it disclosed
to our ravished eyes. So well known is this apparatus, so familiar are these phenomena to every
one, that my courage nearly fails me when I think that I have ventured to address so able an
audience, that I have ventured to entertain you with that same old subject. Here in reality is the
same apparatus, and here are the same phenomena, only the apparatus is operated somewhat
differently, the phenomena are presented in a different aspect. Some of the results we find as
expected, others surprise us, but all captivate our attention, for in scientific investigation each
novel result achieved may be the centre of a new departure, each novel fact learned may lead to
important developments.
Usually in operating an induction coil we have set up a vibration of moderate frequency in the
primary, either by means of an interrupter or break, or by the use of an alternator. Earlier English
investigators, to mention only Spottiswoode and J. E. H. Gordon, have used a rapid break in
connection with the coil. Our knowledge and experience of to-day enables us to see clearly why
these coils under the conditions of the test did not disclose any remarkable phenomena, and why
able experimenters failed to perceive many of the curious effects which have since been observed.
In the experiments such as performed this evening, we operate the coil either from a specially
constructed alternator capable of giving many thousands of reversals of current per second, or, by
disruptively discharging a condenser through the primary, we set up a vibration in the secondary
circuit of a frequency of many hundred thousand or millions per second, if we so desire; and in
using either of these means we enter a field as yet unexplored.
It is impossible to pursue an investigation in any novel line without finally making some
interesting observation or learning some useful fact. That this statement is applicable to the subject
of this lecture the many curious and unexpected phenomena which we observe afford a convincing
proof. By way of illustration, take for instance the most obvious phenomena, those of the
discharge of the induction coil.
Here is a coil which is operated by currents vibrating with extreme rapidity, obtained by
disruptively discharging a Leyden jar. It would not surprise a student were the lecturer to say that
the secondary of this coil consists of a small length of comparatively stout wire; it would not
surprise him were the lecturer to state that, in spite of this, the coil is capable of giving any

potential which the best insulation of the turns is able to withstand; but although he may be
prepared, and even be indifferent as to the anticipated result, yet the aspect of the discharge of the
coil will surprise and interest him. Every one is familiar with the discharge of an ordinary coil; it
need not be reproduced here. But, by way of contrast, here is a form of discharge of a coil, the
primary current of which is vibrating several hundred thousand times per second. The discharge of
an ordinary coil appears as a simple line or band of light. The discharge of this coil appears in the
form of powerful brushes and luminous streams issuing from all points of the two straight wires
attached to the terminals of the secondary. (Fig. 130.)

Fig. 130. Fig. 131.

Now compare this phenomenon which you have just witnessed with the discharge of a Holtz or
Wimshurst machine—that other interesting appliance so dear to the experimenter. What a
difference there is between these phenomena! And yet, had I made the necessary arrangements—
which could have been made easily, were it not that they would interfere with other experiments—
I could have produced with this coil sparks which, had I the coil hidden from your view and only
two knobs exposed, even the keenest observer among you would find it difficult, if not impossible,
to distinguish from those of an influence or friction machine. This may be done in many ways—
for instance, by operating the induction coil which charges the condenser from an alternating-
current machine of very low frequency, and preferably adjusting the discharge circuit so that there
are no oscillations set up in it. We then obtain in the secondary circuit, if the knobs are of the

required size and properly set, a more or less rapid succession of sparks of great intensity and
small quantity, which possess the same brilliancy, and are accompanied by the same sharp
crackling sound, as those obtained from a friction or influence machine.
Another way is to pass through two primary circuits, having a common secondary, two
currents of a slightly different period, which produce in the secondary circuit sparks occurring at
comparatively long intervals. But, even with the means at hand this evening, I may succeed in
imitating the spark of a Holtz machine. For this purpose I establish between the terminals of the
coil which charges the condenser a long, unsteady arc, which is periodically interrupted by the
upward current of air produced by it. To increase the current of air I place on each side of the arc,
and close to it, a large plate of mica. The condenser charged from this coil discharges into the
primary circuit of a second coil through a small air gap, which is necessary to produce a sudden
rush of current through the primary. The scheme of connections in the present experiment is
indicated in Fig. 131.
G is an ordinarily constructed alternator, supplying the primary P of an induction coil, the
secondary S of which charges the condensers or jars C C. The terminals of the secondary are
connected to the inside coatings of the jars, the outer coatings being connected to the ends of the
primary p p of a second induction coil. This primary p p has a small air gap a b.
The secondary s of this coil is provided with knobs or spheres K K of the proper size and set at
a distance suitable for the experiment.
A long arc is established between the terminals A B of the first induction coil. M M are the mica
plates.
Each time the arc is broken between A and B the jars are quickly charged and discharged
through the primary p p, producing a snapping spark between the knobs K K. Upon the arc forming
between A and B the potential falls, and the jars cannot be charged to such high potential as to
break through the air gap a b until the arc is again broken by the draught.
In this manner sudden impulses, at long intervals, are produced in the primary p p, which in the
secondary s give a corresponding number of impulses of great intensity. If the secondary knobs or
spheres, K K, are of the proper size, the sparks show much resemblance to those of a Holtz
machine.
But these two effects, which to the eye appear so very different, are only two of the many
discharge phenomena. We only need to change the conditions of the test, and again we make other
observations of interest.
When, instead of operating the induction coil as in the last two experiments, we operate it from
a high frequency alternator, as in the next experiment, a systematic study of the phenomena is
rendered much more easy. In such case, in varying the strength and frequency of the currents
through the primary, we may observe five distinct forms of discharge, which I have described in
my former paper on the subject before the American Institute of Electrical Engineers, May 20,
1891.
It would take too much time, and it would lead us too far from the subject presented this
evening, to reproduce all these forms, but it seems to me desirable to show you one of them. It is a
brush discharge, which is interesting in more than one respect. Viewed from a near position it
resembles much a jet of gas escaping under great pressure. We know that the phenomenon is due

to the agitation of the molecules near the terminal, and we anticipate that some heat must be
developed by the impact of the molecules against the terminal or against each other. Indeed, we
find that the brush is hot, and only a little thought leads us to the conclusion that, could we but
reach sufficiently high frequencies, we could produce a brush which would give intense light and
heat, and which would resemble in every particular an ordinary flame, save, perhaps, that both
phenomena might not be due to the same agent—save, perhaps, that chemical affinity might not be
electrical in its nature.
As the production of heat and light is here due to the impact of the molecules, or atoms of air,
or something else besides, and, as we can augment the energy simply by raising the potential, we
might, even with frequencies obtained from a dynamo machine, intensify the action to such a
degree as to bring the terminal to melting heat. But with such low frequencies we would have to
deal always with something of the nature of an electric current. If I approach a conducting object
to the brush, a thin little spark passes, yet, even with the frequencies used this evening, the
tendency to spark is not very great. So, for instance, if I hold a metallic sphere at some distance
above the terminal, you may see the whole space between the terminal and sphere illuminated by
the streams without the spark passing; and with the much higher frequencies obtainable by the
disruptive discharge of a condenser, were it not for the sudden impulses, which are comparatively
few in number, sparking would not occur even at very small distances. However, with
incomparably higher frequencies, which we may yet find means to produce efficiently, and
provided that electric impulses of such high frequencies could be transmitted through a conductor,
the electrical characteristics of the brush discharge would completely vanish—no spark would
pass, no shock would be felt—yet we would still have to deal with an electric phenomenon, but in
the broad, modern interpretation of the word. In my first paper, before referred to, I have pointed
out the curious properties of the brush, and described the best manner of producing it, but I have
thought it worth while to endeavor to express myself more clearly in regard to this phenomenon,
because of its absorbing interest.
When a coil is operated with currents of very high frequency, beautiful brush effects may be
produced, even if the coil be of comparatively small dimensions. The experimenter may vary them
in many ways, and, if it were for nothing else, they afford a pleasing sight. What adds to their
interest is that they may be produced with one single terminal as well as with two—in fact, often
better with one than with two.
But of all the discharge phenomena observed, the most pleasing to the eye, and the most
instructive, are those observed with a coil which is operated by means of the disruptive discharge
of a condenser. The power of the brushes, the abundance of the sparks, when the conditions are
patiently adjusted, is often amazing. With even a very small coil, if it be so well insulated as to
stand a difference of potential of several thousand volts per turn, the sparks may be so abundant
that the whole coil may appear a complete mass of fire.
Curiously enough the sparks, when the terminals of the coil are set at a considerable distance,
seem to dart in every possible direction as though the terminals were perfectly independent of each
other. As the sparks would soon destroy the insulation, it is necessary to prevent them. This is best
done by immersing the coil in a good liquid insulator, such as boiled-out oil. Immersion in a liquid
may be considered almost an absolute necessity for the continued and successful working of such a
coil.
It is, of course, out of the question, in an experimental lecture, with only a few minutes at
disposal for the performance of each experiment, to show these discharge phenomena to

advantage, as, to produce each phenomenon at its best, a very careful adjustment is required. But
even if imperfectly produced, as they are likely to be this evening, they are sufficiently striking to
interest an intelligent audience.
Before showing some of these curious effects I must, for the sake of completeness, give a short
description of the coil and other apparatus used in the experiments with the disruptive discharge
this evening.

Fig. 132.
It is contained in a box B (Fig. 132) of thick boards of hard wood, covered on the outside with
a zinc sheet Z, which is carefully soldered all around. It might be advisable, in a strictly scientific
investigation, when accuracy is of great importance, to do away with the metal cover, as it might
introduce many errors, principally on account of its complex action upon the coil, as a condenser
of very small capacity and as an electrostatic and electromagnetic screen. When the coil is used for
such experiments as are here contemplated, the employment of the metal cover offers some
practical advantages, but these are not of sufficient importance to be dwelt upon.
The coil should be placed symmetrically to the metal cover, and the space between should, of
course, not be too small, certainly not less than, say, five centimetres, but much more if possible;

especially the two sides of the zinc box, which are at right angles to the axis of the coil, should be
sufficiently remote from the latter, as otherwise they might impair its action and be a source of
loss.
The coil consists of two spools of hard rubber R R, held apart at a distance of 10 centimetres by
bolts C and nuts n, likewise of hard rubber. Each spool comprises a tube T of approximately 8
centimetres inside diameter, and 3 millimetres thick, upon which are screwed two flanges F F, 24
centimetres square, the space between the flanges being about 3 centimetres. The secondary, S S, of
the best gutta percha-covered wire, has 26 layers, 10 turns in each, giving for each half a total of
260 turns. The two halves are wound oppositely and connected in series, the connection between
both being made over the primary. This disposition, besides being convenient, has the advantage
that when the coil is well balanced—that is, when both of its terminals T1, T1, are connected to
bodies or devices of equal capacity—there is not much danger of breaking through to the primary,
and the insulation between the primary and the secondary need not be thick. In using the coil it is
advisable to attach to both terminals devices of nearly equal capacity, as, when the capacity of the
terminals is not equal, sparks will be apt to pass to the primary. To avoid this, the middle point of
the secondary may be connected to the primary, but this is not always practicable.
The primary P P is wound in two parts, and oppositely, upon a wooden spool w, and the four
ends are led out of the oil through hard rubber tubes t t. The ends of the secondary T1 T1, are also
led out of the oil through rubber tubes t1 t1 of great thickness. The primary and secondary layers
are insulated by cotton cloth, the thickness of the insulation, of course, bearing some proportion to
the difference of potential between the turns of the different layers. Each half of the primary has
four layers, 24 turns in each, this giving a total of 96 turns. When both the parts are connected in
series, this gives a ratio of conversion of about 1:2.7, and with the primaries in multiple, 1:5.4; but
in operating with very rapidly alternating currents this ratio does not convey even an approximate
idea of the ratio of the e. m. f's. in the primary and secondary circuits. The coil is held in position in
the oil on wooden supports, there being about 5 centimetres thickness of oil all round. Where the
oil is not specially needed, the space is filled with pieces of wood, and for this purpose principally
the wooden box B surrounding the whole is used.
The construction here shown is, of course, not the best on general principles, but I believe it is
a good and convenient one for the production of effects in which an excessive potential and a very
small current are needed.
In connection with the coil I use either the ordinary form of discharger or a modified form. In
the former I have introduced two changes which secure some advantages, and which are obvious.
If they are mentioned, it is only in the hope that some experimenter may find them of use.
One of the changes is that the adjustable knobs A and B (Fig. 133), of the discharger are held in
jaws of brass, J J, by spring pressure, this allowing of turning them successively into different
positions, and so doing away with the tedious process of frequent polishing up.

Fig. 133.
The other change consists in the employment of a strong electromagnet N S, which is placed
with its axis at right angles to the line joining the knobs A and B, and produces a strong magnetic
field between them. The pole pieces of the magnet are movable and properly formed so as to
protrude between the brass knobs, in order to make the field as intense as possible; but to prevent
the discharge from jumping to the magnet the pole pieces are protected by a layer of mica, M M, of
sufficient thickness; s1 s1 and s2 s2 are screws for fastening the wires. On each side one of the
screws is for large and the other for small wires. L L are screws for fixing in position the rods R R,
which support the knobs.
In another arrangement with the magnet I take the discharge between the rounded pole pieces
themselves, which in such case are insulated and preferably provided with polished brass caps.
The employment of an intense magnetic field is of advantage principally when the induction
coil or transformer which charges the condenser is operated by currents of very low frequency. In
such a case the number of the fundamental discharges between the knobs may be so small as to
render the currents produced in the secondary unsuitable for many experiments. The intense
magnetic field then serves to blow out the arc between the knobs as soon as it is formed, and the
fundamental discharges occur in quicker succession.

Fig. 134.
Instead of the magnet, a draught or blast of air may be employed with some advantage. In this
case the arc is preferably established between the knobs A B, in Fig. 131 (the knobs a b being
generally joined, or entirely done away with), as in this disposition the arc is long and unsteady,
and is easily affected by the draught.
When a magnet is employed to break the arc, it is better to choose the connection indicated
diagrammatically in Fig. 134, as in this case the currents forming the arc are much more powerful,
and the magnetic field exercises a greater influence. The use of the magnet permits, however, of
the arc being replaced by a vacuum tube, but I have encountered great difficulties in working with
an exhausted tube.
The other form of discharger used in these and similar experiments is indicated in Figs. 135
and 136. It consists of a number of brass pieces c c (Fig. 135), each of which comprises a spherical
middle portion m with an extension e below—which is merely used to fasten the piece in a lathe
when polishing up the discharging surface—and a column above, which consists of a knurled
flange f surmounted by a threaded stem l carrying a nut n, by means of which a wire is fastened to
the column. The flange f conveniently serves for holding the brass piece when fastening the wire,
and also for turning it in any position when it becomes necessary to present a fresh discharging
surface. Two stout strips of hard rubber R R, with planed grooves g g (Fig. 136) to fit the middle
portion of the pieces c c, serve to clamp the latter and hold them firmly in position by means of
two bolts C C (of which only one is shown) passing through the ends of the strips.

Fig. 135.

Fig. 136.
In the use of this kind of discharger I have found three principal advantages over the ordinary
form. First, the dielectric strength of a given total width of air space is greater when a great many
small air gaps are used instead of one, which permits of working with a smaller length of air gap,
and that means smaller loss and less deterioration of the metal; secondly, by reason of splitting the
arc up into smaller arcs, the polished surfaces are made to last much longer; and, thirdly, the
apparatus affords some gauge in the experiments. I usually set the pieces by putting between them
sheets of uniform thickness at a certain very small distance which is known from the experiments
of Sir William Thomson to require a certain electromotive force to be bridged by the spark.

It should, of course, be remembered that the sparking distance is much diminished as the
frequency is increased. By taking any number of spaces the experimenter has a rough idea of the
electromotive force, and he finds it easier to repeat an experiment, as he has not the trouble of
setting the knobs again and again. With this kind of discharger I have been able to maintain an
oscillating motion without any spark being visible with the naked eye between the knobs, and they
would not show a very appreciable rise in temperature. This form of discharge also lends itself to
many arrangements of condensers and circuits which are often very convenient and time-saving. I
have used it preferably in a disposition similar to that indicated in Fig. 131, when the currents
forming the arc are small.
I may here mention that I have also used dischargers with single or multiple air gaps, in which
the discharge surfaces were rotated with great speed. No particular advantage was, however,
gained by this method, except in cases where the currents from the condenser were large and the
keeping cool of the surfaces was necessary, and in cases when, the discharge not being oscillating
of itself, the arc as soon as established was broken by the air current, thus starting the vibration at
intervals in rapid succession. I have also used mechanical interrupters in many ways. To avoid the
difficulties with frictional contacts, the preferred plan adopted was to establish the arc and rotate
through it at great speed a rim of mica provided with many holes and fastened to a steel plate. It is
understood, of course, that the employment of a magnet, air current, or other interrupter, produces
no effect worth noticing, unless the self-induction, capacity and resistance are so related that there
are oscillations set up upon each interruption.
I will now endeavor to show you some of the most noteworthy of these discharge phenomena.
I have stretched across the room two ordinary cotton covered wires, each about seven metres in
length. They are supported on insulating cords at a distance of about thirty centimetres. I attach
now to each of the terminals of the coil one of the wires, and set the coil in action. Upon turning
the lights off in the room you see the wires strongly illuminated by the streams issuing abundantly
from their whole surface in spite of the cotton covering, which may even be very thick. When the
experiment is performed under good conditions, the light from the wires is sufficiently intense to
allow distinguishing the objects in a room. To produce the best result it is, of course, necessary to
adjust carefully the capacity of the jars, the arc between the knobs and the length of the wires. My
experience is that calculation of the length of the wires leads, in such case, to no result whatever.
The experimenter will do best to take the wires at the start very long, and then adjust by cutting off
first long pieces, and then smaller and smaller ones as he approaches the right length.
A convenient way is to use an oil condenser of very small capacity, consisting of two small
adjustable metal plates, in connection with this and similar experiments. In such case I take wires
rather short and at the beginning set the condenser plates at maximum distance. If the streams from
the wires increase by approach of the plates, the length of the wires is about right; if they diminish,
the wires are too long for that frequency and potential. When a condenser is used in connection
with experiments with such a coil, it should be an oil condenser by all means, as in using an air
condenser considerable energy might be wasted. The wires leading to the plates in the oil should
be very thin, heavily coated with some insulating compound, and provided with a conducting
covering—this preferably extending under the surface of the oil. The conducting cover should not
be too near the terminals, or ends, of the wire, as a spark would be apt to jump from the wire to it.
The conducting coating is used to diminish the air losses, in virtue of its action as an electrostatic
screen. As to the size of the vessel containing the oil, and the size of the plates, the experimenter

gains at once an idea from a rough trial. The size of the plates in oil is, however, calculable, as the
dielectric losses are very small.
In the preceding experiment it is of considerable interest to know what relation the quantity of
the light emitted bears to the frequency and potential of the electric impulses. My opinion is that
the heat as well as light effects produced should be proportionate, under otherwise equal
conditions of test, to the product of frequency and square of potential, but the experimental
verification of the law, whatever it may be, would be exceedingly difficult. One thing is certain, at
any rate, and that is, that in augmenting the potential and frequency we rapidly intensify the
streams; and, though it may be very sanguine, it is surely not altogether hopeless to expect that we
may succeed in producing a practical illuminant on these lines. We would then be simply using
burners or flames, in which there would be no chemical process, no consumption of material, but
merely a transfer of energy, and which would, in all probability, emit more light and less heat than
ordinary flames.

Fig. 137.
The luminous intensity of the streams is, of course, considerably increased when they are
focused upon a small surface. This may be shown by the following experiment:

I attach to one of the terminals of the coil a wire w (Fig. 137), bent in a circle of about 30
centimetres in diameter, and to the other terminal I fasten a small brass sphere s, the surface of the
wire being preferably equal to the surface of the sphere, and the centre of the latter being in a line
at right angles to the plane of the wire circle and passing through its centre. When the discharge is
established under proper conditions, a luminous hollow cone is formed, and in the dark one-half of
the brass sphere is strongly illuminated, as shown in the cut.
By some artifice or other it is easy to concentrate the streams upon small surfaces and to
produce very strong light effects. Two thin wires may thus be rendered intensely luminous.
In order to intensify the streams the wires should be very thin and short; but as in this case
their capacity would be generally too small for the coil—at least for such a one as the present—it
is necessary to augment the capacity to the required value, while, at the same time, the surface of
the wires remains very small. This may be done in many ways.

Fig. 138.
Here, for instance, I have two plates, R R, of hard rubber (Fig. 138), upon which I have glued
two very thin wires w w, so as to form a name. The wires may be bare or covered with the best
insulation—it is immaterial for the success of the experiment. Well insulated wires, if anything, are
preferable. On the back of each plate, indicated by the shaded portion, is a tinfoil coating t t. The
plates are placed in line at a sufficient distance to prevent a spark passing from one wire to the
other. The two tinfoil coatings I have joined by a conductor C, and the two wires I presently
connect to the terminals of the coil. It is now easy, by varying the strength and frequency of the
currents through the primary, to find a point at which the capacity of the system is best suited to
the conditions, and the wires become so strongly luminous that, when the light in the room is
turned off the name formed by them appears in brilliant letters.

It is perhaps preferable to perform this experiment with a coil operated from an alternator of
high frequency, as then, owing to the harmonic rise and fall, the streams are very uniform, though
they are less abundant than when produced with such a coil as the present one. This experiment,
however, may be performed with low frequencies, but much less satisfactorily.

Fig. 139.
When two wires, attached to the terminals of the coil, are set at the proper distance, the streams
between them may be so intense as to produce a continuous luminous sheet. To show this
phenomenon I have here two circles, C and c (Fig. 139), of rather stout wire, one being about 80
centimetres and the other 30 centimetres in diameter. To each of the terminals of the coil I attach
one of the circles. The supporting wires are so bent that the circles may be placed in the same
plane, coinciding as nearly as possible. When the light in the room is turned off and the coil set to
work, you see the whole space between the wires uniformly filled with streams, forming a
luminous disc, which could be seen from a considerable distance, such is the intensity of the
streams. The outer circle could have been much larger than the present one; in fact, with this coil I
have used much larger circles, and I have been able to produce a strongly luminous sheet, covering
an area of more than one square metre, which is a remarkable effect with this very small coil. To
avoid uncertainty, the circle has been taken smaller, and the area is now about 0.43 square metre.

The frequency of the vibration, and the quickness of succession of the sparks between the
knobs, affect to a marked degree the appearance of the streams. When the frequency is very low,
the air gives way in more or less the same manner, as by a steady difference of potential, and the
streams consist of distinct threads, generally mingled with thin sparks, which probably correspond
to the successive discharges occurring between the knobs. But when the frequency is extremely
high, and the arc of the discharge produces a very loud and smooth sound—showing both that
oscillation takes place and that the sparks succeed each other with great rapidity—then the
luminous streams formed are perfectly uniform. To reach this result very small coils and jars of
small capacity should be used. I take two tubes of thick Bohemian glass, about 5 centimetres in
diameter and 20 centimetres long. In each of the tubes I slip a primary of very thick copper wire.
On the top of each tube I wind a secondary of much thinner gutta-percha covered wire. The two
secondaries I connect in series, the primaries preferably in multiple arc. The tubes are then placed
in a large glass vessel, at a distance of 10 to 15 centimetres from each other, on insulating supports,
and the vessel is filled with boiled-out oil, the oil reaching about an inch above the tubes. The free
ends of the secondary are lifted out of the coil and placed parallel to each other at a distance of
about ten centimetres. The ends which are scraped should be dipped in the oil. Two four-pint jars
joined in series may be used to discharge through the primary. When the necessary adjustments in
the length and distance of the wires above the oil and in the arc of discharge are made, a luminous
sheet is produced between the wires which is perfectly smooth and textureless, like the ordinary
discharge through a moderately exhausted tube.
I have purposely dwelt upon this apparently insignificant experiment. In trials of this kind the
experimenter arrives at the startling conclusion that, to pass ordinary luminous discharges through
gases, no particular degree of exhaustion is needed, but that the gas may be at ordinary or even
greater pressure. To accomplish this, a very high frequency is essential; a high potential is likewise
required, but this is merely an incidental necessity. These experiments teach us that, in
endeavoring to discover novel methods of producing light by the agitation of atoms, or molecules,
of a gas, we need not limit our research to the vacuum tube, but may look forward quite seriously
to the possibility of obtaining the light effects without the use of any vessel whatever, with air at
ordinary pressure.
Such discharges of very high frequency, which render luminous the air at ordinary pressures,
we have probably occasion often to witness in Nature. I have no doubt that if, as many believe, the
aurora borealis is produced by sudden cosmic disturbances, such as eruptions at the sun's surface,
which set the electrostatic charge of the earth in an extremely rapid vibration, the red glow
observed is not confined to the upper rarefied strata of the air, but the discharge traverses, by
reason of its very high frequency, also the dense atmosphere in the form of a glow, such as we
ordinarily produce in a slightly exhausted tube. If the frequency were very low, or even more so, if
the charge were not at all vibrating, the dense air would break down as in a lightning discharge.
Indications of such breaking down of the lower dense strata of the air have been repeatedly
observed at the occurrence of this marvelous phenomenon; but if it does occur, it can only be
attributed to the fundamental disturbances, which are few in number, for the vibration produced by
them would be far too rapid to allow a disruptive break. It is the original and irregular impulses
which affect the instruments; the superimposed vibrations probably pass unnoticed.
When an ordinary low frequency discharge is passed through moderately rarefied air, the air
assumes a purplish hue. If by some means or other we increase the intensity of the molecular, or
atomic, vibration, the gas changes to a white color. A similar change occurs at ordinary pressures
with electric impulses of very high frequency. If the molecules of the air around a wire are

moderately agitated, the brush formed is reddish or violet; if the vibration is rendered sufficiently
intense, the streams become white. We may accomplish this in various ways. In the experiment
before shown with the two wires across the room, I have endeavored to secure the result by
pushing to a high value both the frequency and potential; in the experiment with the thin wires
glued on the rubber plate I have concentrated the action upon a very small surface—in other
words, I have worked with a great electric density.

Fig. 140.
A most curious form of discharge is observed with such a coil when the frequency and
potential are pushed to the extreme limit. To perform the experiment, every part of the coil should
be heavily insulated, and only two small spheres—or, better still, two sharp-edged metal discs (d d,
Fig. 140) of no more than a few centimetres in diameter—should be exposed to the air. The coil
here used is immersed in oil, and the ends of the secondary reaching out of the oil are covered with
an air-tight cover of hard rubber of great thickness. All cracks, if there are any, should be carefully
stopped up, so that the brush discharge cannot form anywhere except on the small spheres or plates
which are exposed to the air. In this case, since there are no large plates or other bodies of capacity
attached to the terminals, the coil is capable of an extremely rapid vibration. The potential may be
raised by increasing, as far as the experimenter judges proper, the rate of change of the primary
current. With a coil not widely differing from the present, it is best to connect the two primaries in
multiple arc; but if the secondary should have a much greater number of turns the primaries should
preferably be used in series, as otherwise the vibration might be too fast for the secondary. It
occurs under these conditions that misty white streams break forth from the edges of the discs and
spread out phantom-like into space. With this coil, when fairly well produced, they are about 25 to
30 centimetres long. When the hand is held against them no sensation is produced, and a spark,
causing a shock, jumps from the terminal only upon the hand being brought much nearer. If the
oscillation of the primary current is rendered intermittent by some means or other, there is a
corresponding throbbing of the streams, and now the hand or other conducting object may be
brought in still greater proximity to the terminal without a spark being caused to jump.

Among the many beautiful phenomena which may be produced with such a coil, I have here
selected only those which appear to possess some features of novelty, and lead us to some
conclusions of interest. One will not find it at all difficult to produce in the laboratory, by means of
it, many other phenomena which appeal to the eye even more than these here shown, but present
no particular feature of novelty.
Early experimenters describe the display of sparks produced by an ordinary large induction
coil upon an insulating plate separating the terminals. Quite recently Siemens performed some
experiments in which fine effects were obtained, which were seen by many with interest. No doubt
large coils, even if operated with currents of low frequencies, are capable of producing beautiful
effects. But the largest coil ever made could not, by far, equal the magnificent display of streams
and sparks obtained from such a disruptive discharge coil when properly adjusted. To give an idea,
a coil such as the present one will cover easily a plate of one metre in diameter completely with the
streams. The best way to perform such experiments is to take a very thin rubber or a glass plate
and glue on one side of it a narrow ring of tinfoil of very large diameter, and on the other a circular
washer, the centre of the latter coinciding with that of the ring, and the surfaces of both being
preferably equal, so as to keep the coil well balanced. The washer and ring should be connected to
the terminals by heavily insulated thin wires. It is easy in observing the effect of the capacity to
produce a sheet of uniform streams, or a fine network of thin silvery threads, or a mass of loud
brilliant sparks, which completely cover the plate.
Since I have advanced the idea of the conversion by means of the disruptive discharge, in my
paper before the American Institute of Electrical Engineers at the beginning of the past year, the
interest excited in it has been considerable. It affords us a means for producing any potentials by
the aid of inexpensive coils operated from ordinary systems of distribution, and—what is perhaps
more appreciated—it enables us to convert currents of any frequency into currents of any other
lower or higher frequency. But its chief value will perhaps be found in the help which it will afford
us in the investigations of the phenomena of phosphorescence, which a disruptive discharge coil is
capable of exciting in innumerable cases where ordinary coils, even the largest, would utterly fail.
Considering its probable uses for many practical purposes, and its possible introduction into
laboratories for scientific research, a few additional remarks as to the construction of such a coil
will perhaps not be found superfluous.
It is, of course, absolutely necessary to employ in such a coil wires provided with the best
insulation.
Good coils may be produced by employing wires covered with several layers of cotton, boiling
the coil a long time in pure wax, and cooling under moderate pressure. The advantage of such a
coil is that it can be easily handled, but it cannot probably give as satisfactory results as a coil
immersed in pure oil. Besides, it seems that the presence of a large body of wax affects the coil
disadvantageously, whereas this does not seem to be the case with oil. Perhaps it is because the
dielectric losses in the liquid are smaller.
I have tried at first silk and cotton covered wires with oil immersions, but I have been
gradually led to use gutta-percha covered wires, which proved most satisfactory. Gutta-percha
insulation adds, of course, to the capacity of the coil, and this, especially if the coil be large, is a
great disadvantage when extreme frequencies are desired; but, on the other hand, gutta-percha will
withstand much more than an equal thickness of oil, and this advantage should be secured at any
price. Once the coil has been immersed, it should never be taken out of the oil for more than a few

hours, else the gutta-percha will crack up and the coil will not be worth half as much as before.
Gutta-percha is probably slowly attacked by the oil, but after an immersion of eight to nine months
I have found no ill effects.
I have obtained two kinds of gutta-percha wire known in commerce: in one the insulation
sticks tightly to the metal, in the other it does not. Unless a special method is followed to expel all
air, it is much safer to use the first kind. I wind the coil within an oil tank so that all interstices are
filled up with the oil. Between the layers I use cloth boiled out thoroughly in oil, calculating the
thickness according to the difference of potential between the turns. There seems not to be a very
great difference whatever kind of oil is used; I use paraffine or linseed oil.
To exclude more perfectly the air, an excellent way to proceed, and easily practicable with
small coils, is the following: Construct a box of hardwood of very thick boards which have been
for a long time boiled in oil. The boards should be so joined as to safely withstand the external air
pressure. The coil being placed and fastened in position within the box, the latter is closed with a
strong lid, and covered with closely fitting metal sheets, the joints of which are soldered very
carefully. On the top two small holes are drilled, passing through the metal sheet and the wood,
and in these holes two small glass tubes are inserted and the joints made air-tight. One of the tubes
is connected to a vacuum pump, and the other with a vessel containing a sufficient quantity of
boiled-out oil. The latter tube has a very small hole at the bottom, and is provided with a stopcock.
When a fairly good vacuum has been obtained, the stopcock is opened and the oil slowly fed in.
Proceeding in this manner, it is impossible that any big bubbles, which are the principal danger,
should remain between the turns. The air is most completely excluded, probably better than by
boiling out, which, however, when gutta-percha coated wires are used, is not practicable.
For the primaries I use ordinary line wire with a thick cotton coating. Strands of very thin
insulated wires properly interlaced would, of course, be the best to employ for the primaries, but
they are not to be had.
In an experimental coil the size of the wires is not of great importance. In the coil here used the
primary is No. 12 and the secondary No. 24 Brown & Sharpe gauge wire; but the sections may be
varied considerably. It would only imply different adjustments; the results aimed at would not be
materially affected.
I have dwelt at some length upon the various forms of brush discharge because, in studying
them, we not only observe phenomena which please our eye, but also afford us food for thought,
and lead us to conclusions of practical importance. In the use of alternating currents of very high
tension, too much precaution cannot be taken to prevent the brush discharge. In a main conveying
such currents, in an induction coil or transformer, or in a condenser, the brush discharge is a source
of great danger to the insulation. In a condenser, especially, the gaseous matter must be most
carefully expelled, for in it the charged surfaces are near each other, and if the potentials are high,
just as sure as a weight will fall if let go, so the insulation will give way if a single gaseous bubble
of some size be present, whereas, if all gaseous matter were carefully excluded, the condenser
would safely withstand a much higher difference of potential. A main conveying alternating
currents of very high tension may be injured merely by a blow hole or small crack in the
insulation, the more so as a blowhole is apt to contain gas at low pressure; and as it appears almost
impossible to completely obviate such little imperfections, I am led to believe that in our future
distribution of electrical energy by currents of very high tension, liquid insulation will be used.
The cost is a great drawback, but if we employ an oil as an insulator the distribution of electrical
energy with something like 100,000 volts, and even more, becomes, at least with higher

frequencies, so easy that it could be hardly called an engineering feat. With oil insulation and
alternate current motors, transmissions of power can be affected with safety and upon an industrial
basis at distances of as much as a thousand miles.
A peculiar property of oils, and liquid insulation in general, when subjected to rapidly
changing electric stresses, is to disperse any gaseous bubbles which may be present, and diffuse
them through its mass, generally long before any injurious break can occur. This feature may be
easily observed with an ordinary induction coil by taking the primary out, plugging up the end of
the tube upon which the secondary is wound, and filling it with some fairly transparent insulator,
such as paraffine oil. A primary of a diameter something like six millimetres smaller than the
inside of the tube may be inserted in the oil. When the coil is set to work one may see, looking
from the top through the oil, many luminous points—air bubbles which are caught by inserting the
primary, and which are rendered luminous in consequence of the violent bombardment. The
occluded air, by its impact against the oil, heats it; the oil begins to circulate, carrying some of the
air along with it, until the bubbles are dispersed and the luminous points disappear. In this manner,
unless large bubbles are occluded in such way that circulation is rendered impossible, a damaging
break is averted, the only effect being a moderate warming up of the oil. If, instead of the liquid, a
solid insulation, no matter how thick, were used, a breaking through and injury of the apparatus
would be inevitable.
The exclusion of gaseous matter from any apparatus in which the dielectric is subjected to
more or less rapidly changing electric forces is, however, not only desirable in order to avoid a
possible injury of the apparatus, but also on account of economy. In a condenser, for instance, as
long as only a solid or only a liquid dielectric is used, the loss is small; but if a gas under ordinary
or small pressure be present the loss may be very great. Whatever the nature of the force acting in
the dielectric may be, it seems that in a solid or liquid the molecular displacement produced by the
force is small: hence the product of force and displacement is insignificant, unless the force be
very great; but in a gas the displacement, and therefore this product, is considerable; the molecules
are free to move, they reach high speeds, and the energy of their impact is lost in heat or otherwise.
If the gas be strongly compressed, the displacement due to the force is made smaller, and the
losses are reduced.
In most of the succeeding experiments I prefer, chiefly on account of the regular and positive
action, to employ the alternator before referred to. This is one of the several machines constructed
by me for the purpose of these investigations. It has 384 pole projections, and is capable of giving
currents of a frequency of about 10,000 per second. This machine has been illustrated and briefly
described in my first paper before the American Institute of Electrical Engineers, May 20th, 1891,
to which I have already referred. A more detailed description, sufficient to enable any engineer to
build a similar machine, will be found in several electrical journals of that period.
The induction coils operated from the machine are rather small, containing from 5,000 to
15,000 turns in the secondary. They are immersed in boiled-out linseed oil, contained in wooden
boxes covered with zinc sheet.
I have found it advantageous to reverse the usual position of the wires, and to wind, in these
coils, the primaries on the top; thus allowing the use of a much larger primary, which, of course,
reduces the danger of overheating and increases the output of the coil. I make the primary on each
side at least one centimetre shorter than the secondary, to prevent the breaking through on the
ends, which would surely occur unless the insulation on the top of the secondary be very thick, and
this, of course, would be disadvantageous.

When the primary is made movable, which is necessary in some experiments, and many times
convenient for the purposes of adjustment, I cover the secondary with wax, and turn it off in a
lathe to a diameter slightly smaller than the inside of the primary coil. The latter I provide with a
handle reaching out of the oil, which serves to shift it in any position along the secondary.
I will now venture to make, in regard to the general manipulation of induction coils, a few
observations bearing upon points which have not been fully appreciated in earlier experiments
with such coils, and are even now often overlooked.
The secondary of the coil possesses usually such a high self-induction that the current through
the wire is inappreciable, and may be so even when the terminals are joined by a conductor of
small resistance. If capacity is added to the terminals, the self-induction is counteracted, and a
stronger current is made to flow through the secondary, though its terminals are insulated from
each other. To one entirely unacquainted with the properties of alternating currents nothing will
look more puzzling. This feature was illustrated in the experiment performed at the beginning with
the top plates of wire gauze attached to the terminals and the rubber plate. When the plates of wire
gauze were close together, and a small arc passed between them, the arc prevented a strong current
from passing through the secondary, because it did away with the capacity on the terminals; when
the rubber plate was inserted between, the capacity of the condenser formed counteracted the self-
induction of the secondary, a stronger current passed now, the coil performed more work, and the
discharge was by far more powerful.
The first thing, then, in operating the induction coil is to combine capacity with the secondary
to overcome the self-induction. If the frequencies and potentials are very high, gaseous matter
should be carefully kept away from the charged surfaces. If Leyden jars are used, they should be
immersed in oil, as otherwise considerable dissipation may occur if the jars are greatly strained.
When high frequencies are used, it is of equal importance to combine a condenser with the
primary. One may use a condenser connected to the ends of the primary or to the terminals of the
alternator, but the latter is not to be recommended, as the machine might be injured. The best way
is undoubtedly to use the condenser in series with the primary and with the alternator, and to adjust
its capacity so as to annul the self-induction of both the latter. The condenser should be adjustable
by very small steps, and for a finer adjustment a small oil condenser with movable plates may be
used conveniently.
I think it best at this juncture to bring before you a phenomenon, observed by me some time
ago, which to the purely scientific investigator may perhaps appear more interesting than any of
the results which I have the privilege to present to you this evening.
It may be quite properly ranked among the brush phenomena—in fact, it is a brush, formed at,
or near, a single terminal in high vacuum.

Fig. 141. Fig. 142.

In bulbs provided with a conducting terminal, though it be of aluminum, the brush has but an
ephemeral existence, and cannot, unfortunately, be indefinitely preserved in its most sensitive
state, even in a bulb devoid of any conducting electrode. In studying the phenomenon, by all
means a bulb having no leading-in wire should be used. I have found it best to use bulbs
constructed as indicated in Figs. 141 and 142.
In Fig. 141 the bulb comprises an incandescent lamp globe L, in the neck of which is sealed a
barometer tube b, the end of which is blown out to form a small sphere s. This sphere should be
sealed as closely as possible in the centre of the large globe. Before sealing, a thin tube t, of
aluminum sheet, may be slipped in the barometer tube, but it is not important to employ it.
The small hollow sphere s is filled with some conducting powder, and a wire w is cemented in
the neck for the purpose of connecting the conducting powder with the generator.
The construction shown in Fig. 142 was chosen in order to remove from the brush any
conducting body which might possibly affect it. The bulb consists in this case of a lamp globe L,
which has a neck n, provided with a tube b and small sphere s, sealed to it, so that two entirely
independent compartments are formed, as indicated in the drawing. When the bulb is in use the
neck n is provided with a tinfoil coating, which is connected to the generator and acts inductively
upon the moderately rarefied and highly conducted gas inclosed in the neck. From there the

current passes through the tube b into the small sphere s, to act by induction upon the gas
contained in the globe L.
It is of advantage to make the tube t very thick, the hole through it very small, and to blow the
sphere s very thin. It is of the greatest importance that the sphere s be placed in the centre of the
globe L.

Fig. 143.
Figs. 143, 144 and 145 indicate different forms, or stages, of the brush. Fig. 143 shows the
brush as it first appears in a bulb provided with a conducting terminal; but, as in such a bulb it very
soon disappears—often after a few minutes—I will confine myself to the description of the
phenomenon as seen in a bulb without conducting electrode. It is observed under the following
conditions:
When the globe L (Figs. 141 and 142) is exhausted to a very high degree, generally the bulb is
not excited upon connecting the wire w (Fig. 141) or the tinfoil coating of the bulb (Fig. 142) to
the terminal of the induction coil. To excite it, it is usually sufficient to grasp the globe L with the
hand. An intense phosphorescence then spreads at first over the globe, but soon gives place to a
white, misty light. Shortly afterward one may notice that the luminosity is unevenly distributed in
the globe, and after passing the current for some time the bulb appears as in Fig. 144. From this
stage the phenomenon will gradually pass to that indicated in Fig. 145, after some minutes, hours,
days or weeks, according as the bulb is worked. Warming the bulb or increasing the potential
hastens the transit.

Fig. 144. Fig. 145.

When the brush assumes the form indicated in Fig. 145, it may be brought to a state of extreme
sensitiveness to electrostatic and magnetic influence. The bulb hanging straight down from a wire,
and all objects being remote from it, the approach of the observer at a few paces from the bulb will
cause the brush to fly to the opposite side, and if he walks around the bulb it will always keep on
the opposite side. It may begin to spin around the terminal long before it reaches that sensitive
stage. When it begins to turn around, principally, but also before, it is affected by a magnet, and at
a certain stage it is susceptible to magnetic influence to an astonishing degree. A small permanent
magnet, with its poles at a distance of no more than two centimetres, will affect it visibly at a
distance of two metres, slowing down or accelerating the rotation according to how it is held
relatively to the brush. I think I have observed that at the stage when it is most sensitive to
magnetic, it is not most sensitive to electrostatic, influence. My explanation is, that the
electrostatic attraction between the brush and the glass of the bulb, which retards the rotation,
grows much quicker than the magnetic influence when the intensity of the stream is increased.
When the bulb hangs with the globe L down, the rotation is always clockwise. In the southern
hemisphere it would occur in the opposite direction and on the equator the brush should not turn at
all. The rotation may be reversed by a magnet kept at some distance. The brush rotates best,
seemingly, when it is at right angles to the lines of force of the earth. It very likely rotates, when at
its maximum speed, in synchronism with the alternations, say, 10,000 times a second. The rotation
can be slowed down or accelerated by the approach or receding of the observer, or any conducting
body, but it cannot be reversed by putting the bulb in any position. When it is in the state of the
highest sensitiveness and the potential or frequency be varied, the sensitiveness is rapidly
diminished. Changing either of these but little will generally stop the rotation. The sensitiveness is

likewise affected by the variations of temperature. To attain great sensitiveness it is necessary to
have the small sphere s in the centre of the globe L, as otherwise the electrostatic action of the
glass of the globe will tend to stop the rotation. The sphere s should be small and of uniform
thickness; any dissymmetry of course has the effect to diminish the sensitiveness.
The fact that the brush rotates in a definite direction in a permanent magnetic field seems to
show that in alternating currents of very high frequency the positive and negative impulses are not
equal, but that one always preponderates over the other.
Of course, this rotation in one direction may be due to the action of the two elements of the
same current upon each other, or to the action of the field produced by one of the elements upon
the other, as in a series motor, without necessarily one impulse being stronger than the other. The
fact that the brush turns, as far as I could observe, in any position, would speak for this view. In
such case it would turn at any point of the earth's surface. But, on the other hand, it is then hard to
explain why a permanent magnet should reverse the rotation, and one must assume the
preponderance of impulses of one kind.
As to the causes of the formation of the brush or stream, I think it is due to the electrostatic
action of the globe and the dissymmetry of the parts. If the small bulb s and the globe L were
perfect concentric spheres, and the glass throughout of the same thickness and quality, I think the
brush would not form, as the tendency to pass would be equal on all sides. That the formation of
the stream is due to an irregularity is apparent from the fact that it has the tendency to remain in
one position, and rotation occurs most generally only when it is brought out of this position by
electrostatic or magnetic influence. When in an extremely sensitive state it rests in one position,
most curious experiments may be performed with it. For instance, the experimenter may, by
selecting a proper position, approach the hand at a certain considerable distance to the bulb, and he
may cause the brush to pass off by merely stiffening the muscles of the arm. When it begins to
rotate slowly, and the hands are held at a proper distance, it is impossible to make even the
slightest motion without producing a visible effect upon the brush. A metal plate connected to the
other terminal of the coil affects it at a great distance, slowing down the rotation often to one turn a
second.
I am firmly convinced that such a brush, when we learn how to produce it properly, will prove
a valuable aid in the investigation of the nature of the forces acting in an electrostatic or magnetic
field. If there is any motion which is measurable going on in the space, such a brush ought to
reveal it. It is, so to speak, a beam of light, frictionless, devoid of inertia.
I think that it may find practical applications in telegraphy. With such a brush it would be
possible to send dispatches across the Atlantic, for instance, with any speed, since its sensitiveness
may be so great that the slightest changes will affect it. If it were possible to make the stream more
intense and very narrow, its deflections could be easily photographed.
I have been interested to find whether there is a rotation of the stream itself, or whether there is
simply a stress traveling around the bulb. For this purpose I mounted a light mica fan so that its
vanes were in the path of the brush. If the stream itself was rotating the fan would be spun around.
I could produce no distinct rotation of the fan, although I tried the experiment repeatedly; but as
the fan exerted a noticeable influence on the stream, and the apparent rotation of the latter was, in
this case, never quite satisfactory, the experiment did not appear to be conclusive.
I have been unable to produce the phenomenon with the disruptive discharge coil, although
every other of these phenomena can be well produced by it—many, in fact, much better than with

coils operated from an alternator.
It may be possible to produce the brush by impulses of one direction, or even by a steady
potential, in which case it would be still more sensitive to magnetic influence.
In operating an induction coil with rapidly alternating currents, we realize with astonishment,
for the first time, the great importance of the relation of capacity, self-induction and frequency as
regards the general results. The effects of capacity are the most striking, for in these experiments,
since the self-induction and frequency both are high, the critical capacity is very small, and need
be but slightly varied to produce a very considerable change. The experimenter may bring his body
in contact with the terminals of the secondary of the coil, or attach to one or both terminals
insulated bodies of very small bulk, such as bulbs, and he may produce a considerable rise or fall
of potential, and greatly affect the flow of the current through the primary. In the experiment
before shown, in which a brush appears at a wire attached to one terminal, and the wire is vibrated
when the experimenter brings his insulated body in contact with the other terminal of the coil, the
sudden rise of potential was made evident.
I may show you the behavior of the coil in another manner which possesses a feature of some
interest. I have here a little light fan of aluminum sheet, fastened to a needle and arranged to rotate
freely in a metal piece screwed to one of the terminals of the coil. When the coil is set to work, the
molecules of the air are rhythmically attracted and repelled. As the force with which they are
repelled is greater than that with which they are attracted, it results that there is a repulsion exerted
on the surfaces of the fan. If the fan were made simply of a metal sheet, the repulsion would be
equal on the opposite sides, and would produce no effect. But if one of the opposing surfaces is
screened, or if, generally speaking, the bombardment on this side is weakened in some way or
other, there remains the repulsion exerted upon the other, and the fan is set in rotation. The
screening is best effected by fastening upon one of the opposing sides of the fan insulated
conducting coatings, or, if the fan is made in the shape of an ordinary propeller screw, by fastening
on one side, and close to it, an insulated metal plate. The static screen may, however, be omitted,
and simply a thickness of insulating material fastened to one of the sides of the fan.
To show the behavior of the coil, the fan may be placed upon the terminal and it will readily
rotate when the coil is operated by currents of very high frequency. With a steady potential, of
course, and even with alternating currents of very low frequency, it would not turn, because of the
very slow exchange of air and, consequently, smaller bombardment; but in the latter case it might
turn if the potential were excessive. With a pin wheel, quite the opposite rule holds good; it rotates
best with a steady potential, and the effort is the smaller the higher the frequency. Now, it is very
easy to adjust the conditions so that the potential is normally not sufficient to turn the fan, but that
by connecting the other terminal of the coil with an insulated body it rises to a much greater value,
so as to rotate the fan, and it is likewise possible to stop the rotation by connecting to the terminal
a body of different size, thereby diminishing the potential.
Instead of using the fan in this experiment, we may use the "electric" radiometer with similar
effect. But in this case it will be found that the vanes will rotate only at high exhaustion or at
ordinary pressures; they will not rotate at moderate pressures, when the air is highly conducting.
This curious observation was made conjointly by Professor Crookes and myself. I attribute the
result to the high conductivity of the air, the molecules of which then do not act as independent
carriers of electric charges, but act all together as a single conducting body. In such case, of course,
if there is any repulsion at all of the molecules from the vanes, it must be very small. It is possible,
however, that the result is in part due to the fact that the greater part of the discharge passes from

the leading-in wire through the highly conducting gas, instead of passing off from the conducting
vanes.
In trying the preceding experiment with the electric radiometer the potential should not exceed
a certain limit, as then the electrostatic attraction between the vanes and the glass of the bulb may
be so great as to stop the rotation.
A most curious feature of alternate currents of high frequencies and potentials is that they
enable us to perform many experiments by the use of one wire only. In many respects this feature
is of great interest.
In a type of alternate current motor invented by me some years ago I produced rotation by
inducing, by means of a single alternating current passed through a motor circuit, in the mass or
other circuits of the motor, secondary currents, which, jointly with the primary or inducing current,
created a moving field of force. A simple but crude form of such a motor is obtained by winding
upon an iron core a primary, and close to it a secondary coil, joining the ends of the latter and
placing a freely movable metal disc within the influence of the field produced by both. The iron
core is employed for obvious reasons, but it is not essential to the operation. To improve the motor,
the iron core is made to encircle the armature. Again to improve, the secondary coil is made to
partly overlap the primary, so that it cannot free itself from a strong inductive action of the latter,
repel its lines as it may. Once more to improve, the proper difference of phase is obtained between
the primary and secondary currents by a condenser, self-induction, resistance or equivalent
windings.
I had discovered, however, that rotation is produced by means of a single coil and core; my
explanation of the phenomenon, and leading thought in trying the experiment, being that there
must be a true time lag in the magnetization of the core. I remember the pleasure I had when, in
the writings of Professor Ayrton, which came later to my hand, I found the idea of the time lag
advocated. Whether there is a true time lag, or whether the retardation is due to eddy currents
circulating in minute paths, must remain an open question, but the fact is that a coil wound upon
an iron core and traversed by an alternating current creates a moving field of force, capable of
setting an armature in rotation. It is of some interest, in conjunction with the historical Arago
experiment, to mention that in lag or phase motors I have produced rotation in the opposite
direction to the moving field, which means that in that experiment the magnet may not rotate, or
may even rotate in the opposite direction to the moving disc. Here, then, is a motor
(diagrammatically illustrated in Fig. 146), comprising a coil and iron core, and a freely movable
copper disc in proximity to the latter.

Fig. 146.
To demonstrate a novel and interesting feature, I have, for a reason which I will explain,
selected this type of motor. When the ends of the coil are connected to the terminals of an
alternator the disc is set in rotation. But it is not this experiment, now well known, which I desire
to perform. What I wish to show you is that this motor rotates with one single connection between
it and the generator; that is to say, one terminal of the motor is connected to one terminal of the
generator—in this case the secondary of a high-tension induction coil—the other terminals of
motor and generator being insulated in space. To produce rotation it is generally (but not
absolutely) necessary to connect the free end of the motor coil to an insulated body of some size.
The experimenter's body is more than sufficient. If he touches the free terminal with an object held
in the hand, a current passes through the coil and the copper disc is set in rotation. If an exhausted
tube is put in series with the coil, the tube lights brilliantly, showing the passage of a strong
current. Instead of the experimenter's body, a small metal sheet suspended on a cord may be used
with the same result. In this case the plate acts as a condenser in series with the coil. It counteracts
the self-induction of the latter and allows a strong current to pass. In such a combination, the
greater the self-induction of the coil the smaller need be the plate, and this means that a lower
frequency, or eventually a lower potential, is required to operate the motor. A single coil wound
upon a core has a high self-induction; for this reason, principally, this type of motor was chosen to
perform the experiment. Were a secondary closed coil wound upon the core, it would tend to
diminish the self-induction, and then it would be necessary to employ a much higher frequency

and potential. Neither would be advisable, for a higher potential would endanger the insulation of
the small primary coil, and a higher frequency would result in a materially diminished torque.
It should be remarked that when such a motor with a closed secondary is used, it is not at all
easy to obtain rotation with excessive frequencies, as the secondary cuts off almost completely the
lines of the primary—and this, of course, the more, the higher the frequency—and allows the
passage of but a minute current. In such a case, unless the secondary is closed through a condenser,
it is almost essential, in order to produce rotation, to make the primary and secondary coils overlap
each other more or less.
But there is an additional feature of interest about this motor, namely, it is not necessary to
have even a single connection between the motor and generator, except, perhaps, through the
ground; for not only is an insulated plate capable of giving off energy into space, but it is likewise
capable of deriving it from an alternating electrostatic field, though in the latter case the available
energy is much smaller. In this instance one of the motor terminals is connected to the insulated
plate or body located within the alternating electrostatic field, and the other terminal preferably to
the ground.
It is quite possible, however, that such "no wire" motors, as they might be called, could be
operated by conduction through the rarefied air at considerable distances. Alternate currents,
especially of high frequencies, pass with astonishing freedom through even slightly rarefied gases.
The upper strata of the air are rarefied. To reach a number of miles out into space requires the
overcoming of difficulties of a merely mechanical nature. There is no doubt that with the
enormous potentials obtainable by the use of high frequencies and oil insulation, luminous
discharges might be passed through many miles of rarefied air, and that, by thus directing the
energy of many hundreds or thousands of horse-power, motors or lamps might be operated at
considerable distances from stationary sources. But such schemes are mentioned merely as
possibilities. We shall have no need to transmit power in this way. We shall have no need to
transmit power at all. Ere many generations pass, our machinery will be driven by a power
obtainable at any point of the universe. This idea is not novel. Men have been led to it long ago by
instinct or reason. It has been expressed in many ways, and in many places, in the history of old
and new. We find it in the delightful myth of Antheus, who derives power from the earth; we find
it among the subtle speculations of one of your splendid mathematicians, and in many hints and
statements of thinkers of the present time. Throughout space there is energy. Is this energy static or
kinetic? If static our hopes are in vain; if kinetic—and this we know it is, for certain—then it is a
mere question of time when men will succeed in attaching their machinery to the very wheelwork
of nature. Of all, living or dead, Crookes came nearest to doing it. His radiometer will turn in the
light of day and in the darkness of the night; it will turn everywhere where there is heat, and heat is
everywhere. But, unfortunately, this beautiful little machine, while it goes down to posterity as the
most interesting, must likewise be put on record as the most inefficient machine ever invented!
The preceding experiment is only one of many equally interesting experiments which may be
performed by the use of only one wire with alternations of high potential and frequency. We may
connect an insulated line to a source of such currents, we may pass an inappreciable current over
the line, and on any point of the same we are able to obtain a heavy current, capable of fusing a
thick copper wire. Or we may, by the help of some artifice, decompose a solution in any
electrolytic cell by connecting only one pole of the cell to the line or source of energy. Or we may,
by attaching to the line, or only bringing into its vicinity, light up an incandescent lamp, an
exhausted tube, or a phosphorescent bulb.

However impracticable this plan of working may appear in many cases, it certainly seems
practicable, and even recommendable, in the production of light. A perfected lamp would require
but little energy, and if wires were used at all we ought to be able to supply that energy without a
return wire.
It is now a fact that a body may be rendered incandescent or phosphorescent by bringing it
either in single contact or merely in the vicinity of a source of electric impulses of the proper
character, and that in this manner a quantity of light sufficient to afford a practical illuminant may
be produced. It is, therefore, to say the least, worth while to attempt to determine the best
conditions and to invent the best appliances for attaining this object.
Some experiences have already been gained in this direction, and I will dwell on them briefly,
in the hope that they might prove useful.
The heating of a conducting body inclosed in a bulb, and connected to a source of rapidly
alternating electric impulses, is dependent on so many things of a different nature, that it would be
difficult to give a generally applicable rule under which the maximum heating occurs. As regards
the size of the vessel, I have lately found that at ordinary or only slightly differing atmospheric
pressures, when air is a good insulator, and hence practically the same amount of energy by a
certain potential and frequency is given off from the body, whether the bulb be small or large, the
body is brought to a higher temperature if enclosed in a small bulb, because of the better
confinement of heat in this case.
At lower pressures, when air becomes more or less conducting, or if the air be sufficiently
warmed to become conducting, the body is rendered more intensely incandescent in a large bulb,
obviously because, under otherwise equal conditions of test, more energy may be given off from
the body when the bulb is large.
At very high degrees of exhaustion, when the matter in the bulb becomes "radiant," a large
bulb has still an advantage, but a comparatively slight one, over the small bulb.
Finally, at excessively high degrees of exhaustion, which cannot be reached except by the
employment of special means, there seems to be, beyond a certain and rather small size of vessel,
no perceptible difference in the heating.
These observations were the result of a number of experiments, of which one, showing the
effect of the size of the bulb at a high degree of exhaustion, may be described and shown here, as it
presents a feature of interest. Three spherical bulbs of 2 inches, 3 inches and 4 inches diameter
were taken, and in the centre of each was mounted an equal length of an ordinary incandescent
lamp filament of uniform thickness. In each bulb the piece of filament was fastened to the leading-
in wire of platinum, contained in a glass stem sealed in the bulb; care being taken, of course, to
make everything as nearly alike as possible. On each glass stem in the inside of the bulb was
slipped a highly polished tube made of aluminum sheet, which fitted the stem and was held on it
by spring pressure. The function of this aluminum tube will be explained subsequently. In each
bulb an equal length of filament protruded above the metal tube. It is sufficient to say now that
under these conditions equal lengths of filament of the same thickness—in other words, bodies of
equal bulk—were brought to incandescence. The three bulbs were sealed to a glass tube, which
was connected to a Sprengel pump. When a high vacuum had been reached, the glass tube carrying
the bulbs was sealed off. A current was then turned on successively on each bulb, and it was found
that the filaments came to about the same brightness, and, if anything, the smallest bulb, which
was placed midway between the two larger ones, may have been slightly brighter. This result was

expected, for when either of the bulbs was connected to the coil the luminosity spread through the
other two, hence the three bulbs constituted really one vessel. When all the three bulbs were
connected in multiple arc to the coil, in the largest of them the filament glowed brightest, in the
next smaller it was a little less bright, and in the smallest it only came to redness. The bulbs were
then sealed off and separately tried. The brightness of the filaments was now such as would have
been expected on the supposition that the energy given off was proportionate to the surface of the
bulb, this surface in each case representing one of the coatings of a condenser. Accordingly, there
was less difference between the largest and the middle sized than between the latter and the
smallest bulb.
An interesting observation was made in this experiment. The three bulbs were suspended from
a straight bare wire connected to a terminal of a coil, the largest bulb being placed at the end of the
wire, at some distance from it the smallest bulb, and at an equal distance from the latter the
middle-sized one. The carbons glowed then in both the larger bulbs about as expected, but the
smallest did not get its share by far. This observation led me to exchange the position of the bulbs,
and I then observed that whichever of the bulbs was in the middle was by far less bright than it
was in any other position. This mystifying result was, of course, found to be due to the
electrostatic action between the bulbs. When they were placed at a considerable distance, or when
they were attached to the corners of an equilateral triangle of copper wire, they glowed in about
the order determined by their surfaces.
As to the shape of the vessel, it is also of some importance, especially at high degrees of
exhaustion. Of all the possible constructions, it seems that a spherical globe with the refractory
body mounted in its centre is the best to employ. By experience it has been demonstrated that in
such a globe a refractory body of a given bulk is more easily brought to incandescence than when
differently shaped bulbs are used. There is also an advantage in giving to the incandescent body
the shape of a sphere, for self-evident reasons. In any case the body should be mounted in the
centre, where the atoms rebounding from the glass collide. This object is best attained in the
spherical bulb; but it is also attained in a cylindrical vessel with one or two straight filaments
coinciding with its axis, and possibly also in parabolical or spherical bulbs with refractory body or
bodies placed in the focus or foci of the same; though the latter is not probable, as the electrified
atoms should in all cases rebound normally from the surface they strike, unless the speed were
excessive, in which case they would probably follow the general law of reflection. No matter what
shape the vessel may have, if the exhaustion be low, a filament mounted in the globe is brought to
the same degree of incandescence in all parts; but if the exhaustion be high and the bulb be
spherical or pear-shaped, as usual, focal points form and the filament is heated to a higher degree
at or near such points.
To illustrate the effect, I have here two small bulbs which are alike, only one is exhausted to a
low and the other to a very high degree. When connected to the coil, the filament in the former
glows uniformly throughout all its length; whereas in the latter, that portion of the filament which
is in the centre of the bulb glows far more intensely than the rest. A curious point is that the
phenomenon occurs even if two filaments are mounted in a bulb, each being connected to one
terminal of the coil, and, what is still more curious, if they be very near together, provided the
vacuum be very high. I noted in experiments with such bulbs that the filaments would give way
usually at a certain point, and in the first trials I attributed it to a defect in the carbon. But when the
phenomenon occurred many times in succession I recognized its real cause.

In order to bring a refractory body inclosed in a bulb to incandescence, it is desirable, on
account of economy, that all the energy supplied to the bulb from the source should reach without
loss the body to be heated; from there, and from nowhere else, it should be radiated. It is, of
course, out of the question to reach this theoretical result, but it is possible by a proper construction
of the illuminating device to approximate it more or less.
For many reasons, the refractory body is placed in the centre of the bulb, and it is usually
supported on a glass stem containing the leading-in wire. As the potential of this wire is alternated,
the rarefied gas surrounding the stem is acted upon inductively, and the glass stem is violently
bombarded and heated. In this manner by far the greater portion of the energy supplied to the bulb
—especially when exceedingly high frequencies are used—may be lost for the purpose
contemplated. To obviate this loss, or at least to reduce it to a minimum, I usually screen the
rarefied gas surrounding the stem from the inductive action of the leading-in wire by providing the
stem with a tube or coating of conducting material. It seems beyond doubt that the best among
metals to employ for this purpose is aluminum, on account of its many remarkable properties. Its
only fault is that it is easily fusible, and, therefore, its distance from the incandescing body should
be properly estimated. Usually, a thin tube, of a diameter somewhat smaller than that of the glass
stem, is made of the finest aluminum sheet, and slipped on the stem. The tube is conveniently
prepared by wrapping around a rod fastened in a lathe a piece of aluminum sheet of proper size,
grasping the sheet firmly with clean chamois leather or blotting paper, and spinning the rod very
fast. The sheet is wound tightly around the rod, and a highly polished tube of one or three layers of
the sheet is obtained. When slipped on the stem, the pressure is generally sufficient to prevent it
from slipping off, but, for safety, the lower edge of the sheet may be turned inside. The upper
inside corner of the sheet—that is, the one which is nearest to the refractory incandescent body—
should be cut out diagonally, as it often happens that, in consequence of the intense heat, this
corner turns toward the inside and comes very near to, or in contact with, the wire, or filament,
supporting the refractory body. The greater part of the energy supplied to the bulb is then used up
in heating the metal tube, and the bulb is rendered useless for the purpose. The aluminum sheet
should project above the glass stem more or less—one inch or so—or else, if the glass be too close
to the incandescing body, it may be strongly heated and become more or less conducting,
whereupon it may be ruptured, or may, by its conductivity, establish a good electrical connection
between the metal tube and the leading-in wire, in which case, again, most of the energy will be
lost in heating the former. Perhaps the best way is to make the top of the glass tube, for about an
inch, of a much smaller diameter. To still further reduce the danger arising from the heating of the
glass stem, and also with the view of preventing an electrical connection between the metal tube
and the electrode, I preferably wrap the stem with several layers of thin mica, which extends at
least as far as the metal tube. In some bulbs I have also used an outside insulating cover.
The preceding remarks are only made to aid the experimenter in the first trials, for the
difficulties which he encounters he may soon find means to overcome in his own way.
To illustrate the effect of the screen, and the advantage of using it, I have here two bulbs of the
same size, with their stems, leading-in wires and incandescent lamp filaments tied to the latter, as
nearly alike as possible. The stem of one bulb is provided with an aluminum tube, the stem of the
other has none. Originally the two bulbs were joined by a tube which was connected to a Sprengel
pump. When a high vacuum had been reached, first the connecting tube, and then the bulbs, were
sealed off; they are therefore of the same degree of exhaustion. When they are separately
connected to the coil giving a certain potential, the carbon filament in the bulb provided with the
aluminum screen is rendered highly incandescent, while the filament in the other bulb may, with

the same potential, not even come to redness, although in reality the latter bulb takes generally
more energy than the former. When they are both connected together to the terminal, the difference
is even more apparent, showing the importance of the screening. The metal tube placed on the
stem containing the leading-in wire performs really two distinct functions: First, it acts more or
less as an electrostatic screen, thus economizing the energy supplied to the bulb; and, second, to
whatever extent it may fail to act electrostatically, it acts mechanically, preventing the
bombardment, and consequently intense heating and possible deterioration of the slender support
of the refractory incandescent body, or of the glass stem containing the leading-in wire. I say
slender support, for it is evident that in order to confine the heat more completely to the
incandescing body its support should be very thin, so as to carry away the smallest possible
amount of heat by conduction. Of all the supports used I have found an ordinary incandescent
lamp filament to be the best, principally because among conductors it can withstand the highest
degree of heat.
The effectiveness of the metal tube as an electrostatic screen depends largely on the degree of
exhaustion.
At excessively high degrees of exhaustion—which are reached by using great care and special
means in connection with the Sprengel pump—when the matter in the globe is in the ultra-radiant
state, it acts most perfectly. The shadow of the upper edge of the tube is then sharply defined upon
the bulb.
At a somewhat lower degree of exhaustion, which is about the ordinary "non-striking" vacuum,
and generally as long as the matter moves predominantly in straight lines, the screen still does
well. In elucidation of the preceding remark it is necessary to state that what is a "non-striking"
vacuum for a coil operated as ordinarily, by impulses, or currents, of low frequency, is not so, by
far, when the coil is operated by currents of very high frequency. In such case the discharge may
pass with great freedom through the rarefied gas through which a low frequency discharge may not
pass, even though the potential be much higher. At ordinary atmospheric pressures just the reverse
rule holds good: the higher the frequency, the less the spark discharge is able to jump between the
terminals, especially if they are knobs or spheres of some size.
Finally, at very low degrees of exhaustion, when the gas is well conducting, the metal tube not
only does not act as an electrostatic screen, but even is a drawback, aiding to a considerable extent
the dissipation of the energy laterally from the leading-in wire. This, of course, is to be expected.
In this case, namely, the metal tube is in good electrical connection with the leading-in wire, and
most of the bombardment is directed upon the tube. As long as the electrical connection is not
good, the conducting tube is always of some advantage, for although it may not greatly economize
energy, still it protects the support of the refractory button, and is the means of concentrating more
energy upon the same.
To whatever extent the aluminum tube performs the function of a screen, its usefulness is
therefore limited to very high degrees of exhaustion when it is insulated from the electrode—that
is, when the gas as a whole is non-conducting, and the molecules, or atoms, act as independent
carriers of electric charges.
In addition to acting as a more or less effective screen, in the true meaning of the word, the
conducting tube or coating may also act, by reason of its conductivity, as a sort of equalizer or
dampener of the bombardment against the stem. To be explicit, I assume the action to be as
follows: Suppose a rhythmical bombardment to occur against the conducting tube by reason of its

imperfect action as a screen, it certainly must happen that some molecules, or atoms, strike the
tube sooner than others. Those which come first in contact with it give up their superfluous charge,
and the tube is electrified, the electrification instantly spreading over its surface. But this must
diminish the energy lost in the bombardment, for two reasons: first, the charge given up by the
atoms spreads over a great area, and hence the electric density at any point is small, and the atoms
are repelled with less energy than they would be if they struck against a good insulator; secondly,
as the tube is electrified by the atoms which first come in contact with it, the progress of the
following atoms against the tube is more or less checked by the repulsion which the electrified
tube must exert upon the similarly electrified atoms. This repulsion may perhaps be sufficient to
prevent a large portion of the atoms from striking the tube, but at any rate it must diminish the
energy of their impact. It is clear that when the exhaustion is very low, and the rarefied gas well
conducting, neither of the above effects can occur, and, on the other hand, the fewer the atoms,
with the greater freedom they move; in other words, the higher the degree of exhaustion, up to a
limit, the more telling will be both the effects.

Fig. 147. Fig. 148.

What I have just said may afford an explanation of the phenomenon observed by Prof.
Crookes, namely, that a discharge through a bulb is established with much greater facility when an
insulator than when a conductor is present in the same. In my opinion, the conductor acts as a
dampener of the motion of the atoms in the two ways pointed out; hence, to cause a visible
discharge to pass through the bulb, a much higher potential is needed if a conductor, especially of
much surface, be present.
For the sake of elucidating of some of the remarks before made, I must now refer to Figs. 147,
148 and 149, which illustrate various arrangements with a type of bulb most generally used.

Fig. 147 is a section through a spherical bulb L, with the glass stem s, contains the leading-in
wire w, which has a lamp filament l fastened to it, serving to support the refractory button m in the
centre. M is a sheet of thin mica wound in several layers around the stem s, and a is the aluminum
tube.
Fig. 148 illustrates such a bulb in a somewhat more advanced stage of perfection. A metallic
tube S is fastened by means of some cement to the neck of the tube. In the tube is screwed a plug P,
of insulating material, in the centre of which is fastened a metallic terminal t, for the connection to
the leading-in wire w. This terminal must be well insulated from the metal tube S; therefore, if the
cement used is conducting—and most generally it is sufficiently so—the space between the plug P
and the neck of the bulb should be filled with some good insulating material, such as mica powder.
Fig. 149 shows a bulb made for experimental purposes. In this bulb the aluminum tube is
provided with an external connection, which serves to investigate the effect of the tube under
various conditions. It is referred to chiefly to suggest a line of experiment followed.
Since the bombardment against the stem containing the leading-in wire is due to the inductive
action of the latter upon the rarefied gas, it is of advantage to reduce this action as far as
practicable by employing a very thin wire, surrounded by a very thick insulation of glass or other
material, and by making the wire passing through the rarefied gas as short as practicable. To
combine these features I employ a large tube T (Fig. 150), which protrudes into the bulb to some
distance, and carries on the top a very short glass stem s, into which is sealed the leading-in wire
w, and I protect the top of the glass stem against the heat by a small aluminum tube a and a layer
of mica underneath the same, as usual. The wire w, passing through the large tube to the outside of
the bulb, should be well insulated—with a glass tube, for instance—and the space between ought
to be filled out with some excellent insulator. Among many insulating powders I have found that
mica powder is the best to employ. If this precaution is not taken, the tube T, protruding into the
bulb, will surely be cracked in consequence of the heating by the brushes which are apt to form in
the upper part of the tube, near the exhausted globe, especially if the vacuum be excellent, and
therefore the potential necessary to operate the lamp be very high.

Fig. 149. Fig. 150.

Fig. 151 illustrates a similar arrangement, with a large tube T protruding into the part of the
bulb containing the refractory button m. In this case the wire leading from the outside into the bulb
is omitted, the energy required being supplied through condenser coatings C C. The insulating
packing P should in this construction be tightly fitting to the glass, and rather wide, or otherwise
the discharge might avoid passing through the wire w, which connects the inside condenser coating
to the incandescent button m.
The molecular bombardment against the glass stem in the bulb is a source of great trouble. As
an illustration I will cite a phenomenon only too frequently and unwillingly observed. A bulb,
preferably a large one, may be taken, and a good conducting body, such as a piece of carbon, may
be mounted in it upon a platinum wire sealed in the glass stem. The bulb may be exhausted to a
fairly high degree, nearly to the point when phosphorescence begins to appear. When the bulb is
connected with the coil, the piece of carbon, if small, may become highly incandescent at first, but
its brightness immediately diminishes, and then the discharge may break through the glass
somewhere in the middle of the stem, in the form of bright sparks, in spite of the fact that the
platinum wire is in good electrical connection with the rarefied gas through the piece of carbon or
metal at the top. The first sparks are singularly bright, recalling those drawn from a clear surface of
mercury. But, as they heat the glass rapidly, they, of course, lose their brightness, and cease when
the glass at the ruptured place becomes incandescent, or generally sufficiently hot to conduct.
When observed for the first time the phenomenon must appear very curious, and shows in a
striking manner how radically different alternate currents, or impulses, of high frequency behave,
as compared with steady currents, or currents of low frequency. With such currents—namely, the
latter—the phenomenon would of course not occur. When frequencies such as are obtained by
mechanical means are used, I think that the rupture of the glass is more or less the consequence of

the bombardment, which warms it up and impairs its insulating power; but with frequencies
obtainable with condensers I have no doubt that the glass may give way without previous heating.
Although this appears most singular at first, it is in reality what we might expect to occur. The
energy supplied to the wire leading into the bulb is given off partly by direct action through the
carbon button, and partly by inductive action through the glass surrounding the wire. The case is
thus analogous to that in which a condenser shunted by a conductor of low resistance is connected
to a source of alternating current. As long as the frequencies are low, the conductor gets the most
and the condenser is perfectly safe; but when the frequency becomes excessive, the role of the
conductor may become quite insignificant. In the latter case the difference of potential at the
terminals of the condenser may become so great as to rupture the dielectric, notwithstanding the
fact that the terminals are joined by a conductor of low resistance.
It is, of course, not necessary, when it is desired to produce the incandescence of a body
inclosed in a bulb by means of these currents, that the body should be a conductor, for even a
perfect non-conductor may be quite as readily heated. For this purpose it is sufficient to surround a
conducting electrode with a non-conducting material, as, for instance, in the bulb described before
in Fig. 150, in which a thin incandescent lamp filament is coated with a non-conductor, and
supports a button of the same material on the top. At the start the bombardment goes on by
inductive action through the non-conductor, until the same is sufficiently heated to become
conducting, when the bombardment continues in the ordinary way.

Fig. 151. Fig. 152.

A different arrangement used in some of the bulbs constructed is illustrated in Fig. 152. In this
instance a non-conductor m is mounted in a piece of common arc light carbon so as to project
some small distance above the latter. The carbon piece is connected to the leading-in wire passing
through a glass stem, which is wrapped with several layers of mica. An aluminum tube a is
employed as usual for screening. It is so arranged that it reaches very nearly as high as the carbon
and only the non-conductor m projects a little above it. The bombardment goes at first against the
upper surface of carbon, the lower parts being protected by the aluminum tube. As soon, however,
as the non-conductor m is heated it is rendered good conducting, and then it becomes the centre of
the bombardment, being most exposed to the same.
I have also constructed during these experiments many such single-wire bulbs with or without
internal electrode, in which the radiant matter was projected against, or focused upon, the body to
be rendered incandescent. Fig. 153 (page 263) illustrates one of the bulbs used. It consists of a
spherical globe L, provided with a long neck n, on top, for increasing the action in some cases by
the application of an external conducting coating. The globe L is blown out on the bottom into a
very small bulb b, which serves to hold it firmly in a socket S of insulating material into which it is
cemented. A fine lamp filament f, supported on a wire w, passes through the centre of the globe L.
The filament is rendered incandescent in the middle portion, where the bombardment proceeding
from the lower inside surface of the globe is most intense. The lower portion of the globe, as far as
the socket S reaches, is rendered conducting, either by a tinfoil coating or otherwise, and the
external electrode is connected to a terminal of the coil.
The arrangement diagrammatically indicated in Fig. 153 was found to be an inferior one when
it was desired to render incandescent a filament or button supported in the centre of the globe, but
it was convenient when the object was to excite phosphorescence.
In many experiments in which bodies of different kind were mounted in the bulb as, for
instance, indicated in Fig. 152, some observations of interest were made.
It was found, among other things, that in such cases, no matter where the bombardment began,
just as soon as a high temperature was reached there was generally one of the bodies which
seemed to take most of the bombardment upon itself, the other, or others, being thereby relieved.
The quality appeared to depend principally on the point of fusion, and on the facility with which
the body was "evaporated," or, generally speaking, disintegrated—meaning by the latter term not
only the throwing off of atoms, but likewise of large lumps. The observation made was in
accordance with generally accepted notions. In a highly exhausted bulb, electricity is carried off
from the electrode by independent carriers, which are partly the atoms, or molecules, of the
residual atmosphere, and partly the atoms, molecules, or lumps thrown off from the electrode. If
the electrode is composed of bodies of different character, and if one of these is more easily
disintegrated than the other, most of the electricity supplied is carried off from that body, which is
then brought to a higher temperature than the others, and this the more, as upon an increase of the
temperature the body is still more easily disintegrated.
It seems to me quite probable that a similar process takes place in the bulb even with a
homogeneous electrode, and I think it to be the principal cause of the disintegration. There is
bound to be some irregularity, even if the surface is highly polished, which, of course, is
impossible with most of the refractory bodies employed as electrodes. Assume that a point of the
electrode gets hotter; instantly most of the discharge passes through that point, and a minute patch
it probably fused and evaporated. It is now possible that in consequence of the violent
disintegration the spot attacked sinks in temperature, or that a counter force is created, as in an arc;

at any rate, the local tearing off meets with the limitations incident to the experiment, whereupon
the same process occurs on another place. To the eye the electrode appears uniformly brilliant, but
there are upon it points constantly shifting and wandering around, of a temperature far above the
mean, and this materially hastens the process of deterioration. That some such thing occurs, at
least when the electrode is at a lower temperature, sufficient experimental evidence can be
obtained in the following manner: Exhaust a bulb to a very high degree, so that with a fairly high
potential the discharge cannot pass—that is, not a luminous one, for a weak invisible discharge
occurs always, in all probability. Now raise slowly and carefully the potential, leaving the primary
current on no more than for an instant. At a certain point, two, three, or half a dozen
phosphorescent spots will appear on the globe. These places of the glass are evidently more
violently bombarded than others, this being due to the unevenly distributed electric density,
necessitated, of course, by sharp projections, or, generally speaking, irregularities of the electrode.
But the luminous patches are constantly changing in position, which is especially well observable
if one manages to produce very few, and this indicates that the configuration of the electrode is
rapidly changing.
From experiences of this kind I am led to infer that, in order to be most durable, the refractory
button in the bulb should be in the form of a sphere with a highly polished surface. Such a small
sphere could be manufactured from a diamond or some other crystal, but a better way would be to
fuse, by the employment of extreme degrees of temperature, some oxide—as, for instance, zirconia
—into a small drop, and then keep it in the bulb at a temperature somewhat below its point of
fusion.
Interesting and useful results can, no doubt, be reached in the direction of extreme degrees of
heat. How can such high temperatures be arrived at? How are the highest degrees of heat reached
in nature? By the impact of stars, by high speeds and collisions. In a collision any rate of heat
generation may be attained. In a chemical process we are limited. When oxygen and hydrogen
combine, they fall, metaphorically speaking, from a definite height. We cannot go very far with a
blast, nor by confining heat in a furnace, but in an exhausted bulb we can concentrate any amount
of energy upon a minute button. Leaving practicability out of consideration, this, then, would be
the means which, in my opinion, would enable us to reach the highest temperature. But a great
difficulty when proceeding in this way is encountered, namely, in most cases the body is carried
off before it can fuse and form a drop. This difficulty exists principally with an oxide, such as
zirconia, because it cannot be compressed in so hard a cake that it would not be carried off quickly.
I have endeavored repeatedly to fuse zirconia, placing it in a cup of arc light carbon, as indicated
in Fig. 152. It glowed with a most intense light, and the stream of the particles projected out of the
carbon cup was of a vivid white; but whether it was compressed in a cake or made into a paste
with carbon, it was carried off before it could be fused. The carbon cup, containing zirconia, had to
be mounted very low in the neck of a large bulb, as the heating of the glass by the projected
particles of the oxide was so rapid that in the first trial the bulb was cracked almost in an instant,
when the current was turned on. The heating of the glass by the projected particles was found to be
always greater when the carbon cup contained a body which was rapidly carried off—I presume,
because in such cases, with the same potential, higher speeds were reached, and also because, per
unit of time, more matter was projected—that is, more particles would strike the glass.
The before-mentioned difficulty did not exist, however, when the body mounted in the carbon
cup offered great resistance to deterioration. For instance, when an oxide was first fused in an
oxygen blast, and then mounted in the bulb, it melted very readily into a drop.

Generally, during the process of fusion, magnificent light effects were noted, of which it would
be difficult to give an adequate idea. Fig. 152 is intended to illustrate the effect observed with a
ruby drop. At first one may see a narrow funnel of white light projected against the top of the
globe, where it produces an irregularly outlined phosphorescent patch. When the point of the ruby
fuses, the phosphorescence becomes very powerful; but as the atoms are projected with much
greater speed from the surface of the drop, soon the glass gets hot and "tired," and now only the
outer edge of the patch glows. In this manner an intensely phosphorescent, sharply defined line, l,
corresponding to the outline of the drop, is produced, which spreads slowly over the globe as the
drop gets larger. When the mass begins to boil, small bubbles and cavities are formed, which cause
dark colored spots to sweep across the globe. The bulb may be turned downward without fear of
the drop falling off, as the mass possesses considerable viscosity.
I may mention here another feature of some interest, which I believe to have noted in the
course of these experiments, though the observations do not amount to a certitude. It appeared that
under the molecular impact caused by the rapidly alternating potential, the body was fused and
maintained in that state at a lower temperature in a highly exhausted bulb than was the case at
normal pressure and application of heat in the ordinary way—that is, at least, judging from the
quantity of the light emitted. One of the experiments performed may be mentioned here by way of
illustration. A small piece of pumice stone was stuck on a platinum wire, and first melted to it in a
gas burner. The wire was next placed between two pieces of charcoal, and a burner applied, so as
to produce an intense heat, sufficient to melt down the pumice stone into a small glass-like button.
The platinum wire had to be taken of sufficient thickness, to prevent its melting in the fire. While
in the charcoal fire, or when held in a burner to get a better idea of the degree of heat, the button
glowed with great brilliancy. The wire with the button was then mounted in a bulb, and upon
exhausting the same to a high degree, the current was turned on slowly, so as to prevent the
cracking of the button. The button was heated to the point of fusion, and when it melted, it did not,
apparently, glow with the same brilliancy as before, and this would indicate a lower temperature.
Leaving out of consideration the observer's possible, and even probable, error, the question is, can
a body under these conditions be brought from a solid to a liquid state with the evolution of less
light?
When the potential of a body is rapidly alternated, it is certain that the structure is jarred. When
the potential is very high, although the vibrations may be few—say 20,000 per second—the effect
upon the structure may be considerable. Suppose, for example, that a ruby is melted into a drop by
a steady application of energy. When it forms a drop, it will emit visible and invisible waves,
which will be in a definite ratio, and to the eye the drop will appear to be of a certain brilliancy.
Next, suppose we diminish to any degree we choose the energy steadily supplied, and, instead,
supply energy which rises and falls according to a certain law. Now, when the drop is formed,
there will be emitted from it three different kinds of vibrations—the ordinary visible, and two
kinds of invisible waves: that is, the ordinary dark waves of all lengths, and, in addition, waves of
a well defined character. The latter would not exist by a steady supply of the energy; still they help
to jar and loosen the structure. If this really be the case, then the ruby drop will emit relatively less
visible and more invisible waves than before. Thus it would seem that when a platinum wire, for
instance, is fused by currents alternating with extreme rapidity, it emits at the point of fusion less
light and more visible radiation than it does when melted by a steady current, though the total
energy used up in the process of fusion is the same in both cases. Or, to cite another example, a
lamp filament is not capable of withstanding as long with currents of extreme frequency as it does
with steady currents, assuming that it be worked at the same luminous intensity. This means that
for rapidly alternating currents the filament should be shorter and thicker. The higher the frequency

—that is, the greater the departure from the steady flow—the worse it would be for the filament.
But if the truth of this remark were demonstrated, it would be erroneous to conclude that such a
refractory button as used in these bulbs would be deteriorated quicker by currents of extremely
high frequency than by steady or low frequency currents. From experience I may say that just the
opposite holds good: the button withstands the bombardment better with currents of very high
frequency. But this is due to the fact that a high frequency discharge passes through a rarefied gas
with much greater freedom than a steady or low frequency discharge, and this will mean that with
the former we can work with a lower potential or with a less violent impact. As long, then, as the
gas is of no consequence, a steady or low frequency current is better; but as soon as the action of
the gas is desired and important, high frequencies are preferable.
In the course of these experiments a great many trials were made with all kinds of carbon
buttons. Electrodes made of ordinary carbon buttons were decidedly more durable when the
buttons were obtained by the application of enormous pressure. Electrodes prepared by depositing
carbon in well known ways did not show up well; they blackened the globe very quickly. From
many experiences I conclude that lamp filaments obtained in this manner can be advantageously
used only with low potentials and low frequency currents. Some kinds of carbon withstand so well
that, in order to bring them to the point of fusion, it is necessary to employ very small buttons. In
this case the observation is rendered very difficult on account of the intense heat produced.
Nevertheless there can be no doubt that all kinds of carbon are fused under the molecular
bombardment, but the liquid state must be one of great instability. Of all the bodies tried there
were two which withstood best—diamond and carborundum. These two showed up about equally,
but the latter was preferable for many reasons. As it is more than likely that this body is not yet
generally known, I will venture to call your attention to it.
It has been recently produced by Mr. E. G. Acheson, of Monongahela City, Pa., U. S. A. It is
intended to replace ordinary diamond powder for polishing precious stones, etc., and I have been
informed that it accomplishes this object quite successfully. I do not know why the name
"carborundum" has been given to it, unless there is something in the process of its manufacture
which justifies this selection. Through the kindness of the inventor, I obtained a short while ago
some samples which I desired to test in regard to their qualities of phosphorescence and capability
of withstanding high degrees of heat.
Carborundum can be obtained in two forms—in the form of "crystals" and of powder. The
former appear to the naked eye dark colored, but are very brilliant; the latter is of nearly the same
color as ordinary diamond powder, but very much finer. When viewed under a microscope the
samples of crystals given to me did not appear to have any definite form, but rather resembled
pieces of broken up egg coal of fine quality. The majority were opaque, but there were some which
were transparent and colored. The crystals are a kind of carbon containing some impurities; they
are extremely hard, and withstand for a long time even an oxygen blast. When the blast is directed
against them they at first form a cake of some compactness, probably in consequence of the fusion
of impurities they contain. The mass withstands for a very long time the blast without further
fusion; but a slow carrying off, or burning, occurs, and, finally, a small quantity of a glass-like
residue is left, which, I suppose, is melted alumina. When compressed strongly they conduct very
well, but not as well as ordinary carbon. The powder, which is obtained from the crystals in some
way, is practically non-conducting. It affords a magnificent polishing material for stones.
The time has been too short to make a satisfactory study of the properties of this product, but
enough experience has been gained in a few weeks I have experimented upon it to say that it does

possess some remarkable properties in many respects. It withstands excessively high degrees of
heat, it is little deteriorated by molecular bombardment, and it does not blacken the globe as
ordinary carbon does. The only difficulty which I have experienced in its use in connection with
these experiments was to find some binding material which would resist the heat and the effect of
the bombardment as successfully as carborundum itself does.
I have here a number of bulbs which I have provided with buttons of carborundum. To make
such a button of carborundum crystals I proceed in the following manner: I take an ordinary lamp
filament and dip its point in tar, or some other thick substance or paint which may be readily
carbonized. I next pass the point of the filament through the crystals, and then hold it vertically
over a hot plate. The tar softens and forms a drop on the point of the filament, the crystals adhering
to the surface of the drop. By regulating the distance from the plate the tar is slowly dried out and
the button becomes solid. I then once more dip the button in tar and hold it again over a plate until
the tar is evaporated, leaving only a hard mass which firmly binds the crystals. When a larger
button is required I repeat the process several times, and I generally also cover the filament a
certain distance below the button with crystals. The button being mounted in a bulb, when a good
vacuum has been reached, first a weak and then a strong discharge is passed through the bulb to
carbonize the tar and expel all gases, and later it is brought to a very intense incandescence.
When the powder is used I have found it best to proceed as follows: I make a thick paint of
carborundum and tar, and pass a lamp filament through the paint. Taking then most of the paint off
by rubbing the filament against a piece of chamois leather, I hold it over a hot plate until the tar
evaporates and the coating becomes firm. I repeat this process as many times as it is necessary to
obtain a certain thickness of coating. On the point of the coated filament I form a button in the
same manner.
There is no doubt that such a button—properly prepared under great pressure—of
carborundum, especially of powder of the best quality, will withstand the effect of the
bombardment fully as well as anything we know. The difficulty is that the binding material gives
way, and the carborundum is slowly thrown off after some time. As it does not seem to blacken the
globe in the least, it might be found useful for coating the filaments of ordinary incandescent
lamps, and I think that it is even possible to produce thin threads or sticks of carborundum which
will replace the ordinary filaments in an incandescent lamp. A carborundum coating seems to be
more durable than other coatings, not only because the carborundum can withstand high degrees of
heat, but also because it seems to unite with the carbon better than any other material I have tried.
A coating of zirconia or any other oxide, for instance, is far more quickly destroyed. I prepared
buttons of diamond dust in the same manner as of carborundum, and these came in durability
nearest to those prepared of carborundum, but the binding paste gave way much more quickly in
the diamond buttons; this, however, I attributed to the size and irregularity of the grains of the
diamond.
It was of interest to find whether carborundum possesses the quality of phosphorescence. One
is, of course, prepared to encounter two difficulties: first, as regards the rough product, the
"crystals," they are good conducting, and it is a fact that conductors do not phosphoresce; second,
the powder, being exceedingly fine, would not be apt to exhibit very prominently this quality, since
we know that when crystals, even such as diamond or ruby, are finely powdered, they lose the
property of phosphorescence to a considerable degree.
The question presents itself here, can a conductor phosphoresce? What is there in such a body
as a metal, for instance, that would deprive it of the quality of phosphoresence, unless it is that

property which characterizes it as a conductor? For it is a fact that most of the phosphorescent
bodies lose that quality when they are sufficiently heated to become more or less conducting.
Then, if a metal be in a large measure, or perhaps entirely, deprived of that property, it should be
capable of phosphoresence. Therefore it is quite possible that at some extremely high frequency,
when behaving practically as a non-conductor, a metal or any other conductor might exhibit the
quality of phosphoresence, even though it be entirely incapable of phosphorescing under the
impact of a low-frequency discharge. There is, however, another possible way how a conductor
might at least appear to phosphoresce.
Considerable doubt still exists as to what really is phosphorescence, and as to whether the
various phenomena comprised under this head are due to the same causes. Suppose that in an
exhausted bulb, under the molecular impact, the surface of a piece of metal or other conductor is
rendered strongly luminous, but at the same time it is found that it remains comparatively cool,
would not this luminosity be called phosphorescence? Now such a result, theoretically at least, is
possible, for it is a mere question of potential or speed. Assume the potential of the electrode, and
consequently the speed of the projected atoms, to be sufficiently high, the surface of the metal
piece, against which the atoms are projected, would be rendered highly incandescent, since the
process of heat generation would be incomparably faster than that of radiating or conducting away
from the surface of the collision. In the eye of the observer a single impact of the atoms would
cause an instantaneous flash, but if the impacts were repeated with sufficient rapidity, they would
produce a continuous impression upon his retina. To him then the surface of the metal would
appear continuously incandescent and of constant luminous intensity, while in reality the light
would be either intermittent, or at least changing periodically in intensity. The metal piece would
rise in temperature until equilibrium was attained—that is, until the energy continuously radiated
would equal that intermittently supplied. But the supplied energy might under such conditions not
be sufficient to bring the body to any more than a very moderate mean temperature, especially if
the frequency of the atomic impacts be very low—just enough that the fluctuation of the intensity
of the light emitted could not be detected by the eye. The body would now, owing to the manner in
which the energy is supplied, emit a strong light, and yet be at a comparatively very low mean
temperature. How should the observer name the luminosity thus produced? Even if the analysis of
the light would teach him something definite, still he would probably rank it under the phenomena
of phosphorescence. It is conceivable that in such a way both conducting and non-conducting
bodies may be maintained at a certain luminous intensity, but the energy required would very
greatly vary with the nature and properties of the bodies.
These and some foregoing remarks of a speculative nature were made merely to bring out
curious features of alternate currents or electric impulses. By their help we may cause a body to
emit more light, while at a certain mean temperature, than it would emit if brought to that
temperature by a steady supply; and, again, we may bring a body to the point of fusion, and cause
it to emit less light than when fused by the application of energy in ordinary ways. It all depends
on how we supply the energy, and what kind of vibrations we set up; in one case the vibrations are
more, in the other less, adapted to affect our sense of vision.
Some effects, which I had not observed before, obtained with carborundum in the first trials, I
attributed to phosphorescence, but in subsequent experiments it appeared that it was devoid of that
quality. The crystals possess a noteworthy feature. In a bulb provided with a single electrode in the
shape of a small circular metal disc, for instance, at a certain degree of exhaustion the electrode is
covered with a milky film, which is separated by a dark space from the glow filling the bulb.
When the metal disc is covered with carborundum crystals, the film is far more intense, and snow-

white. This I found later to be merely an effect of the bright surface of the crystals, for when an
aluminum electrode was highly polished, it exhibited more or less the same phenomenon. I made a
number of experiments with the samples of crystals obtained, principally because it would have
been of special interest to find that they are capable of phosphorescence, on account of their being
conducting. I could not produce phosphorescence distinctly, but I must remark that a decisive
opinion cannot be formed until other experimenters have gone over the same ground.
The powder behaved in some experiments as though it contained alumina, but it did not exhibit
with sufficient distinctness the red of the latter. Its dead color brightens considerably under the
molecular impact, but I am now convinced it does not phosphoresce. Still, the tests with the
powder are not conclusive, because powdered carborundum probably does not behave like a
phosphorescent sulphide, for example, which could be finely powdered without impairing the
phosphorescence, but rather like powdered ruby or diamond, and therefore it would be necessary,
in order to make a decisive test, to obtain it in a large lump and polish up the surface.
If the carborundum proves useful in connection with these and similar experiments, its chief
value will be found in the production of coatings, thin conductors, buttons, or other electrodes
capable of withstanding extremely high degrees of heat.
The production of a small electrode, capable of withstanding enormous temperatures, I regard
as of the greatest importance in the manufacture of light. It would enable us to obtain, by means of
currents of very high frequencies, certainly 20 times, if not more, the quantity of light which is
obtained in the present incandescent lamp by the same expenditure of energy. This estimate may
appear to many exaggerated, but in reality I think it is far from being so. As this statement might
be misunderstood, I think it is necessary to expose clearly the problem with which, in this line of
work, we are confronted, and the manner in which, in my opinion, a solution will be arrived at.
Any one who begins a study of the problem will be apt to think that what is wanted in a lamp
with an electrode is a very high degree of incandescence of the electrode. There he will be
mistaken. The high incandescence of the button is a necessary evil, but what is really wanted is the
high incandescence of the gas surrounding the button. In other words, the problem in such a lamp
is to bring a mass of gas to the highest possible incandescence. The higher the incandescence, the
quicker the mean vibration, the greater is the economy of the light production. But to maintain a
mass of gas at a high degree of incandescence in a glass vessel, it will always be necessary to keep
the incandescent mass away from the glass; that is, to confine it as much as possible to the central
portion of the globe.
In one of the experiments this evening a brush was produced at the end of a wire. The brush
was a flame, a source of heat and light. It did not emit much perceptible heat, nor did it glow with
an intense light; but is it the less a flame because it does not scorch my hand? Is it the less a flame
because it does not hurt my eyes by its brilliancy? The problem is precisely to produce in the bulb
such a flame, much smaller in size, but incomparably more powerful. Were there means at hand
for producing electric impulses of a sufficiently high frequency, and for transmitting them, the bulb
could be done away with, unless it were used to protect the electrode, or to economize the energy
by confining the heat. But as such means are not at disposal, it becomes necessary to place the
terminal in the bulb and rarefy the air in the same. This is done merely to enable the apparatus to
perform the work which it is not capable of performing at ordinary air pressure. In the bulb we are
able to intensify the action to any degree—so far that the brush emits a powerful light.

The intensity of the light emitted depends principally on the frequency and potential of the
impulses, and on the electric density on the surface of the electrode. It is of the greatest importance
to employ the smallest possible button, in order to push the density very far. Under the violent
impact of the molecules of the gas surrounding it, the small electrode is of course brought to an
extremely high temperature, but around it is a mass of highly incandescent gas, a flame
photosphere, many hundred times the volume of the electrode. With a diamond, carborundum or
zirconia button the photosphere can be as much as one thousand times the volume of the button.
Without much reflection one would think that in pushing so far the incandescence of the electrode
it would be instantly volatilized. But after a careful consideration one would find that,
theoretically, it should not occur, and in this fact—which, moreover, is experimentally
demonstrated—lies principally the future value of such a lamp.
At first, when the bombardment begins, most of the work is performed on the surface of the
button, but when a highly conducting photosphere is formed the button is comparatively relieved.
The higher the incandescence of the photosphere, the more it approaches in conductivity to that of
the electrode, and the more, therefore, the solid and the gas form one conducting body. The
consequence is that the further the incandescence is forced the more work, comparatively, is
performed on the gas, and the less on the electrode. The formation of a powerful photosphere is
consequently the very means for protecting the electrode. This protection, of course, is a relative
one, and it should not be thought that by pushing the incandescence higher the electrode is actually
less deteriorated. Still, theoretically, with extreme frequencies, this result must be reached, but
probably at a temperature too high for most of the refractory bodies known. Given, then, an
electrode which can withstand to a very high limit the effect of the bombardment and outward
strain, it would be safe, no matter how much it was forced beyond that limit. In an incandescent
lamp quite different considerations apply. There the gas is not at all concerned; the whole of the
work is performed on the filament; and the life of the lamp diminishes so rapidly with the increase
of the degree of incandescence that economical reasons compel us to work it at a low
incandescence. But if an incandescent lamp is operated with currents of very high frequency, the
action of the gas cannot be neglected, and the rules for the most economical working must be
considerably modified.
In order to bring such a lamp with one or two electrodes to a great perfection, it is necessary to
employ impulses of very high frequency. The high frequency secures, among others, two chief
advantages, which have a most important bearing upon the economy of the light production. First,
the deterioration of the electrode is reduced by reason of the fact that we employ a great many
small impacts, instead of a few violent ones, which quickly shatter the structure; secondly, the
formation of a large photosphere is facilitated.
In order to reduce the deterioration of the electrode to the minimum, it is desirable that the
vibration be harmonic, for any suddenness hastens the process of destruction. An electrode lasts
much longer when kept at incandescence by currents, or impulses, obtained from a high frequency
alternator, which rise and fall more or less harmonically, than by impulses obtained from a
disruptive discharge coil. In the latter case there is no doubt that most of the damage is done by the
fundamental sudden discharges.
One of the elements of loss in such a lamp is the bombardment of the globe. As the potential is
very high, the molecules are projected with great speed; they strike the glass, and usually excite a
strong phosphorescence. The effect produced is very pretty, but for economical reasons it would be
perhaps preferable to prevent, or at least reduce to a minimum, the bombardment against the globe,

as in such case it is, as a rule, not the object to excite phosphorescence, and as some loss of energy
results from the bombardment. This loss in the bulb is principally dependent on the potential of the
impulses and on the electric density on the surface of the electrode. In employing very high
frequencies the loss of energy by the bombardment is greatly reduced, for, first, the potential
needed to perform a given amount of work is much smaller; and, secondly, by producing a highly
conducting photosphere around the electrode, the same result is obtained as though the electrode
were much larger, which is equivalent to a smaller electric density. But be it by the diminution of
the maximum potential or of the density, the gain is effected in the same manner, namely, by
avoiding violent shocks, which strain the glass much beyond its limit of elasticity. If the frequency
could be brought high enough, the loss due to the imperfect elasticity of the glass would be
entirely negligible. The loss due to bombardment of the globe may, however, be reduced by using
two electrodes instead of one. In such case each of the electrodes may be connected to one of the
terminals; or else, if it is preferable to use only one wire, one electrode may be connected to one
terminal and the other to the ground or to an insulated body of some surface, as, for instance, a
shade on the lamp. In the latter case, unless some judgment is used, one of the electrodes might
glow more intensely than the other.
But on the whole I find it preferable, when using such high frequencies, to employ only one
electrode and one connecting wire. I am convinced that the illuminating device of the near future
will not require for its operation more than one lead, and, at any rate, it will have no leading-in
wire, since the energy required can be as well transmitted through the glass. In experimental bulbs
the leading-in wire is not generally used on account of convenience, as in employing condenser
coatings in the manner indicated in Fig. 151, for example, there is some difficulty in fitting the
parts, but these difficulties would not exist if a great many bulbs were manufactured; otherwise the
energy can be conveyed through the glass as well as through a wire, and with these high
frequencies the losses are very small. Such illustrating devices will necessarily involve the use of
very high potentials, and this, in the eyes of practical men, might be an objectionable feature. Yet,
in reality, high potentials are not objectionable—certainly not in the least so far as the safety of the
devices is concerned.
There are two ways of rendering an electric appliance safe. One is to use low potentials, the
other is to determine the dimensions of the apparatus so that it is safe, no matter how high a
potential is used. Of the two, the latter seems to me the better way, for then the safety is absolute,
unaffected by any possible combination of circumstances which might render even a low-potential
appliance dangerous to life and property. But the practical conditions require not only the judicious
determination of the dimensions of the apparatus; they likewise necessitate the employment of
energy of the proper kind. It is easy, for instance, to construct a transformer capable of giving,
when operated from an ordinary alternate current machine of low tension, say 50,000 volts, which
might be required to light a highly exhausted phosphorescent tube, so that, in spite of the high
potential, it is perfectly safe, the shock from it producing no inconvenience. Still such a
transformer would be expensive, and in itself inefficient; and, besides, what energy was obtained
from it would not be economically used for the production of light. The economy demands the
employment of energy in the form of extremely rapid vibrations. The problem of producing light
has been likened to that of maintaining a certain high-pitch note by means of a bell. It should be
said a barely audible note; and even these words would not express it, so wonderful is the
sensitiveness of the eye. We may deliver powerful blows at long intervals, waste a good deal of
energy, and still not get what we want; or we may keep up the note by delivering frequent taps, and
get nearer to the object sought by the expenditure of much less energy. In the production of light,
as far as the illuminating device is concerned, there can be only one rule—that is, to use as high

frequencies as can be obtained; but the means for the production and conveyance of impulses of
such character impose, at present at least, great limitations. Once it is decided to use very high
frequencies, the return wire becomes unnecessary, and all the appliances are simplified. By the use
of obvious means the same result is obtained as though the return wire were used. It is sufficient
for this purpose to bring in contact with the bulb, or merely in the vicinity of the same, an
insulated body of some surface. The surface need, of course, be the smaller, the higher the
frequency and potential used, and necessarily, also, the higher the economy of the lamp or other
device.
This plan of working has been resorted to on several occasions this evening. So, for instance,
when the incandescence of a button was produced by grasping the bulb with the hand, the body of
the experimenter merely served to intensify the action. The bulb used was similar to that illustrated
in Fig. 148, and the coil was excited to a small potential, not sufficient to bring the button to
incandescence when the bulb was hanging from the wire; and incidentally, in order to perform the
experiment in a more suitable manner, the button was taken so large that a perceptible time had to
elapse before, upon grasping the bulb, it could be rendered incandescent. The contact with the bulb
was, of course, quite unnecessary. It is easy, by using a rather large bulb with an exceedingly small
electrode, to adjust the conditions so that the latter is brought to bright incandescence by the mere
approach of the experimenter within a few feet of the bulb, and that the incandescence subsides
upon his receding.

Fig. 153. Fig. 154.

In another experiment, when phosphorescence was excited, a similar bulb was used. Here
again, originally, the potential was not sufficient to excite phosphorescence until the action was
intensified—in this case, however, to present a different feature, by touching the socket with a
metallic object held in the hand. The electrode in the bulb was a carbon button so large that it
could not be brought to incandescence, and thereby spoil the effect produced by phosphorescence.
Again, in another of the early experiments, a bulb was used, as illustrated in Fig. 141. In this
instance, by touching the bulb with one or two fingers, one or two shadows of the stem inside were
projected against the glass, the touch of the finger producing the same results as the application of
an external negative electrode under ordinary circumstances.
In all these experiments the action was intensified by augmenting the capacity at the end of the
lead connected to the terminal. As a rule, it is not necessary to resort to such means, and would be
quite unnecessary with still higher frequencies; but when it is desired, the bulb, or tube, can be
easily adapted to the purpose.
In Fig. 153, for example, an experimental bulb, L, is shown, which is provided with a neck, n,
on the top, for the application of an external tinfoil coating, which may be connected to a body of
larger surface. Such a lamp as illustrated in Fig. 154 may also be lighted by connecting the tinfoil
coating on the neck n to the terminal, and the leading-in wire, w, to an insulated plate. If the bulb
stands in a socket upright, as shown in the cut, a shade of conducting material may be slipped in
the neck, n, and the action thus magnified.
A more perfected arrangement used in some of these bulbs is illustrated in Fig. 155. In this
case the construction of the bulb is as shown and described before, when reference was made to
Fig. 148. A zinc sheet, Z, with a tubular extension, T, is applied over the metallic socket, S. The
bulb hangs downward from the terminal, t, the zinc sheet, Z, performing the double office of
intensifier and reflector. The reflector is separated from the terminal, t, by an extension of the
insulating plug, P.
A similar disposition with a phosphorescent tube is illustrated in Fig. 156. The tube, T, is
prepared from two short tubes of different diameter, which are sealed on the ends. On the lower
end is placed an inside conducting coating, C, which connects to the wire w. The wire has a hook
on the upper end for suspension, and passes through the centre of the inside tube, which is filled
with some good and tightly packed insulator. On the outside of the upper end of the tube, T, is
another conducting coating, C1, upon which is slipped a metallic reflector Z, which should be
separated by a thick insulation from the end of wire w.
The economical use of such a reflector or intensifier would require that all energy supplied to
an air condenser should be recoverable, or, in other words, that there should not be any losses,
neither in the gaseous medium nor through its action elsewhere. This is far from being so, but,
fortunately, the losses may be reduced to anything desired. A few remarks are necessary on this
subject, in order to make the experiences gathered in the course of these investigations perfectly
clear.

Fig. 155.
Suppose a small helix with many well insulated turns, as in experiment Fig. 146, has one of its
ends connected to one of the terminals of the induction coil, and the other to a metal plate, or, for
the sake of simplicity, a sphere, insulated in space. When the coil is set to work, the potential of the
sphere is alternated, and a small helix now behaves as though its free end were connected to the
other terminal of the induction coil. If an iron rod be held within a small helix, it is quickly
brought to a high temperature, indicating the passage of a strong current through the helix. How
does the insulated sphere act in this case? It can be a condenser, storing and returning the energy
supplied to it, or it can be a mere sink of energy, and the conditions of the experiment determine
whether it is rather one than the other. The sphere being charged to a high potential, it acts
inductively upon the surrounding air, or whatever gaseous medium there might be. The molecules,
or atoms, which are near the sphere, are of course more attracted, and move through a greater
distance than the farther ones. When the nearest molecules strike the sphere, they are repelled, and
collisions occur at all distances within the inductive action of the sphere. It is now clear that, if the
potential be steady, but little loss of energy can be caused in this way, for the molecules which are
nearest to the sphere, having had an additional charge imparted to them by contact, are not
attracted until they have parted, if not with all, at least with most of the additional charge, which
can be accomplished only after a great many collisions. From the fact, that with a steady potential
there is but little loss in dry air, one must come to such a conclusion. When the potential of a
sphere, instead of being steady, is alternating, the conditions are entirely different. In this case a
rhythmical bombardment occurs, no matter whether the molecules, after coming in contact with

the sphere, lose the imparted charge or not; what is more, if the charge is not lost, the impacts are
only the more violent. Still, if the frequency of the impulses be very small, the loss caused by the
impacts and collisions would not be serious, unless the potential were excessive. But when
extremely high frequencies and more or less high potentials are used, the loss may very great. The
total energy lost per unit of time is proportionate to the product of the number of impacts per
second, or the frequency and the energy lost in each impact. But the energy of an impact must be
proportionate to the square of the electric density of the sphere, since the charge imparted to the
molecule is proportionate to that density. I conclude from this that the total energy lost must be
proportionate to the product of the frequency and the square of the electric density; but this law
needs experimental confirmation. Assuming the preceding considerations to be true, then, by
rapidly alternating the potential of a body immersed in an insulating gaseous medium, any amount
of energy may be dissipated into space. Most of that energy then, I believe, is not dissipated in the
form of long ether waves, propagated to considerable distance, as is thought most generally, but is
consumed—in the case of an insulated sphere, for example—in impact and collisional losses—that
is, heat vibrations—on the surface and in the vicinity of the sphere. To reduce the dissipation, it is
necessary to work with a small electric density—the smaller, the higher the frequency.

Fig. 156.

But since, on the assumption before made, the loss is diminished with the square of the density,
and since currents of very high frequencies involve considerable waste when transmitted through
conductors, it follows that, on the whole, it is better to employ one wire than two. Therefore, if
motors, lamps, or devices of any kind are perfected, capable of being advantageously operated by
currents of extremely high frequency, economical reasons will make it advisable to use only one
wire, especially if the distances are great.
When energy is absorbed in a condenser, the same behaves as though its capacity were
increased. Absorption always exists more or less, but generally it is small and of no consequence
as long as the frequencies are not very great. In using extremely high frequencies, and, necessarily
in such case, also high potentials, the absorption—or, what is here meant more particularly by this
term, the loss of energy due to the presence of a gaseous medium—is an important factor to be
considered, as the energy absorbed in the air condenser may be any fraction of the supplied energy.
This would seem to make it very difficult to tell from the measured or computed capacity of an air
condenser its actual capacity or vibration period, especially if the condenser is of very small
surface and is charged to a very high potential. As many important results are dependent upon the
correctness of the estimation of the vibration period, this subject demands the most careful scrutiny
of other investigators. To reduce the probable error as much as possible in experiments of the kind
alluded to, it is advisable to use spheres or plates of large surface, so as to make the density
exceedingly small. Otherwise, when it is practicable, an oil condenser should be used in
preference. In oil or other liquid dielectrics there are seemingly no such losses as in gaseous
media. It being impossible to exclude entirely the gas in condensers with solid dielectrics, such
condensers should be immersed in oil, for economical reasons, if nothing else; they can then be
strained to the utmost, and will remain cool. In Leyden jars the loss due to air is comparatively
small, as the tinfoil coatings are large, close together, and the charged surfaces not directly
exposed; but when the potentials are very high, the loss may be more or less considerable at, or
near, the upper edge of the foil, where the air is principally acted upon. If the jar be immersed in
boiled-out oil, it will be capable of performing four times the amount of work which it can for any
length of time when used in the ordinary way, and the loss will be inappreciable.
It should not be thought that the loss in heat in an air condenser is necessarily associated with
the formation of visible streams or brushes. If a small electrode, inclosed in an unexhausted bulb,
is connected to one of the terminals of the coil, streams can be seen to issue from the electrode,
and the air in the bulb is heated; if instead of a small electrode a large sphere is inclosed in the
bulb, no streams are observed, still the air is heated.
Nor should it be thought that the temperature of an air condenser would give even an
approximate idea of the loss in heat incurred, as in such case heat must be given off much more
quickly, since there is, in addition to the ordinary radiation, a very active carrying away of heat by
independent carriers going on, and since not only the apparatus, but the air at some distance from it
is heated in consequence of the collisions which must occur.
Owing to this, in experiments with such a coil, a rise of temperature can be distinctly observed
only when the body connected to the coil is very small. But with apparatus on a larger scale, even
a body of considerable bulk would be heated, as, for instance, the body of a person; and I think
that skilled physicians might make observations of utility in such experiments, which, if the
apparatus were judiciously designed, would not present the slightest danger.
A question of some interest, principally to meteorologists, presents itself here. How does the
earth behave? The earth is an air condenser, but is it a perfect or a very imperfect one—a mere sink

of energy? There can be little doubt that to such small disturbance as might be caused in an
experiment, the earth behaves as an almost perfect condenser. But it might be different when its
charge is set in vibration by some sudden disturbance occurring in the heavens. In such case, as
before stated, probably only little of the energy of the vibrations set up would be lost into space in
the form of long ether radiations, but most of the energy, I think, would spend itself in molecular
impacts and collisions, and pass off into space in the form of short heat, and possibly light, waves.
As both the frequency of the vibrations of the charge and the potential are in all probability
excessive, the energy converted into heat may be considerable. Since the density must be unevenly
distributed, either in consequence of the irregularity of the earth's surface, or on account of the
condition of the atmosphere in various places, the effect produced would accordingly vary from
place to place. Considerable variations in the temperature and pressure of the atmosphere may in
this manner be caused at any point of the surface of the earth. The variations may be gradual or
very sudden, according to the nature of the general disturbance, and may produce rain and storms,
or locally modify the weather in any way.
From the remarks before made, one may see what an important factor of loss the air in the
neighborhood of a charged surface becomes when the electric density is great and the frequency of
the impulses excessive. But the action, as explained, implies that the air is insulating—that is, that
it is composed of independent carriers immersed in an insulating medium. This is the case only
when the air is at something like ordinary or greater, or at extremely small, pressure. When the air
is slightly rarefied and conducting, then true conduction losses occur also. In such case, of course,
considerable energy may be dissipated into space even with a steady potential, or with impulses of
low frequency, if the density is very great.
When the gas is at very low pressure, an electrode is heated more because higher speeds can be
reached. If the gas around the electrode is strongly compressed, the displacements, and
consequently the speeds, are very small, and the heating is insignificant. But if in such case the
frequency could be sufficiently increased, the electrode would be brought to a high temperature as
well as if the gas were at very low pressure; in fact, exhausting the bulb is only necessary because
we cannot produce, (and possibly not convey) currents of the required frequency.
Returning to the subject of electrode lamps, it is obviously of advantage in such a lamp to
confine as much as possible the heat to the electrode by preventing the circulation of the gas in the
bulb. If a very small bulb be taken, it would confine the heat better than a large one, but it might
not be of sufficient capacity to be operated from the coil, or, if so, the glass might get too hot. A
simple way to improve in this direction is to employ a globe of the required size, but to place a
small bulb, the diameter of which is properly estimated, over the refractory button contained in the
globe. This arrangement is illustrated in Fig. 157.

Fig. 157. Fig. 158.

The globe L has in this case a large neck n, allowing the small bulb b to slip through. Otherwise
the construction is the same as shown in Fig. 147, for example. The small bulb is conveniently
supported upon the stem s, carrying the refractory button m. It is separated from the aluminum
tube a by several layers of mica M, in order to prevent the cracking of the neck by the rapid heating
of the aluminum tube upon a sudden turning on of the current. The inside bulb should be as small
as possible when it is desired to obtain light only by incandescence of the electrode. If it is desired
to produce phosphorescence, the bulb should be larger, else it would be apt to get too hot, and the
phosphorescence would cease. In this arrangement usually only the small bulb shows
phosphorescence, as there is practically no bombardment against the outer globe. In some of these
bulbs constructed as illustrated in Fig. 157, the small tube was coated with phosphorescent paint,
and beautiful effects were obtained. Instead of making the inside bulb large, in order to avoid
undue heating, it answers the purpose to make the electrode m larger. In this case the bombardment
is weakened by reason of the smaller electric density.
Many bulbs were constructed on the plan illustrated in Fig. 158. Here a small bulb b,
containing the refractory button m, upon being exhausted to a very high degree was sealed in a
large globe L, which was then moderately exhausted and sealed off. The principal advantage of this
construction was that it allowed of reaching extremely high vacua, and, at the same time of using a
large bulb. It was found, in the course of experiments with bulbs such as illustrated in Fig. 158,
that it was well to make the stem s, near the seal at e, very thick, and the leading-in wire w thin, as
it occurred sometimes that the stem at e was heated and the bulb was cracked. Often the outer
globe L was exhausted only just enough to allow the discharge to pass through, and the space

between the bulbs appeared crimson, producing a curious effect. In some cases, when the
exhaustion in globe L was very low, and the air good conducting, it was found necessary, in order
to bring the button m to high incandescence, to place, preferably on the upper part of the neck of
the globe, a tinfoil coating which was connected to an insulated body, to the ground, or to the other
terminal of the coil, as the highly conducting air weakened the effect somewhat, probably by being
acted upon inductively from the wire w, where it entered the bulb at e. Another difficulty—which,
however, is always present when the refractory button is mounted in a very small bulb—existed in
the construction illustrated in Fig. 158, namely, the vacuum in the bulb b would be impaired in a
comparatively short time.
The chief idea in the two last described constructions was to confine the heat to the central
portion of the globe by preventing the exchange of air. An advantage is secured, but owing to the
heating of the inside bulb and slow evaporation of the glass, the vacuum is hard to maintain, even
if the construction illustrated in Fig. 157 be chosen, in which both bulbs communicate.
But by far the better way—the ideal way—would be to reach sufficiently high frequencies. The
higher the frequency, the slower would be the exchange of the air, and I think that a frequency may
be reached, at which there would be no exchange whatever of the air molecules around the
terminal. We would then produce a flame in which there would be no carrying away of material,
and a queer flame it would be, for it would be rigid! With such high frequencies the inertia of the
particles would come into play. As the brush, or flame, would gain rigidity in virtue of the inertia
of the particles, the exchange of the latter would be prevented. This would necessarily occur, for,
the number of impulses being augmented, the potential energy of each would diminish, so that
finally only atomic vibrations could be set up, and the motion of translation through measurable
space would cease. Thus an ordinary gas burner connected to a source of rapidly alternating
potential might have its efficiency augmented to a certain limit, and this for two reasons—because
of the additional vibration imparted, and because of a slowing down of the process of carrying off.
But the renewal being rendered difficult, a renewal being necessary to maintain the burner, a
continued increase of the frequency of the impulses, assuming they could be transmitted to and
impressed upon the flame, would result in the "extinction" of the latter, meaning by this term only
the cessation of the chemical process.
I think, however, that in the case of an electrode immersed in a fluid insulating medium, and
surrounded by independent carriers of electric charges, which can be acted upon inductively, a
sufficient high frequency of the impulses would probably result in a gravitation of the gas all
around toward the electrode. For this it would be only necessary to assume that the independent
bodies are irregularly shaped; they would then turn toward the electrode their side of the greatest
electric density, and this would be a position in which the fluid resistance to approach would be
smaller than that offered to the receding.
The general opinion, I do not doubt, is that it is out of the question to reach any such
frequencies as might—assuming some of the views before expressed to be true—produce any of
the results which I have pointed out as mere possibilities. This may be so, but in the course of
these investigations, from the observation of many phenomena, I have gained the conviction that
these frequencies would be much lower than one is apt to estimate at first. In a flame we set up
light vibrations by causing molecules, or atoms, to collide. But what is the ratio of the frequency of
the collisions and that of the vibrations set up? Certainly it must be incomparably smaller than that
of the strokes of the bell and the sound vibrations, or that of the discharges and the oscillations of
the condenser. We may cause the molecules of the gas to collide by the use of alternate electric

impulses of high frequency, and so we may imitate the process in a flame; and from experiments
with frequencies which we are now able to obtain, I think that the result is producible with
impulses which are transmissible through a conductor.
In connection with thoughts of a similar nature, it appeared to me of great interest to
demonstrate the rigidity of a vibrating gaseous column. Although with such low frequencies as,
say 10,000 per second, which I was able to obtain without difficulty from a specially constructed
alternator, the task looked discouraging at first, I made a series of experiments. The trials with air
at ordinary pressure led to no result, but with air moderately rarefied I obtain what I think to be an
unmistakable experimental evidence of the property sought for. As a result of this kind might lead
able investigators to conclusions of importance, I will describe one of the experiments performed.
It is well known that when a tube is slightly exhausted, the discharge may be passed through it
in the form of a thin luminous thread. When produced with currents of low frequency, obtained
from a coil operated as usual, this thread is inert. If a magnet be approached to it, the part near the
same is attracted or repelled, according to the direction of the lines of force of the magnet. It
occurred to me that if such a thread would be produced with currents of very high frequency, it
should be more or less rigid, and as it was visible it could be easily studied. Accordingly I prepared
a tube about one inch in diameter and one metre long, with outside coating at each end. The tube
was exhausted to a point at which, by a little working, the thread discharge could be obtained. It
must be remarked here that the general aspect of the tube, and the degree of exhaustion, are quite
other than when ordinary low frequency currents are used. As it was found preferable to work with
one terminal, the tube prepared was suspended from the end of a wire connected to the terminal,
the tinfoil coating being connected to the wire, and to the lower coating sometimes a small
insulated plate was attached. When the thread was formed, it extended through the upper part of
the tube and lost itself in the lower end. If it possessed rigidity it resembled, not exactly an elastic
cord stretched tight between two supports, but a cord suspended from a height with a small weight
attached at the end. When the finger or a small magnet was approached to the upper end of the
luminous thread, it could be brought locally out of position by electrostatic or magnetic action; and
when the disturbing object was very quickly removed, an analogous result was produced, as
though a suspended cord would be displaced and quickly released near the point of suspension. In
doing this the luminous thread was set in vibration, and two very sharply marked nodes, and a
third indistinct one, were formed. The vibration, once set up, continued for fully eight minutes,
dying gradually out. The speed of the vibration often varied perceptibly, and it could be observed
that the electrostatic attraction of the glass affected the vibrating thread; but it was clear that the
electrostatic action was not the cause of the vibration, for the thread was most generally stationary,
and could always be set in vibration by passing the finger quickly near the upper part of the tube.
With a magnet the thread could be split in two and both parts vibrated. By approaching the hand to
the lower coating of the tube, or insulation plate if attached, the vibration was quickened; also, as
far as I could see, by raising the potential or frequency. Thus, either increasing the frequency or
passing a stronger discharge of the same frequency corresponded to a tightening of the cord. I did
not obtain any experimental evidence with condenser discharges. A luminous band excited in the
bulb by repeated discharges of a Leyden jar must possess rigidity, and if deformed and suddenly
released, should vibrate. But probably the amount of vibrating matter is so small that in spite of the
extreme speed, the inertia cannot prominently assert itself. Besides, the observation in such a case
is rendered extremely difficult on account of the fundamental vibration.
The demonstration of the fact—which still needs better experimental confirmation—that a
vibrating gaseous column possesses rigidity, might greatly modify the views of thinkers. When

with low frequencies and insignificant potentials indications of that property may be noted, how
must a gaseous medium behave under the influence of enormous electrostatic stresses which may
be active in the interstellar space, and which may alternate with inconceivable rapidity? The
existence of such an electrostatic, rhythmically throbbing force—of a vibrating electrostatic field
—would show a possible way how solids might have formed from the ultra-gaseous uterus, and
how transverse and all kinds of vibrations may be transmitted through a gaseous medium filling all
space. Then, ether might be a true fluid, devoid of rigidity, and at rest, it being merely necessary as
a connecting link to enable interaction. What determines the rigidity of a body? It must be the
speed and the amount of motive matter. In a gas the speed maybe considerable, but the density is
exceedingly small; in a liquid the speed would be likely to be small, though the density may be
considerable; and in both cases the inertia resistance offered to displacement is practically nil. But
place a gaseous (or liquid) column in an intense, rapidly alternating electrostatic field, set the
particles vibrating with enormous speeds, then the inertia resistance asserts itself. A body might
move with more or less freedom through the vibrating mass, but as a whole it would be rigid.
There is a subject which I must mention in connection with these experiments: it is that of high
vacua. This is a subject, the study of which is not only interesting, but useful, for it may lead to
results of great practical importance. In commercial apparatus, such as incandescent lamps,
operated from ordinary systems of distribution, a much higher vacuum than is obtained at present
would not secure a very great advantage. In such a case the work is performed on the filament, and
the gas is little concerned; the improvement, therefore, would be but trifling. But when we begin to
use very high frequencies and potentials, the action of the gas becomes all important, and the
degree of exhaustion materially modifies the results. As long as ordinary coils, even very large
ones, were used, the study of the subject was limited, because just at a point when it became most
interesting it had to be interrupted on account of the "non-striking" vacuum being reached. But at
present we are able to obtain from a small disruptive discharge coil potentials much higher than
even the largest coil was capable of giving, and, what is more, we can make the potential alternate
with great rapidity. Both of these results enable us now to pass a luminous discharge through
almost any vacua obtainable, and the field of our investigations is greatly extended. Think we as
we may, of all the possible directions to develop a practical illuminant, the line of high vacua
seems to be the most promising at present. But to reach extreme vacua the appliances must be
much more improved, and ultimate perfection will not be attained until we shall have discharged
the mechanical and perfected an electrical vacuum pump. Molecules and atoms can be thrown out
of a bulb under the action of an enormous potential: this will be the principle of the vacuum pump
of the future. For the present, we must secure the best results we can with mechanical appliances.
In this respect, it might not be out of the way to say a few words about the method of, and
apparatus for, producing excessively high degrees of exhaustion of which I have availed myself in
the course of these investigations. It is very probable that other experimenters have used similar
arrangements; but as it is possible that there may be an item of interest in their description, a few
remarks, which will render this investigation more complete, might be permitted.

Fig. 159.
The apparatus is illustrated in a drawing shown in Fig. 159. S represents a Sprengel pump,
which has been specially constructed to better suit the work required. The stop-cock which is
usually employed has been omitted, and instead of it a hollow stopper s has been fitted in the neck
of the reservoir R. This stopper has a small hole h, through which the mercury descends; the size of
the outlet o being properly determined with respect to the section of the fall tube t, which is sealed
to the reservoir instead of being connected to it in the usual manner. This arrangement overcomes
the imperfections and troubles which often arise from the use of the stopcock on the reservoir and
the connections of the latter with the fall tube.

The pump is connected through a U-shaped tube t to a very large reservoir R1. Especial care
was taken in fitting the grinding surfaces of the stoppers p and p1, and both of these and the
mercury caps above them were made exceptionally long. After the U-shaped tube was fitted and
put in place, it was heated, so as to soften and take off the strain resulting from imperfect fitting.
The U-shaped tube was provided with a stopcock C, and two ground connections g and g1,—one
for a small bulb b, usually containing caustic potash, and the other for the receiver r, to be
exhausted.
The reservoir R1, was connected by means of a rubber tube to a slightly larger reservoir R2,
each of the two reservoirs being provided with a stopcock C1 and C2, respectively. The reservoir R2

could be raised and lowered by a wheel and rack, and the range of its motion was so determined
that when it was filled with mercury and the stopcock C2 closed, so as to form a Torricellian
vacuum in it when raised, it could be lifted so high that the reservoir R1 would stand a little above
stopcock C1; and when this stopcock was closed and the reservoir R2 descended, so as to form a
Torricellian vacuum in reservoir R1, it could be lowered so far as to completely empty the latter,
the mercury filling the reservoir R2 up to a little above stopcock C2.

The capacity of the pump and of the connections was taken as small as possible relatively to
the volume of reservoir, R1, since, of course, the degree of exhaustion depended upon the ratio of
these quantities.
With this apparatus I combined the usual means indicated by former experiments for the
production of very high vacua. In most of the experiments it was most convenient to use caustic
potash. I may venture to say, in regard to its use, that much time is saved and a more perfect action
of the pump insured by fusing and boiling the potash as soon as, or even before, the pump settles
down. If this course is not followed, the sticks, as ordinarily employed, may give off moisture at a
certain very slow rate, and the pump may work for many hours without reaching a very high
vacuum. The potash was heated either by a spirit lamp or by passing a discharge through it, or by
passing a current through a wire contained in it. The advantage in the latter case was that the
heating could be more rapidly repeated.
Generally the process of exhaustion was the following:—At the start, the stop-cocks C and C1
being open, and all other connections closed, the reservoir R2 was raised so far that the mercury
filled the reservoir R1 and a part of the narrow connecting U-shaped tube. When the pump was set
to work, the mercury would, of course, quickly rise in the tube, and reservoir R2 was lowered, the
experimenter keeping the mercury at about the same level. The reservoir R2 was balanced by a
long spring which facilitated the operation, and the friction of the parts was generally sufficient to
keep it in almost any position. When the Sprengel pump had done its work, the reservoir R2 was
further lowered and the mercury descended in R1 and filled R2, whereupon stopcock C2 was closed.
The air adhering to the walls of R1 and that absorbed by the mercury was carried off, and to free
the mercury of all air the reservoir R2 was for a long time worked up and down. During this
process some air, which would gather below stopcock C2, was expelled from R2 by lowering it far
enough and opening the stopcock, closing the latter again before raising the reservoir. When all the
air had been expelled from the mercury, and no air would gather in R2 when it was lowered, the
caustic potash was resorted to. The reservoir R2 was now again raised until the mercury in R1,
stood above stopcock C1. The caustic potash was fused and boiled, and moisture partly carried off
by the pump and partly re-absorbed; and this process of heating and cooling was repeated many
times, and each time, upon the moisture being absorbed or carried off, the reservoir R2 was for a
long time raised and lowered. In this manner all the moisture was carried off from the mercury,
and both the reservoirs were in proper condition to be used. The reservoir R2 was then again raised
to the top, and the pump was kept working for a long time. When the highest vacuum obtainable
with the pump had been reached, the potash bulb was usually wrapped with cotton which was
sprinkled with ether so as to keep the potash at a very low temperature, then the reservoir R2 was
lowered, and upon reservoir R1 being emptied the receiver was quickly sealed up.

When a new bulb was put on, the mercury was always raised above stopcock C1, which was
closed, so as to always keep the mercury and both the reservoirs in fine condition, and the mercury
was never withdrawn from R1 except when the pump had reached the highest degree of exhaustion.
It is necessary to observe this rule if it is desired to use the apparatus to advantage.
By means of this arrangement I was able to proceed very quickly, and when the apparatus was
in perfect order it was possible to reach the phosphorescent stage in a small bulb in less than
fifteen minutes, which is certainly very quick work for a small laboratory arrangement requiring
all in all about 100 pounds of mercury. With ordinary small bulbs the ratio of the capacity of the
pump, receiver, and connections, and that of reservoir R was about 1 to 20, and the degrees of
exhaustion reached were necessarily very high, though I am unable to make a precise and reliable
statement how far the exhaustion was carried.
What impresses the investigator most in the course of these experiences is the behavior of
gases when subjected to great rapidly alternating electrostatic stresses. But he must remain in
doubt as to whether the effects observed are due wholly to the molecules, or atoms, of the gas
which chemical analysis discloses to us, or whether there enters into play another medium of a
gaseous nature, comprising atoms, or molecules, immersed in a fluid pervading the space. Such a
medium surely must exist, and I am convinced that, for instance, even if air were absent, the
surface and neighborhood of a body in space would be heated by rapidly alternating the potential
of the body; but no such heating of the surface or neighborhood could occur if all free atoms were
removed and only a homogeneous, incompressible, and elastic fluid—such as ether is supposed to
be—would remain, for then there would be no impacts, no collisions. In such a case, as far as the
body itself is concerned, only frictional losses in the inside could occur.
It is a striking fact that the discharge through a gas is established with ever-increasing freedom
as the frequency of the impulses is augmented. It behaves in this respect quite contrarily to a
metallic conductor. In the latter the impedance enters prominently into play as the frequency is
increased, but the gas acts much as a series of condensers would; the facility with which the
discharge passes through, seems to depend on the rate of change of potential. If it acts so, then in a
vacuum tube even of great length, and no matter how strong the current, self-induction could not
assert itself to any appreciable degree. We have, then, as far as we can now see, in the gas a
conductor which is capable of transmitting electric impulses of any frequency which we may be
able to produce. Could the frequency be brought high enough, then a queer system of electric
distribution, which would be likely to interest gas companies, might be realized: metal pipes filled
with gas—the metal being the insulator, the gas the conductor—supplying phosphorescent bulbs,
or perhaps devices as yet uninvented. It is certainly possible to take a hollow core of copper, rarefy
the gas in the same, and by passing impulses of sufficiently high frequency through a circuit
around it, bring the gas inside to a high degree of incandescence; but as to the nature of the forces
there would be considerable uncertainty, for it would be doubtful whether with such impulses the
copper core would act as a static screen. Such paradoxes and apparent impossibilities we encounter
at every step in this line of work, and therein lies, to a great extent, the charm of the study.
I have here a short and wide tube which is exhausted to a high degree and covered with a
substantial coating of bronze, the coating barely allowing the light to shine through. A metallic
cap, with a hook for suspending the tube, is fastened around the middle portion of the latter, the
clasp being in contact with the bronze coating. I now want to light the gas inside by suspending the
tube on a wire connected to the coil. Any one who would try the experiment for the first time, not
having any previous experience, would probably take care to be quite alone when making the trial,

for fear that he might become the joke of his assistants. Still, the bulb lights in spite of the metal
coating, and the light can be distinctly perceived through the latter. A long tube covered with
aluminum bronze lights when held in one hand—the other touching the terminal of the coil—quite
powerfully. It might be objected that the coatings are not sufficiently conducting; still, even if they
were highly resistant, they ought to screen the gas. They certainly screen it perfectly in a condition
of rest, but far from perfectly when the charge is surging in the coating. But the loss of energy
which occurs within the tube, notwithstanding the screen, is occasioned principally by the presence
of the gas. Were we to take a large hollow metallic sphere and fill it with a perfect, incompressible,
fluid dielectric, there would be no loss inside of the sphere, and consequently the inside might be
considered as perfectly screened, though the potential be very rapidly alternating. Even were the
sphere filled with oil, the loss would be incomparably smaller than when the fluid is replaced by a
gas, for in the latter case the force produces displacements; that means impact and collisions in the
inside.
No matter what the pressure of the gas may be, it becomes an important factor in the heating of
a conductor when the electric density is great and the frequency very high. That in the heating of
conductors by lightning discharges, air is an element of great importance, is almost as certain as an
experimental fact. I may illustrate the action of the air by the following experiment: I take a short
tube which is exhausted to a moderate degree and has a platinum wire running through the middle
from one end to the other. I pass a steady or low frequency current through the wire, and it is
heated uniformly in all parts. The heating here is due to conduction, or frictional losses, and the
gas around the wire has—as far as we can see—no function to perform. But now let me pass
sudden discharges, or high frequency currents, through the wire. Again the wire is heated, this time
principally on the ends and least in the middle portion; and if the frequency of the impulses, or the
rate of change, is high enough, the wire might as well be cut in the middle as not, for practically all
heating is due to the rarefied gas. Here the gas might only act as a conductor of no impedance
diverting the current from the wire as the impedance of the latter is enormously increased, and
merely heating the ends of the wire by reason of their resistance to the passage of the discharge.
But it is not at all necessary that the gas in the tube should be conducting; it might be at an
extremely low pressure, still the ends of the wire would be heated—as, however, is ascertained by
experience—only the two ends would in such case not be electrically connected through the
gaseous medium. Now what with these frequencies and potentials occurs in an exhausted tube,
occurs in the lightning discharges at ordinary pressure. We only need remember one of the facts
arrived at in the course of these investigations, namely, that to impulses of very high frequency the
gas at ordinary pressure behaves much in the same manner as though it were at moderately low
pressure. I think that in lightning discharges frequently wires or conducting objects are volatilized
merely because air is present, and that, were the conductor immersed in an insulating liquid, it
would be safe, for then the energy would have to spend itself somewhere else. From the behavior
of gases under sudden impulses of high potential, I am led to conclude that there can be no surer
way of diverting a lightning discharge than by affording it a passage through a volume of gas, if
such a thing can be done in a practical manner.
There are two more features upon which I think it necessary to dwell in connection with these
experiments—the "radiant state" and the "non-striking vacuum."
Any one who has studied Crookes' work must have received the impression that the "radiant
state" is a property of the gas inseparably connected with an extremely high degree of exhaustion.
But it should be remembered that the phenomena observed in an exhausted vessel are limited to
the character and capacity of the apparatus which is made use of. I think that in a bulb a molecule,

or atom, does not precisely move in a straight line because it meets no obstacle, but because the
velocity imparted to it is sufficient to propel it in a sensibly straight line. The mean free path is one
thing, but the velocity—the energy associated with the moving body—is another, and under
ordinary circumstances I believe that it is a mere question of potential or speed. A disruptive
discharge coil, when the potential is pushed very far, excites phosphorescence and projects
shadows, at comparatively low degrees of exhaustion. In a lightning discharge, matter moves in
straight lines at ordinary pressure when the mean free path is exceedingly small, and frequently
images of wires or other metallic objects have been produced by the particles thrown off in straight
lines.
I have prepared a bulb to illustrate by an experiment the correctness of these assertions. In a
globe L, Fig. 160, I have mounted upon a lamp filament f a piece of lime l. The lamp filament is
connected with a wire which leads into the bulb, and the general construction of the latter is as
indicated in Fig. 148, before described. The bulb being suspended from a wire connected to the
terminal of the coil, and the latter being set to work, the lime piece l and the projecting parts of the
filament f are bombarded. The degree of exhaustion is just such that with the potential the coil is
capable of giving, phosphorescence of the glass is produced, but disappears as soon as the vacuum
is impaired. The lime containing moisture, and moisture being given off as soon as heating occurs,
the phosphorescence lasts only for a few moments. When the lime has been sufficiently heated,
enough moisture has been given off to impair materially the vacuum of the bulb. As the
bombardment goes on, one point of the lime piece is more heated than other points, and the result
is that finally practically all the discharge passes through that point which is intensely heated, and
a white stream of lime particles (Fig. 160) then breaks forth from that point. This stream is
composed of "radiant" matter, yet the degree of exhaustion is low. But the particles move in
straight lines because the velocity imparted to them is great, and this is due to three causes—to the
great electric density, the high temperature of the small point, and the fact that the particles of the
lime are easily torn and thrown off—far more easily than those of carbon. With frequencies such
as we are able to obtain, the particles are bodily thrown off and projected to a considerable
distance; but with sufficiently high frequencies no such thing would occur; in such case only a
stress would spread or a vibration would be propagated through the bulb. It would be out of the
question to reach any such frequency on the assumption that the atoms move with the speed of
light; but I believe that such a thing is impossible; for this an enormous potential would be
required. With potentials which we are able to obtain, even with a disruptive discharge coil, the
speed must be quite insignificant.

Fig. 160.
As to the "non-striking vacuum," the point to be noted is, that it can occur only with low frequency impulses,
and it is necessitated by the impossibility of carrying off enough energy with such impulses in high vacuum, since
the few atoms which are around the terminal upon coming in contact with the same, are repelled and kept at a
distance for a comparatively long period of time, and not enough work can be performed to render the effect
perceptible to the eye. If the difference of potential between the terminals is raised, the dielectric breaks down. But
with very high frequency impulses there is no necessity for such breaking down, since any amount of work can be
performed by continually agitating the atoms in the exhausted vessel, provided the frequency is high enough. It is
easy to reach—even with frequencies obtained from an alternator as here used—a stage at which the discharge
does not pass between two electrodes in a narrow tube, each of these being connected to one of the terminals of the
coil, but it is difficult to reach a point at which a luminous discharge would not occur around each electrode.
A thought which naturally presents itself in connection with high frequency currents, is to make use of their
powerful electrodynamic inductive action to produce light effects in a sealed glass globe. The leading-in wire is
one of the defects of the present incandescent lamp, and if no other improvement were made, that imperfection at
least should be done away with. Following this thought, I have carried on experiments in various directions, of
which some were indicated in my former paper. I may here mention one or two more lines of experiment which
have been followed up.
Many bulbs were constructed as shown in Fig. 161 and Fig. 162.

Fig. 161. Fig. 162.

In Fig. 161, a wide tube, T, was sealed to a smaller W shaped tube U, of phosphorescent glass. In the tube T,
was placed a coil C, of aluminum wire, the ends of which were provided with small spheres, t and t1, of aluminum,
and reached into the U tube. The tube T was slipped into a socket containing a primary coil, through which usually
the discharges of Leyden jars were directed, and the rarefied gas in the small U tube was excited to strong
luminosity by the high-tension current induced in the coil C. When Leyden jar discharges were used to induce
currents in the coil C, it was found necessary to pack the tube T tightly with insulating powder, as a discharge
would occur frequently between the turns of the coil, especially when the primary was thick and the air gap,
through which the jars discharged, large, and no little trouble was experienced in this way.
In Fig. 162 is illustrated another form of the bulb constructed. In this case a tube T is sealed to a globe L. The
tube contains a coil C, the ends of which pass through two small glass tubes t and t1, which are sealed to the tube T.
Two refractory buttons m and m1, are mounted on lamp filaments which are fastened to the ends of the wires
passing through the glass tubes t and t1. Generally in bulbs made on this plan the globe L communicated with the
tube T. For this purpose the ends of the small tubes t and t1 were heated just a trifle in the burner, merely to hold
the wires, but not to interfere with the communication. The tube T, with the small tubes, wires through the same,
and the refractory buttons m and m1, were first prepared, and then sealed to globe L, whereupon the coil C was
slipped in and the connections made to its ends. The tube was then packed with insulating powder, jamming the
latter as tight as possible up to very nearly the end; then it was closed and only a small hole left through which the
remainder of the powder was introduced, and finally the end of the tube was closed. Usually in bulbs constructed
as shown in Fig. 162 an aluminum tube a was fastened to the upper end s of each of the tubes t and t1 in order to
protect that end against the heat. The buttons m and m1 could be brought to any degree of incandescence by
passing the discharges of Leyden jars around the coil C. In such bulbs with two buttons a very curious effect is
produced by the formation of the shadows of each of the two buttons.
Another line of experiment, which has been assiduously followed, was to induce by electro-dynamic induction
a current or luminous discharge in an exhausted tube or bulb. This matter has received such able treatment at the
hands of Prof. J. J. Thomson, that I could add but little to what he has made known, even had I made it the special

subject of this lecture. Still, since experiments in this line have gradually led me to the present views and results, a
few words must be devoted here to this subject.
It has occurred, no doubt, to many that as a vacuum tube is made longer, the electromotive force per unit length
of the tube, necessary to pass a luminous discharge through the latter, becomes continually smaller; therefore, if
the exhausted tube be made long enough, even with low frequencies a luminous discharge could be induced in
such a tube closed upon itself. Such a tube might be placed around a hall or on a ceiling, and at once a simple
appliance capable of giving considerable light would be obtained. But this would be an appliance hard to
manufacture and extremely unmanageable. It would not do to make the tube up of small lengths, because there
would be with ordinary frequencies considerable loss in the coatings, and besides, if coatings were used, it would
be better to supply the current directly to the tube by connecting the coatings to a transformer. But even if all
objections of such nature were removed, with low frequencies the light conversion itself would be inefficient, as I
have before stated. In using extremely high frequencies the length of the secondary—in other words, the size of the
vessel—can be reduced as much as desired, and the efficiency of the light conversion is increased, provided that
means are invented for efficiently obtaining such high frequencies. Thus one is led, from theoretical and practical
considerations, to the use of high frequencies, and this means high electromotive forces and small currents in the
primary. When one works with condenser charges—and they are the only means up to the present known for
reaching these extreme frequencies—one gets to electromotive forces of several thousands of volts per turn of the
primary. We cannot multiply the electro-dynamic inductive effect by taking more turns in the primary, for we
arrive at the conclusion that the best way is to work with one single turn—though we must sometimes depart from
this rule—and we must get along with whatever inductive effect we can obtain with one turn. But before one has
long experimented with the extreme frequencies required to set up in a small bulb an electromotive force of several
thousands of volts, one realizes the great importance of electrostatic effects, and these effects grow relatively to the
electro-dynamic in significance as the frequency is increased.
Now, if anything is desirable in this case, it is to increase the frequency, and this would make it still worse for
the electrodynamic effects. On the other hand, it is easy to exalt the electrostatic action as far as one likes by taking
more turns on the secondary, or combining self-induction and capacity to raise the potential. It should also be
remembered that, in reducing the current to the smallest value and increasing the potential, the electric impulses of
high frequency can be more easily transmitted through a conductor.
These and similar thoughts determined me to devote more attention to the electrostatic phenomena, and to
endeavor to produce potentials as high as possible, and alternating as fast as they could be made to alternate. I then
found that I could excite vacuum tubes at considerable distance from a conductor connected to a properly
constructed coil, and that I could, by converting the oscillatory current of a conductor to a higher potential,
establish electrostatic alternating fields which acted through the whole extent of the room, lighting up a tube no
matter where it was held in space. I thought I recognized that I had made a step in advance, and I have persevered
in this line; but I wish to say that I share with all lovers of science and progress the one and only desire—to reach a
result of utility to men in any direction to which thought or experiment may lead me. I think that this departure is
the right one, for I cannot see, from the observation of the phenomena which manifest themselves as the frequency
is increased, what there would remain to act between two circuits conveying, for instance, impulses of several
hundred millions per second, except electrostatic forces. Even with such trifling frequencies the energy would be
practically all potential, and my conviction has grown strong that, to whatever kind of motion light may be due, it
is produced by tremendous electrostatic stresses vibrating with extreme rapidity.

Fig. 163. Fig. 164.

Of all these phenomena observed with currents, or electric impulses, of high frequency, the most fascinating
for an audience are certainly those which are noted in an electrostatic field acting through considerable distance;
and the best an unskilled lecturer can do is to begin and finish with the exhibition of these singular effects. I take a
tube in my hand and move it about, and it is lighted wherever I may hold it; throughout space the invisible forces
act. But I may take another tube and it might not light, the vacuum being very high. I excite it by means of a
disruptive discharge coil, and now it will light in the electrostatic field. I may put it away for a few weeks or
months, still it retains the faculty of being excited. What change have I produced in the tube in the act of exciting
it? If a motion imparted to atoms, it is difficult to perceive how it can persist so long without being arrested by
frictional losses; and if a strain exerted in the dielectric, such as a simple electrification would produce, it is easy to
see how it may persist indefinitely, but very difficult to understand why such a condition should aid the excitation
when we have to deal with potentials which are rapidly alternating.
Since I have exhibited these phenomena for the first time, I have obtained some other interesting effects. For
instance, I have produced the incandescence of a button, filament, or wire enclosed in a tube. To get to this result it
was necessary to economize the energy which is obtained from the field, and direct most of it on the small body to
be rendered incandescent. At the beginning the task appeared difficult, but the experiences gathered permitted me
to reach the result easily. In Fig. 163 and Fig. 164, two such tubes are illustrated, which are prepared for the
occasion. In Fig. 163 a short tube T1, sealed to another long tube T, is provided with a stem s, with a platinum wire
sealed in the latter. A very thin lamp filament l, is fastened to this wire and connection to the outside is made

through a thin copper wire w. The tube is provided with outside and inside coatings, C and C1, respectively, and is
filled as far as the coatings reach with conducting, and the space above with insulating, powder. These coatings are
merely used to enable me to perform two experiments with the tube—namely, to produce the effect desired either
by direct connection of the body of the experimenter or of another body to the wire w, or by acting inductively
through the glass. The stem s is provided with an aluminum tube a, for purposes before explained, and only a small
part of the filament reaches out of this tube. By holding the tube T1 anywhere in the electrostatic field, the filament
is rendered incandescent.
A more interesting piece of apparatus is illustrated in Fig. 164. The construction is the same as before, only
instead of the lamp filament a small platinum wire p, sealed in a stem s, and bent above it in a circle, is connected
to the copper wire w, which is joined to an inside coating C. A small stem s1 is provided with a needle, on the point
of which is arranged, to rotate very freely, a very light fan of mica v. To prevent the fan from falling out, a thin
stem of glass g, is bent properly and fastened to the aluminum tube. When the glass tube is held anywhere in the
electrostatic field the platinum wire becomes incandescent, and the mica vanes are rotated very fast.
Intense phosphorescence may be excited in a bulb by merely connecting it to a plate within the field, and the
plate need not be any larger than an ordinary lamp shade. The phosphorescence excited with these currents is
incomparably more powerful than with ordinary apparatus. A small phosphorescent bulb, when attached to a wire
connected to a coil, emits sufficient light to allow reading ordinary print at a distance of five to six paces. It was of
interest to see how some of the phosphorescent bulbs of Professor Crookes would behave with these currents, and
he has had the kindness to lend me a few for the occasion. The effects produced are magnificent, especially by the
sulphide of calcium and sulphide of zinc. With the disruptive discharge coil they glow intensely merely by holding
them in the hand and connecting the body to the terminal of the coil.
To whatever results investigations of this kind may lead, the chief interest lies, for the present, in the
possibilities they offer for the production of an efficient illuminating device. In no branch of electric industry is an
advance more desired than in the manufacture of light. Every thinker, when considering the barbarous methods
employed, the deplorable losses incurred in our best systems of light production, must have asked himself, What is
likely to be the light of the future? Is it to be an incandescent solid, as in the present lamp, or an incandescent gas,
or a phosphorescent body, or something like a burner, but incomparably more efficient?
There is little chance to perfect a gas burner; not, perhaps, because human ingenuity has been bent upon that
problem for centuries without a radical departure having been made—though the argument is not devoid of force
—but because in a burner the highest vibrations can never be reached, except by passing through all the low ones.
For how is a flame to proceed unless by a fall of lifted weights? Such process cannot be maintained without
renewal, and renewal is repeated passing from low to high vibrations. One way only seems to be open to improve a
burner, and that is by trying to reach higher degrees of incandescence. Higher incandescence is equivalent to a
quicker vibration: that means more light from the same material, and that again, means more economy. In this
direction some improvements have been made, but the progress is hampered by many limitations. Discarding,
then, the burner, there remains the three ways first mentioned, which are essentially electrical.
Suppose the light of the immediate future to be a solid, rendered incandescent by electricity. Would it not seem
that it is better to employ a small button than a frail filament? From many considerations it certainly must be
concluded that a button is capable of a higher economy, assuming, of course, the difficulties connected with the
operation of such a lamp to be effectively overcome. But to light such a lamp we require a high potential; and to
get this economically, we must use high frequencies.
Such considerations apply even more to the production of light by the incandescence of a gas, or by
phosphorescence. In all cases we require high frequencies and high potentials. These thoughts occurred to me a
long time ago.
Incidentally we gain, by the use of high frequencies, many advantages, such as higher economy in the light
production, the possibility of working with one lead, the possibility of doing away with the leading-in wire, etc.
The question is, how far can we go with frequencies? Ordinary conductors rapidly lose the facility of
transmitting electric impulses when the frequency is greatly increased. Assume the means for the production of
impulses of very great frequency brought to the utmost perfection, every one will naturally ask how to transmit
them when the necessity arises. In transmitting such impulses through conductors we must remember that we have

to deal with pressure and flow, in the ordinary interpretation of these terms. Let the pressure increase to an
enormous value, and let the flow correspondingly diminish, then such impulses—variations merely of pressure, as
it were—can no doubt be transmitted through a wire even if their frequency be many hundreds of millions per
second. It would, of course, be out of question to transmit such impulses through a wire immersed in a gaseous
medium, even if the wire were provided with a thick and excellent insulation, for most of the energy would be lost
in molecular bombardment and consequent heating. The end of the wire connected to the source would be heated,
and the remote end would receive but a trifling part of the energy supplied. The prime necessity, then, if such
electric impulses are to be used, is to find means to reduce as much as possible the dissipation.
The first thought is, to employ the thinnest possible wire surrounded by the thickest practicable insulation. The
next thought is to employ electrostatic screens. The insulation of the wire may be covered with a thin conducting
coating and the latter connected to the ground. But this would not do, as then all the energy would pass through the
conducting coating to the ground and nothing would get to the end of the wire. If a ground connection is made it
can only be made through a conductor offering an enormous impedance, or through a condenser of extremely
small capacity. This, however, does not do away with other difficulties.
If the wave length of the impulses is much smaller than the length of the wire, then corresponding short waves
will be set up in the conducting coating, and it will be more or less the same as though the coating were directly
connected to earth. It is therefore necessary to cut up the coating in sections much shorter than the wave length.
Such an arrangement does not still afford a perfect screen, but it is ten thousand times better than none. I think it
preferable to cut up the conducting coating in small sections, even if the current waves be much longer than the
coating.
If a wire were provided with a perfect electrostatic screen, it would be the same as though all objects were
removed from it at infinite distance. The capacity would then be reduced to the capacity of the wire itself, which
would be very small. It would then be possible to send over the wire current vibrations of very high frequencies at
enormous distances, without affecting greatly the character of the vibrations. A perfect screen is of course out of
the question, but I believe that with a screen such as I have just described telephony could be rendered practicable
across the Atlantic. According to my ideas, the gutta-percha covered wire should be provided with a third
conducting coating subdivided in sections. On the top of this should be again placed a layer of gutta-percha and
other insulation, and on the top of the whole the armor. But such cables will not be constructed, for ere long
intelligence—transmitted without wires—will throb through the earth like a pulse through a living organism. The
wonder is that, with the present state of knowledge and the experiences gained, no attempt is being made to disturb
the electrostatic or magnetic condition of the earth, and transmit, if nothing else, intelligence.
It has been my chief aim in presenting these results to point out phenomena or features of novelty, and to
advance ideas which I am hopeful will serve as starting points of new departures. It has been my chief desire this
evening to entertain you with some novel experiments. Your applause, so frequently and generously accorded, has
told me that I have succeeded.
In conclusion, let me thank you most heartily for your kindness and attention, and assure you that the honor I
have had in addressing such a distinguished audience, the pleasure I have had in presenting these results to a
gathering of so many able men—and among them also some of those in whose work for many years past I have
found enlightenment and constant pleasure—I shall never forget.

CHAPTER XXVIII.

On Light and Other High Frequency Phenomena.[3]
INTRODUCTORY.—SOME THOUGHTS ON THE EYE.

When we look at the world around us, on Nature, we are impressed with its beauty and grandeur. Each thing
we perceive, though it may be vanishingly small, is in itself a world, that is, like the whole of the universe, matter
and force governed by law,—a world, the contemplation of which fills us with feelings of wonder and irresistibly
urges us to ceaseless thought and inquiry. But in all this vast world, of all objects our senses reveal to us, the most
marvellous, the most appealing to our imagination, appears no doubt a highly developed organism, a thinking
being. If there is anything fitted to make us admire Nature's handiwork, it is certainly this inconceivable structure,
which performs its innumerable motions of obedience to external influence. To understand its workings, to get a
deeper insight into this Nature's masterpiece, has ever been for thinkers a fascinating aim, and after many centuries
of arduous research men have arrived at a fair understanding of the functions of its organs and senses. Again, in all
the perfect harmony of its parts, of the parts which constitute the material or tangible of our being, of all its organs
and senses, the eye is the most wonderful. It is the most precious, the most indispensable of our perceptive or
directive organs, it is the great gateway through which all knowledge enters the mind. Of all our organs, it is the
one, which is in the most intimate relation with that which we call intellect. So intimate is this relation, that it is
often said, the very soul shows itself in the eye.
It can be taken as a fact, which the theory of the action of the eye implies, that for each external impression,
that is, for each image produced upon the retina, the ends of the visual nerves, concerned in the conveyance of the
impression to the mind, must be under a peculiar stress or in a vibratory state. It now does not seem improbable
that, when by the power of thought an image is evoked, a distinct reflex action, no matter how weak, is exerted
upon certain ends of the visual nerves, and therefore upon the retina. Will it ever be within human power to
analyze the condition of the retina when disturbed by thought or reflex action, by the help of some optical or other
means of such sensitiveness, that a clear idea of its state might be gained at any time? If this were possible, then
the problem of reading one's thoughts with precision, like the characters of an open book, might be much easier to
solve than many problems belonging to the domain of positive physical science, in the solution of which many, if
not the majority, of scientific men implicitly believe. Helmholtz, has shown that the fundi of the eye are
themselves, luminous, and he was able to see, in total darkness, the movement of his arm by the light of his own
eyes. This is one of the most remarkable experiments recorded in the history of science, and probably only a few
men could satisfactorily repeat it, for it is very likely, that the luminosity of the eyes is associated with uncommon
activity of the brain and great imaginative power. It is fluorescence of brain action, as it were.
Another fact having a bearing on this subject which has probably been noted by many, since it is stated in
popular expressions, but which I cannot recollect to have found chronicled as a positive result of observation is,
that at times, when a sudden idea or image presents itself to the intellect, there is a distinct and sometimes painful
sensation of luminosity produced in the eye, observable even in broad daylight.
The saying then, that the soul shows itself in the eye, is deeply founded, and we feel that it expresses a great
truth. It has a profound meaning even for one who, like a poet or artist, only following his inborn instinct or love
for Nature, finds delight in aimless thoughts and in the mere contemplation of natural phenomena, but a still more
profound meaning for one who, in the spirit of positive scientific investigation, seeks to ascertain the causes of the
effects. It is principally the natural philosopher, the physicist, for whom the eye is the subject of the most intense
admiration.
Two facts about the eye must forcibly impress the mind of the physicist, notwithstanding he may think or say
that it is an imperfect optical instrument, forgetting, that the very conception of that which is perfect or seems so to
him, has been gained through this same instrument. First, the eye is, as far as our positive knowledge goes, the
only organ which is directly affected by that subtile medium, which as science teaches us, must fill all space;
secondly, it is the most sensitive of our organs, incomparably more sensitive to external impressions than any
other.

The organ of hearing implies the impact of ponderable bodies, the organ of smell the transference of detached
material particles, and the organs of taste, and of touch or force, the direct contact, or at least some interference of
ponderable matter, and this is true even in those instances of animal organisms, in which some of these organs are
developed to a degree of truly marvelous perfection. This being so, it seems wonderful that the organ of sight
solely should be capable of being stirred by that, which all our other organs are powerless to detect, yet which
plays an essential part in all natural phenomena, which transmits all energy and sustains all motion and, that most
intricate of all, life, but which has properties such that even a scientifically trained mind cannot help drawing a
distinction between it and all that is called matter. Considering merely this, and the fact that the eye, by its
marvelous power, widens our otherwise very narrow range of perception far beyond the limits of the small world
which is our own, to embrace myriads of other worlds, suns and stars in the infinite depths of the universe, would
make it justifiable to assert, that it is an organ of a higher order. Its performances are beyond comprehension.
Nature as far as we know never produced anything more wonderful. We can get barely a faint idea of its
prodigious power by analyzing what it does and by comparing. When ether waves impinge upon the human body,
they produce the sensations of warmth or cold, pleasure or pain, or perhaps other sensations of which we are not
aware, and any degree or intensity of these sensations, which degrees are infinite in number, hence an infinite
number of distinct sensations. But our sense of touch, or our sense of force, cannot reveal to us these differences in
degree or intensity, unless they are very great. Now we can readily conceive how an organism, such as the human,
in the eternal process of evolution, or more philosophically speaking, adaptation to Nature, being constrained to
the use of only the sense of touch or force, for instance, might develop this sense to such a degree of sensitiveness
or perfection, that it would be capable of distinguishing the minutest differences in the temperature of a body even
at some distance, to a hundredth, or thousandth, or millionth part of a degree. Yet, even this apparently impossible
performance would not begin to compare with that of the eye, which is capable of distinguishing and conveying to
the mind in a single instant innumerable peculiarities of the body, be it in form, or color, or other respects. This
power of the eye rests upon two things, namely, the rectilinear propagation of the disturbance by which it is
effected, and upon its sensitiveness. To say that the eye is sensitive is not saying anything. Compared with it, all
other organs are monstrously crude. The organ of smell which guides a dog on the trail of a deer, the organ of
touch or force which guides an insect in its wanderings, the organ of hearing, which is affected by the slightest
disturbances of the air, are sensitive organs, to be sure, but what are they compared with the human eye! No doubt
it responds to the faintest echoes or reverberations of the medium; no doubt, it brings us tidings from other worlds,
infinitely remote, but in a language we cannot as yet always understand. And why not? Because we live in a
medium filled with air and other gases, vapors and a dense mass of solid particles flying about. These play an
important part in many phenomena; they fritter away the energy of the vibrations before they can reach the eye;
they too, are the carriers of germs of destruction, they get into our lungs and other organs, clog up the channels and
imperceptibly, yet inevitably, arrest the stream of life. Could we but do away with all ponderable matter in the line
of sight of the telescope, it would reveal to us undreamt of marvels. Even the unaided eye, I think, would be
capable of distinguishing in the pure medium, small objects at distances measured probably by hundreds or
perhaps thousands of miles.
But there is something else about the eye which impresses us still more than these wonderful features which
we observed, viewing it from the standpoint of a physicist, merely as an optical instrument,—something which
appeals to us more than its marvelous faculty of being directly affected by the vibrations of the medium, without
interference of gross matter, and more than its inconceivable sensitiveness and discerning power. It is its
significance in the processes of life. No matter what one's views on nature and life may be, he must stand amazed
when, for the first time in his thoughts, he realizes the importance of the eye in the physical processes and mental
performances of the human organism. And how could it be otherwise, when he realizes, that the eye is the means
through which the human race has acquired the entire knowledge it possesses, that it controls all our motions, more
still, all our actions.
There is no way of acquiring knowledge except through the eye. What is the foundation of all philosophical
systems of ancient and modern times, in fact, of all the philosophy of man? I am, I think; I think, therefore I am.
But how could I think and how would I know that I exist, if I had not the eye? For knowledge involves
consciousness; consciousness involves ideas, conceptions; conceptions involve pictures or images, and images the
sense of vision, and therefore the organ of sight. But how about blind men, will be asked? Yes, a blind man may
depict in magnificent poems, forms and scenes from real life, from a world he physically does not see. A blind man
may touch the keys of an instrument with unerring precision, may model the fastest boat, may discover and invent,
calculate and construct, may do still greater wonders—but all the blind men who have done such things have
descended from those who had seeing eyes. Nature may reach the same result in many ways. Like a wave in the

physical world, in the infinite ocean of the medium which pervades all, so in the world of organisms, in life, an
impulse started proceeds onward, at times, may be, with the speed of light, at times, again, so slowly that for ages
and ages it seems to stay, passing through processes of a complexity inconceivable to men, but in all its forms, in
all its stages, its energy ever and ever integrally present. A single ray of light from a distant star falling upon the
eye of a tyrant in bygone times, may have altered the course of his life, may have changed the destiny of nations,
may have transformed the surface of the globe, so intricate, so inconceivably complex are the processes in Nature.
In no way can we get such an overwhelming idea of the grandeur of Nature, as when we consider, that in
accordance with the law of the conservation of energy, throughout the infinite, the forces are in a perfect balance,
and hence the energy of a single thought may determine the motion of a Universe. It is not necessary that every
individual, not even that every generation or many generations, should have the physical instrument of sight, in
order to be able to form images and to think, that is, form ideas or conceptions; but sometime or other, during the
process of evolution, the eye certainly must have existed, else thought, as we understand it, would be impossible;
else conceptions, like spirit, intellect, mind, call it as you may, could not exist. It is conceivable, that in some other
world, in some other beings, the eye is replaced by a different organ, equally or more perfect, but these beings
cannot be men.
Now what prompts us all to voluntary motions and actions of any kind? Again the eye. If I am conscious of the
motion, I must have an idea or conception, that is, an image, therefore the eye. If I am not precisely conscious of
the motion, it is, because the images are vague or indistinct, being blurred by the superimposition of many. But
when I perform the motion, does the impulse which prompts me to the action come from within or from without?
The greatest physicists have not disdained to endeavor to answer this and similar questions and have at times
abandoned themselves to the delights of pure and unrestrained thought. Such questions are generally considered
not to belong to the realm of positive physical science, but will before long be annexed to its domain. Helmholtz
has probably thought more on life than any modern scientist. Lord Kelvin expressed his belief that life's process is
electrical and that there is a force inherent to the organism and determining its motions. Just as much as I am
convinced of any physical truth I am convinced that the motive impulse must come from the outside. For, consider
the lowest organism we know—and there are probably many lower ones—an aggregation of a few cells only. If it
is capable of voluntary motion it can perform an infinite number of motions, all definite and precise. But now a
mechanism consisting of a finite number of parts and few at that, cannot perform an infinite number of definite
motions, hence the impulses which govern its movements must come from the environment. So, the atom, the
ulterior element of the Universe's structure, is tossed about in space, eternally, a play to external influences, like a
boat in a troubled sea. Were it to stop its motion it would die. Matter at rest, if such a thing could exist, would be
matter dead. Death of matter! Never has a sentence of deeper philosophical meaning been uttered. This is the way
in which Prof. Dewar forcibly expresses it in the description of his admirable experiments, in which liquid oxygen
is handled as one handles water, and air at ordinary pressure is made to condense and even to solidify by the
intense cold. Experiments, which serve to illustrate, in his language, the last feeble manifestations of life, the last
quiverings of matter about to die. But human eyes shall not witness such death. There is no death of matter, for
throughout the infinite universe, all has to move, to vibrate, that is, to live.
I have made the preceding statements at the peril of treading upon metaphysical ground, in my desire to
introduce the subject of this lecture in a manner not altogether uninteresting, I may hope, to an audience such as I
have the honor to address. But now, then, returning to the subject, this divine organ of sight, this indispensable
instrument for thought and all intellectual enjoyment, which lays open to us the marvels of this universe, through
which we have acquired what knowledge we possess, and which prompts us to, and controls, all our physical and
mental activity. By what is it affected? By light! What is light?
We have witnessed the great strides which have been made in all departments of science in recent years. So
great have been the advances that we cannot refrain from asking ourselves, Is this all true, or is it but a dream?
Centuries ago men have lived, have thought, discovered, invented, and have believed that they were soaring, while
they were merely proceeding at a snail's pace. So we too may be mistaken. But taking the truth of the observed
events as one of the implied facts of science, we must rejoice in the immense progress already made and still more
in the anticipation of what must come, judging from the possibilities opened up by modern research. There is,
however, an advance which we have been witnessing, which must be particularly gratifying to every lover of
progress. It is not a discovery, or an invention, or an achievement in any particular direction. It is an advance in all
directions of scientific thought and experiment. I mean the generalization of the natural forces and phenomena, the
looming up of a certain broad idea on the scientific horizon. It is this idea which has, however, long ago taken
possession of the most advanced minds, to which I desire to call your attention, and which I intend to illustrate in a

general way, in these experiments, as the first step in answering the question "What is light?" and to realize the
modern meaning of this word.
It is beyond the scope of my lecture to dwell upon the subject of light in general, my object being merely to
bring presently to your notice a certain class of light effects and a number of phenomena observed in pursuing the
study of these effects. But to be consistent in my remarks it is necessary to state that, according to that idea, now
accepted by the majority of scientific men as a positive result of theoretical and experimental investigation, the
various forms or manifestations of energy which were generally designated as "electric" or more precisely
"electromagnetic" are energy manifestations of the same nature as those of radiant heat and light. Therefore the
phenomena of light and heat and others besides these, may be called electrical phenomena. Thus electrical science
has become the mother science of all and its study has become all important. The day when we shall know exactly
what "electricity" is, will chronicle an event probably greater, more important than any other recorded in the
history of the human race. The time will come when the comfort, the very existence, perhaps, of man will depend
upon that wonderful agent. For our existence and comfort we require heat, light and mechanical power. How do we
now get all these? We get them from fuel, we get them by consuming material. What will man do when the forests
disappear, when the coal fields are exhausted? Only one thing, according to our present knowledge will remain;
that is, to transmit power at great distances. Men will go to the waterfalls, to the tides, which are the stores of an
infinitesimal part of Nature's immeasurable energy. There will they harness the energy and transmit the same to
their settlements, to warm their homes by, to give them light, and to keep their obedient slaves, the machines,
toiling. But how will they transmit this energy if not by electricity? Judge then, if the comfort, nay, the very
existence, of man will not depend on electricity. I am aware that this view is not that of a practical engineer, but
neither is it that of an illusionist, for it is certain, that power transmission, which at present is merely a stimulus to
enterprise, will some day be a dire necessity.
It is more important for the student, who takes up the study of light phenomena, to make himself thoroughly
acquainted with certain modern views, than to peruse entire books on the subject of light itself, as disconnected
from these views. Were I therefore to make these demonstrations before students seeking information—and for the
sake of the few of those who may be present, give me leave to so assume—it would be my principal endeavor to
impress these views upon their minds in this series of experiments.
It might be sufficient for this purpose to perform a simple and well-known experiment. I might take a familiar
appliance, a Leyden jar, charge it from a frictional machine, and then discharge it. In explaining to you its
permanent state when charged, and its transitory condition when discharging, calling your attention to the forces
which enter into play and to the various phenomena they produce, and pointing out the relation of the forces and
phenomena, I might fully succeed in illustrating that modern idea. No doubt, to the thinker, this simple experiment
would appeal as much as the most magnificent display. But this is to be an experimental demonstration, and one
which should possess, besides instructive, also entertaining features and as such, a simple experiment, such as the
one cited, would not go very far towards the attainment of the lecturer's aim. I must therefore choose another way
of illustrating, more spectacular certainly, but perhaps also more instructive. Instead of the frictional machine and
Leyden jar, I shall avail myself in these experiments, of an induction coil of peculiar properties, which was
described in detail by me in a lecture before the London Institution of Electrical Engineers, in Feb., 1892. This
induction coil is capable of yielding currents of enormous potential differences, alternating with extreme rapidity.
With this apparatus I shall endeavor to show you three distinct classes of effects, or phenomena, and it is my desire
that each experiment, while serving for the purposes of illustration, should at the same time teach us some novel
truth, or show us some novel aspect of this fascinating science. But before doing this, it seems proper and useful to
dwell upon the apparatus employed, and method of obtaining the high potentials and high-frequency currents
which are made use of in these experiments.

ON THE APPARATUS AND METHOD OF CONVERSION.

These high-frequency currents are obtained in a peculiar manner. The method employed was advanced by me
about two years ago in an experimental lecture before the American Institute of Electrical Engineers. A number of
ways, as practiced in the laboratory, of obtaining these currents either from continuous or low frequency
alternating currents, is diagramatically indicated in Fig. 165, which will be later described in detail. The general
plan is to charge condensers, from a direct or alternate-current source, preferably of high-tension, and to discharge
them disruptively while observing well-known conditions necessary to maintain the oscillations of the current. In
view of the general interest taken in high-frequency currents and effects producible by them, it seems to me

advisable to dwell at some length upon this method of conversion. In order to give you a clear idea of the action, I
will suppose that a continuous-current generator is employed, which is often very convenient. It is desirable that
the generator should possess such high tension as to be able to break through a small air space. If this is not the
case, then auxiliary means have to be resorted to, some of which will be indicated subsequently. When the
condensers are charged to a certain potential, the air, or insulating space, gives way and a disruptive discharge
occurs. There is then a sudden rush of current and generally a large portion of accumulated electrical energy
spends itself. The condensers are thereupon quickly charged and the same process is repeated in more or less rapid
succession. To produce such sudden rushes of current it is necessary to observe certain conditions. If the rate at
which the condensers are discharged is the same as that at which they are charged, then, clearly, in the assumed
case the condensers do not come into play. If the rate of discharge be smaller than the rate of charging, then, again,
the condensers cannot play an important part. But if, on the contrary, the rate of discharging is greater than that of
charging, then a succession of rushes of current is obtained. It is evident that, if the rate at which the energy is
dissipated by the discharge is very much greater than the rate of supply to the condensers, the sudden rushes will
be comparatively few, with long-time intervals between. This always occurs when a condenser of considerable
capacity is charged by means of a comparatively small machine. If the rates of supply and dissipation are not
widely different, then the rushes of current will be in quicker succession, and this the more, the more nearly equal
both the rates are, until limitations incident to each case and depending upon a number of causes are reached. Thus
we are able to obtain from a continuous-current generator as rapid a succession of discharges as we like. Of course,
the higher the tension of the generator, the smaller need be the capacity of the condensers, and for this reason,
principally, it is of advantage to employ a generator of very high tension. Besides, such a generator permits the
attaining of greater rates of vibration.

Fig. 165.
The rushes of current may be of the same direction under the conditions before assumed, but most generally
there is an oscillation superimposed upon the fundamental vibration of the current. When the conditions are so
determined that there are no oscillations, the current impulses are unidirectional and thus a means is provided of
transforming a continuous current of high tension, into a direct current of lower tension, which I think may find
employment in the arts.

This method of conversion is exceedingly interesting and I was much impressed by its beauty when I first
conceived it. It is ideal in certain respects. It involves the employment of no mechanical devices of any kind, and it
allows of obtaining currents of any desired frequency from an ordinary circuit, direct or alternating. The frequency
of the fundamental discharges depending on the relative rates of supply and dissipation can be readily varied
within wide limits, by simple adjustments of these quantities, and the frequency of the superimposed vibration by
the determination of the capacity, self-induction and resistance of the circuit. The potential of the currents, again,
may be raised as high as any insulation is capable of withstanding safely by combining capacity and self-induction
or by induction in a secondary, which need have but comparatively few turns.
As the conditions are often such that the intermittence or oscillation does not readily establish itself, especially
when a direct current source is employed, it is of advantage to associate an interrupter with the arc, as I have, some
time ago, indicated the use of an air-blast or magnet, or other such device readily at hand. The magnet is employed
with special advantage in the conversion of direct currents, as it is then very effective. If the primary source is an
alternate current generator, it is desirable, as I have stated on another occasion, that the frequency should be low,
and that the current forming the arc be large, in order to render the magnet more effective.
A form of such discharger with a magnet which has been found convenient, and adopted after some trials, in
the conversion of direct currents particularly, is illustrated in Fig. 166. N S are the pole pieces of a very strong
magnet which is excited by a coil C. The pole pieces are slotted for adjustment and can be fastened in any position
by screws s s1. The discharge rods d d1, thinned down on the ends in order to allow a closer approach of the
magnetic pole pieces, pass through the columns of brass b b1 and are fastened in position by screws s2 s2. Springs r
r1 and collars c c1 are slipped on the rods, the latter serving to set the points of the rods at a certain suitable
distance by means of screws s3 s3, and the former to draw the points apart. When it is desired to start the arc, one
of the large rubber handles h h1 is tapped quickly with the hand, whereby the points of the rods are brought in
contact but are instantly separated by the springs r r1. Such an arrangement has been found to be often necessary,
namely in cases when the e. m. f. was not large enough to cause the discharge to break through the gap, and also
when it was desirable to avoid short circuiting of the generator by the metallic contact of the rods. The rapidity of
the interruptions of the current with a magnet depends on the intensity of the magnetic field and on the potential
difference at the end of the arc. The interruptions are generally in such quick succession as to produce a musical
sound. Years ago it was observed that when a powerful induction coil is discharged between the poles of a strong
magnet, the discharge produces a loud noise, not unlike a small pistol shot. It was vaguely stated that the spark was
intensified by the presence of the magnetic field. It is now clear that the discharge current, flowing for some time,
was interrupted a great number of times by the magnet, thus producing the sound. The phenomenon is especially
marked when the field circuit of a large magnet or dynamo is broken in a powerful magnetic field.

Fig. 166.
When the current through the gap is comparatively large, it is of advantage to slip on the points of the
discharge rods pieces of very hard carbon and let the arc play between the carbon pieces. This preserves the rods,
and besides has the advantage of keeping the air space hotter, as the heat is not conducted away as quickly through
the carbons, and the result is that a smaller e. m. f. in the arc gap is required to maintain a succession of discharges.

Fig. 167.
Another form of discharger, which may be employed with advantage in some cases, is illustrated in Fig. 167.
In this form the discharge rods d d1 pass through perforations in a wooden box B, which is thickly coated with mica
on the inside, as indicated by the heavy lines. The perforations are provided with mica tubes m m1 of some
thickness, which are preferably not in contact with the rods d d1. The box has a cover C which is a little larger and
descends on the outside of the box. The spark gap is warmed by a small lamp l contained in the box. A plate p
above the lamp allows the draught to pass only through the chimney e of the lamp, the air entering through holes o
o in or near the bottom of the box and following the path indicated by the arrows. When the discharger is in
operation, the door of the box is closed so that the light of the arc is not visible outside. It is desirable to exclude
the light as perfectly as possible, as it interferes with some experiments. This form of discharger is simple and very
effective when properly manipulated. The air being warmed to a certain temperature, has its insulating power
impaired; it becomes dielectrically weak, as it were, and the consequence is that the arc can be established at much
greater distance. The arc should, of course, be sufficiently insulating to allow the discharge to pass through the gap
disruptively. The arc formed under such conditions, when long, may be made extremely sensitive, and the weak
draught through the lamp chimney c is quite sufficient to produce rapid interruptions. The adjustment is made by
regulating the temperature and velocity of the draught. Instead of using the lamp, it answers the purpose to provide
for a draught of warm air in other ways. A very simple way which has been practiced is to enclose the arc in a long
vertical tube, with plates on the top and bottom for regulating the temperature and velocity of the air current. Some
provision had to be made for deadening the sound.
The air may be rendered dielectrically weak also by rarefaction. Dischargers of this kind have likewise been
used by me in connection with a magnet. A large tube is for this purpose provided with heavy electrodes of carbon
or metal, between which the discharge is made to pass, the tube being placed in a powerful magnetic field. The
exhaustion of the tube is carried to a point at which the discharge breaks through easily, but the pressure should be
more than 75 millimetres, at which the ordinary thread discharge occurs. In another form of discharger, combining
the features before mentioned, the discharge was made to pass between two adjustable magnetic pole pieces, the
space between them being kept at an elevated temperature.
It should be remarked here that when such, or interrupting devices of any kind, are used and the currents are
passed through the primary of a disruptive discharge coil, it is not, as a rule, of advantage to produce a number of
interruptions of the current per second greater than the natural frequency of vibration of the dynamo supply circuit,
which is ordinarily small. It should also be pointed out here, that while the devices mentioned in connection with
the disruptive discharge are advantageous under certain conditions, they may be sometimes a source of trouble, as
they produce intermittences and other irregularities in the vibration which it would be very desirable to overcome.
There is, I regret to say, in this beautiful method of conversion a defect, which fortunately is not vital, and
which I have been gradually overcoming. I will best call attention to this defect and indicate a fruitful line of work,
by comparing the electrical process with its mechanical analogue. The process may be illustrated in this manner.
Imagine a tank with a wide opening at the bottom, which is kept closed by spring pressure, but so that it snaps off
suddenly when the liquid in the tank has reached a certain height. Let the fluid be supplied to the tank by means of
a pipe feeding at a certain rate. When the critical height of the liquid is reached, the spring gives way and the
bottom of the tank drops out. Instantly the liquid falls through the wide opening, and the spring, reasserting itself,
closes the bottom again. The tank is now filled, and after a certain time interval the same process is repeated. It is
clear, that if the pipe feeds the fluid quicker than the bottom outlet is capable of letting it pass through, the bottom
will remain off and the tank will still overflow. If the rates of supply are exactly equal, then the bottom lid will
remain partially open and no vibration of the same and of the liquid column will generally occur, though it might,
if started by some means. But if the inlet pipe does not feed the fluid fast enough for the outlet, then there will be
always vibration. Again, in such case, each time the bottom flaps up or down, the spring and the liquid column, if
the pliability of the spring and the inertia of the moving parts are properly chosen, will perform independent
vibrations. In this analogue the fluid may be likened to electricity or electrical energy, the tank to the condenser,
the spring to the dielectric, and the pipe to the conductor through which electricity is supplied to the condenser. To
make this analogy quite complete it is necessary to make the assumption, that the bottom, each time it gives way, is
knocked violently against a non-elastic stop, this impact involving some loss of energy; and that, besides, some
dissipation of energy results due to frictional losses. In the preceding analogue the liquid is supposed to be under a
steady pressure. If the presence of the fluid be assumed to vary rhythmically, this may be taken as corresponding to

the case of an alternating current. The process is then not quite as simple to consider, but the action is the same in
principle.
It is desirable, in order to maintain the vibration economically, to reduce the impact and frictional losses as
much as possible. As regards the latter, which in the electrical analogue correspond to the losses due to the
resistance of the circuits, it is impossible to obviate them entirely, but they can be reduced to a minimum by a
proper selection of the dimensions of the circuits and by the employment of thin conductors in the form of strands.
But the loss of energy caused by the first breaking through of the dielectric—which in the above example
corresponds to the violent knock of the bottom against the inelastic stop—would be more important to overcome.
At the moment of the breaking through, the air space has a very high resistance, which is probably reduced to a
very small value when the current has reached some strength, and the space is brought to a high temperature. It
would materially diminish the loss of energy if the space were always kept at an extremely high temperature, but
then there would be no disruptive break. By warming the space moderately by means of a lamp or otherwise, the
economy as far as the arc is concerned is sensibly increased. But the magnet or other interrupting device does not
diminish the loss in the arc. Likewise, a jet of air only facilitates the carrying off of the energy. Air, or a gas in
general, behaves curiously in this respect. When two bodies charged to a very high potential, discharge
disruptively through an air space, any amount of energy may be carried off by the air. This energy is evidently
dissipated by bodily carriers, in impact and collisional losses of the molecules. The exchange of the molecules in
the space occurs with inconceivable rapidity. A powerful discharge taking place between two electrodes, they may
remain entirely cool, and yet the loss in the air may represent any amount of energy. It is perfectly practicable, with
very great potential differences in the gap, to dissipate several horse-power in the arc of the discharge without even
noticing a small increase in the temperature of the electrodes. All the frictional losses occur then practically in the
air. If the exchange of the air molecules is prevented, as by enclosing the air hermetically, the gas inside of the
vessel is brought quickly to a high temperature, even with a very small discharge. It is difficult to estimate how
much of the energy is lost in sound waves, audible or not, in a powerful discharge. When the currents through the
gap are large, the electrodes may become rapidly heated, but this is not a reliable measure of the energy wasted in
the arc, as the loss through the gap itself may be comparatively small. The air or a gas in general is, at ordinary
pressure at least, clearly not the best medium through which a disruptive discharge should occur. Air or other gas
under great pressure is of course a much more suitable medium for the discharge gap. I have carried on long-
continued experiments in this direction, unfortunately less practicable on account of the difficulties and expense in
getting air under great pressure. But even if the medium in the discharge space is solid or liquid, still the same
losses take place, though they are generally smaller, for just as soon as the arc is established, the solid or liquid is
volatilized. Indeed, there is no body known which would not be disintegrated by the arc, and it is an open question
among scientific men, whether an arc discharge could occur at all in the air itself without the particles of the
electrodes being torn off. When the current through the gap is very small and the arc very long, I believe that a
relatively considerable amount of heat is taken up in the disintegration of the electrodes, which partially on this
account may remain quite cold.
The ideal medium for a discharge gap should only crack, and the ideal electrode should be of some material
which cannot be disintegrated. With small currents through the gap it is best to employ aluminum, but not when
the currents are large. The disruptive break in the air, or more or less in any ordinary medium, is not of the nature
of a crack, but it is rather comparable to the piercing of innumerable bullets through a mass offering great frictional
resistances to the motion of the bullets, this involving considerable loss of energy. A medium which would merely
crack when strained electrostatically—and this possibly might be the case with a perfect vacuum, that is, pure ether
—would involve a very small loss in the gap, so small as to be entirely negligible, at least theoretically, because a
crack may be produced by an infinitely small displacement. In exhausting an oblong bulb provided with two
aluminum terminals, with the greatest care, I have succeeded in producing such a vacuum that the secondary
discharge of a disruptive discharge coil would break disruptively through the bulb in the form of fine spark
streams. The curious point was that the discharge would completely ignore the terminals and start far behind the
two aluminum plates which served as electrodes. This extraordinary high vacuum could only be maintained for a
very short while. To return to the ideal medium, think, for the sake of illustration, of a piece of glass or similar
body clamped in a vice, and the latter tightened more and more. At a certain point a minute increase of the pressure
will cause the glass to crack. The loss of energy involved in splitting the glass may be practically nothing, for
though the force is great, the displacement need be but extremely small. Now imagine that the glass would possess
the property of closing again perfectly the crack upon a minute diminution of the pressure. This is the way the
dielectric in the discharge space should behave. But inasmuch as there would be always some loss in the gap, the
medium, which should be continuous, should exchange through the gap at a rapid rate. In the preceding example,

the glass being perfectly closed, it would mean that the dielectric in the discharge space possesses a great
insulating power; the glass being cracked, it would signify that the medium in the space is a good conductor. The
dielectric should vary enormously in resistance by minute variations of the e. m. f. across the discharge space. This
condition is attained, but in an extremely imperfect manner, by warming the air space to a certain critical
temperature, dependent on the e. m. f. across the gap, or by otherwise impairing the insulating power of the air. But
as a matter of fact the air does never break down disruptively, if this term be rigorously interpreted, for before the
sudden rush of the current occurs, there is always a weak current preceding it, which rises first gradually and then
with comparative suddenness. That is the reason why the rate of change is very much greater when glass, for
instance, is broken through, than when the break takes place through an air space of equivalent dielectric strength.
As a medium for the discharge space, a solid, or even a liquid, would be preferable therefor. It is somewhat
difficult to conceive of a solid body which would possess the property of closing instantly after it has been
cracked. But a liquid, especially under great pressure, behaves practically like a solid, while it possesses the
property of closing the crack. Hence it was thought that a liquid insulator might be more suitable as a dielectric
than air. Following out this idea, a number of different forms of dischargers in which a variety of such insulators,
sometimes under great pressure, were employed, have been experimented upon. It is thought sufficient to dwell in
a few words upon one of the forms experimented upon. One of these dischargers is illustrated in Figs. 168a and
168b.

Fig. 168a. Fig. 168b.

A hollow metal pulley P (Fig. 168a), was fastened upon an arbor a, which by suitable means was rotated at a
considerable speed. On the inside of the pulley, but disconnected from the same, was supported a thin disc h
(which is shown thick for the sake of clearness), of hard rubber in which there were embedded two metal segments
s s with metallic extensions e e into which were screwed conducting terminals t t covered with thick tubes of hard
rubber t t. The rubber disc h with its metallic segments s s, was finished in a lathe, and its entire surface highly
polished so as to offer the smallest possible frictional resistance to the motion through a fluid. In the hollow of the
pulley an insulating liquid such as a thin oil was poured so as to reach very nearly to the opening left in the flange
f, which was screwed tightly on the front side of the pulley. The terminals t t, were connected to the opposite
coatings of a battery of condensers so that the discharge occurred through the liquid. When the pulley was rotated,
the liquid was forced against the rim of the pulley and considerable fluid pressure resulted. In this simple way the
discharge gap was filled with a medium which behaved practically like a solid, which possessed the quality of
closing instantly upon the occurrence of the break, and which moreover was circulating through the gap at a rapid
rate. Very powerful effects were produced by discharges of this kind with liquid interrupters, of which a number of
different forms were made. It was found that, as expected, a longer spark for a given length of wire was obtainable
in this way than by using air as an interrupting device. Generally the speed, and therefore also the fluid pressure,
was limited by reason of the fluid friction, in the form of discharger described, but the practically obtainable speed
was more than sufficient to produce a number of breaks suitable for the circuits ordinarily used. In such instances
the metal pulley P was provided with a few projections inwardly, and a definite number of breaks was then
produced which could be computed from the speed of rotation of the pulley. Experiments were also carried on with
liquids of different insulating power with the view of reducing the loss in the arc. When an insulating liquid is
moderately warmed, the loss in the arc is diminished.

A point of some importance was noted in experiments with various discharges of this kind. It was found, for
instance, that whereas the conditions maintained in these forms were favorable for the production of a great spark
length, the current so obtained was not best suited to the production of light effects. Experience undoubtedly has
shown, that for such purposes a harmonic rise and fall of the potential is preferable. Be it that a solid is rendered
incandescent, or phosphorescent, or be it that energy is transmitted by condenser coating through the glass, it is
quite certain that a harmonically rising and falling potential produces less destructive action, and that the vacuum
is more permanently maintained. This would be easily explained if it were ascertained that the process going on in
an exhausted vessel is of an electrolytic nature.
In the diagrammatical sketch, Fig. 165, which has been already referred to, the cases which are most likely to
be met with in practice are illustrated. One has at his disposal either direct or alternating currents from a supply
station. It is convenient for an experimenter in an isolated laboratory to employ a machine G, such as illustrated,
capable of giving both kinds of currents. In such case it is also preferable to use a machine with multiple circuits,
as in many experiments it is useful and convenient to have at one's disposal currents of different phases. In the
sketch, D represents the direct and A the alternating circuit. In each of these, three branch circuits are shown, all of
which are provided with double line switches s s s s s s. Consider first the direct current conversion; Ia represents
the simplest case. If the e. m. f. of the generator is sufficient to break through a small air space, at least when the
latter is warmed or otherwise rendered poorly insulating, there is no difficulty in maintaining a vibration with fair
economy by judicious adjustment of the capacity, self-induction and resistance of the circuit L containing the
devices l l m. The magnet N, S, can be in this case advantageously combined with the air space. The discharger d d
with the magnet may be placed either way, as indicated by the full or by the dotted lines. The circuit Ia with the
connections and devices is supposed to possess dimensions such as are suitable for the maintenance of a vibration.
But usually the e. m. f. on the circuit or branch Ia will be something like a 100 volts or so, and in this case it is not
sufficient to break through the gap. Many different means may be used to remedy this by raising the e. m. f. across
the gap. The simplest is probably to insert a large self-induction coil in series with the circuit L. When the arc is
established, as by the discharger illustrated in Fig. 166, the magnet blows the arc out the instant it is formed. Now
the extra current of the break, being of high e. m. f., breaks through the gap, and a path of low resistance for the
dynamo current being again provided, there is a sudden rush of current from the dynamo upon the weakening or
subsidence of the extra current. This process is repeated in rapid succession, and in this manner I have maintained
oscillation with as low as 50 volts, or even less, across the gap. But conversion on this plan is not to be
recommended on account of the too heavy currents through the gap and consequent heating of the electrodes;
besides, the frequencies obtained in this way are low, owing to the high self-induction necessarily associated with
the circuit. It is very desirable to have the e. m. f. as high as possible, first, in order to increase the economy of the
conversion, and, secondly, to obtain high frequencies. The difference of potential in this electric oscillation is, of
course, the equivalent of the stretching force in the mechanical vibration of the spring. To obtain very rapid
vibration in a circuit of some inertia, a great stretching force or difference of potential is necessary. Incidentally,
when the e. m. f. is very great, the condenser which is usually employed in connection with the circuit need but
have a small capacity, and many other advantages are gained. With a view of raising the e. m. f. to a many times
greater value than obtainable from ordinary distribution circuits, a rotating transformer g is used, as indicated at
IIa, Fig. 165, or else a separate high potential machine is driven by means of a motor operated from the generator
G. The latter plan is in fact preferable, as changes are easier made. The connections from the high tension winding
are quite similar to those in branch Ia with the exception that a condenser C, which should be adjustable, is
connected to the high tension circuit. Usually, also, an adjustable self-induction coil in series with the circuit has
been employed in these experiments. When the tension of the currents is very high, the magnet ordinarily used in
connection with the discharger is of comparatively small value, as it is quite easy to adjust the dimensions of the
circuit so that oscillation is maintained. The employment of a steady e. m. f. in the high frequency conversion
affords some advantages over the employment of alternating e. m. f., as the adjustments are much simpler and the
action can be easier controlled. But unfortunately one is limited by the obtainable potential difference. The
winding also breaks down easily in consequence of the sparks which form between the sections of the armature or
commutator when a vigorous oscillation takes place. Besides, these transformers are expensive to build. It has been
found by experience that it is best to follow the plan illustrated at IIIa. In this arrangement a rotating transformer g,
is employed to convert the low tension direct currents into low frequency alternating currents, preferably also of
small tension. The tension of the currents is then raised in a stationary transformer T. The secondary S of this
transformer is connected to an adjustable condenser C which discharges through the gap or discharger d d, placed
in either of the ways indicated, through the primary P of a disruptive discharge coil, the high frequency current

being obtained from the secondary S of this coil, as described on previous occasions. This will undoubtedly be
found the cheapest and most convenient way of converting direct currents.
The three branches of the circuit A represent the usual cases met in practice when alternating currents are
converted. In Fig. 1b a condenser C, generally of large capacity, is connected to the circuit L containing the devices
l l, m m. The devices m m are supposed to be of high self-induction so as to bring the frequency of the circuit more
or less to that of the dynamo. In this instance the discharger d d should best have a number of makes and breaks
per second equal to twice the frequency of the dynamo. If not so, then it should have at least a number equal to a
multiple or even fraction of the dynamo frequency. It should be observed, referring to Ib, that the conversion to a
high potential is also effected when the discharger d d, which is shown in the sketch, is omitted. But the effects
which are produced by currents which rise instantly to high values, as in a disruptive discharge, are entirely
different from those produced by dynamo currents which rise and fall harmonically. So, for instance, there might
be in a given case a number of makes and breaks at d d equal to just twice the frequency of the dynamo, or in other
words, there may be the same number of fundamental oscillations as would be produced without the discharge gap,
and there might even not be any quicker superimposed vibration; yet the differences of potential at the various
points of the circuit, the impedance and other phenomena, dependent upon the rate of change, will bear no
similarity in the two cases. Thus, when working with currents discharging disruptively, the element chiefly to be
considered is not the frequency, as a student might be apt to believe, but the rate of change per unit of time. With
low frequencies in a certain measure the same effects may be obtained as with high frequencies, provided the rate
of change is sufficiently great. So if a low frequency current is raised to a potential of, say, 75,000 volts, and the
high tension current passed through a series of high resistance lamp filaments, the importance of the rarefied gas
surrounding the filament is clearly noted, as will be seen later; or, if a low frequency current of several thousand
amperes is passed through a metal bar, striking phenomena of impedance are observed, just as with currents of
high frequencies. But it is, of course, evident that with low frequency currents it is impossible to obtain such rates
of change per unit of time as with high frequencies, hence the effects produced by the latter are much more
prominent. It is deemed advisable to make the preceding remarks, inasmuch as many more recently described
effects have been unwittingly identified with high frequencies. Frequency alone in reality does not mean anything,
except when an undisturbed harmonic oscillation is considered.
In the branch IIIb a similar disposition to that in Ib is illustrated, with the difference that the currents
discharging through the gap d d are used to induce currents in the secondary S of a transformer T. In such case the
secondary should be provided with an adjustable condenser for the purpose of tuning it to the primary.
IIb illustrates a plan of alternate current high frequency conversion which is most frequently used and which is
found to be most convenient. This plan has been dwelt upon in detail on previous occasions and need not be
described here.
Some of these results were obtained by the use of a high frequency alternator. A description of such machines
will be found in my original paper before the American Institute of Electrical Engineers, and in periodicals of that
period, notably in The Electrical Engineer of March 18, 1891.
I will now proceed with the experiments.

ON PHENOMENA PRODUCED BY ELECTROSTATIC FORCE.

The first class of effects I intend to show you are effects produced by electrostatic force. It is the force which
governs the the motion of the atoms, which causes them to collide and develop the life-sustaining energy of heat
and light, and which causes them to aggregate in an infinite variety of ways, according to Nature's fanciful designs,
and to form all these wondrous structures we perceive around us; it is, in fact, if our present views be true, the most
important force for us to consider in Nature. As the term electrostatic might imply a steady electric condition, it
should be remarked, that in these experiments the force is not constant, but varies at a rate which may be
considered moderate, about one million times a second, or thereabouts. This enables me to produce many effects
which are not producible with an unvarying force.
When two conducting bodies are insulated and electrified, we say that an electrostatic force is acting between
them. This force manifests itself in attractions, repulsions and stresses in the bodies and space or medium without.
So great may be the strain exerted in the air, or whatever separates the two conducting bodies, that it may break
down, and we observe sparks or bundles of light or streamers, as they are called. These streamers form abundantly

when the force through the air is rapidly varying. I will illustrate this action of electrostatic force in a novel
experiment in which I will employ the induction coil before referred to. The coil is contained in a trough filled
with oil, and placed under the table. The two ends of the secondary wire pass through the two thick columns of
hard rubber which protrude to some height above the table. It is necessary to insulate the ends or terminals of the
secondary heavily with hard rubber, because even dry wood is by far too poor an insulator for these currents of
enormous potential differences. On one of the terminals of the coil, I have placed a large sphere of sheet brass,
which is connected to a larger insulated brass plate, in order to enable me to perform the experiments under
conditions, which, as you will see, are more suitable for this experiment. I now set the coil to work and approach
the free terminal with a metallic object held in my hand, this simply to avoid burns. As I approach the metallic
object to a distance of eight or ten inches, a torrent of furious sparks breaks forth from the end of the secondary
wire, which passes through the rubber column. The sparks cease when the metal in my hand touches the wire. My
arm is now traversed by a powerful electric current, vibrating at about the rate of one million times a second. All
around me the electrostatic force makes itself felt, and the air molecules and particles of dust flying about are acted
upon and are hammering violently against my body. So great is this agitation of the particles, that when the lights
are turned out you may see streams of feeble light appear on some parts of my body. When such a streamer breaks
out on any part of the body, it produces a sensation like the pricking of a needle. Were the potentials sufficiently
high and the frequency of the vibration rather low, the skin would probably be ruptured under the tremendous
strain, and the blood would rush out with great force in the form of fine spray or jet so thin as to be invisible, just
as oil will when placed on the positive terminal of a Holtz machine. The breaking through of the skin though it
may seem impossible at first, would perhaps occur, by reason of the tissues under the skin being incomparably
better conducting. This, at least, appears plausible, judging from some observations.

Fig. 169.
I can make these streams of light visible to all, by touching with the metallic object one of the terminals as
before, and approaching my free hand to the brass sphere, which is connected to the second terminal of the coil. As
the hand is approached, the air between it and the sphere, or in the immediate neighborhood, is more violently
agitated, and you see streams of light now break forth from my finger tips and from the whole hand (Fig. 169).
Were I to approach the hand closer, powerful sparks would jump from the brass sphere to my hand, which might
be injurious. The streamers offer no particular inconvenience, except that in the ends of the finger tips a burning
sensation is felt. They should not be confounded with those produced by an influence machine, because in many
respects they behave differently. I have attached the brass sphere and plate to one of the terminals in order to
prevent the formation of visible streamers on that terminal, also in order to prevent sparks from jumping at a
considerable distance. Besides, the attachment is favorable for the working of the coil.
The streams of light which you have observed issuing from my hand are due to a potential of about 200,000
volts, alternating in rather irregular intervals, sometimes like a million times a second. A vibration of the same
amplitude, but four times as fast, to maintain which over 3,000,000 volts would be required, would be more than
sufficient to envelop my body in a complete sheet of flame. But this flame would not burn me up; quite contrarily,

the probability is that I would not be injured in the least. Yet a hundredth part of that energy, otherwise directed,
would be amply sufficient to kill a person.
The amount of energy which may thus be passed into the body of a person depends on the frequency and
potential of the currents, and by making both of these very great, a vast amount of energy may be passed into the
body without causing any discomfort, except perhaps, in the arm, which is traversed by a true conduction current.
The reason why no pain in the body is felt, and no injurious effect noted, is that everywhere, if a current be
imagined to flow through the body, the direction of its flow would be at right angles to the surface; hence the body
of the experimenter offers an enormous section to the current, and the density is very small, with the exception of
the arm, perhaps, where the density may be considerable. But if only a small fraction of that energy would be
applied in such a way that a current would traverse the body in the same manner as a low frequency current, a
shock would be received which might be fatal. A direct or low frequency alternating current is fatal, I think,
principally because its distribution through the body is not uniform, as it must divide itself in minute streamlets of
great density, whereby some organs are vitally injured. That such a process occurs I have not the least doubt,
though no evidence might apparently exist, or be found upon examination. The surest to injure and destroy life, is
a continuous current, but the most painful is an alternating current of very low frequency. The expression of these
views, which are the result of long continued experiment and observation, both with steady and varying currents, is
elicited by the interest which is at present taken in this subject, and by the manifestly erroneous ideas which are
daily propounded in journals on this subject.
I may illustrate an effect of the electrostatic force by another striking experiment, but before, I must call your
attention to one or two facts. I have said before, that when the medium between two oppositely electrified bodies is
strained beyond a certain limit it gives way and, stated in popular language, the opposite electric charges unite and
neutralize each other. This breaking down of the medium occurs principally when the force acting between the
bodies is steady, or varies at a moderate rate. Were the variation sufficiently rapid, such a destructive break would
not occur, no matter how great the force, for all the energy would be spent in radiation, convection and mechanical
and chemical action. Thus the spark length, or greatest distance which a spark will jump between the electrified
bodies is the smaller, the greater the variation or time rate of change. But this rule may be taken to be true only in a
general way, when comparing rates which are widely different.

Fig. 170a. Fig. 170b.

I will show you by an experiment the difference in the effect produced by a rapidly varying and a steady or
moderately varying force. I have here two large circular brass plates p p (Fig. 170a and Fig. 170b), supported on
movable insulating stands on the table, connected to the ends of the secondary of a coil similar to the one used
before. I place the plates ten or twelve inches apart and set the coil to work. You see the whole space between the
plates, nearly two cubic feet, filled with uniform light, Fig. 170a. This light is due to the streamers you have seen
in the first experiment, which are now much more intense. I have already pointed out the importance of these
streamers in commercial apparatus and their still greater importance in some purely scientific investigations. Often
they are too weak to be visible, but they always exist, consuming energy and modifying the action of the
apparatus. When intense, as they are at present, they produce ozone in great quantity, and also, as Professor
Crookes has pointed out, nitrous acid. So quick is the chemical action that if a coil, such as this one, is worked for

a very long time it will make the atmosphere of a small room unbearable, for the eyes and throat are attacked. But
when moderately produced, the streamers refresh the atmosphere wonderfully, like a thunder-storm, and exercises
unquestionably a beneficial effect.
In this experiment the force acting between the plates changes in intensity and direction at a very rapid rate. I
will now make the rate of change per unit time much smaller. This I effect by rendering the discharges through the
primary of the induction coil less frequent, and also by diminishing the rapidity of the vibration in the secondary.
The former result is conveniently secured by lowering the e. m. f. over the air gap in the primary circuit, the latter
by approaching the two brass plates to a distance of about three or four inches. When the coil is set to work, you
see no streamers or light between the plates, yet the medium between them is under a tremendous strain. I still
further augment the strain by raising the e. m. f. in the primary circuit, and soon you see the air give way and the
hall is illuminated by a shower of brilliant and noisy sparks, Fig. 170b. These sparks could be produced also with
unvarying force; they have been for many years a familiar phenomenon, though they were usually obtained from
an entirely different apparatus. In describing these two phenomena so radically different in appearance, I have
advisedly spoken of a "force" acting between the plates. It would be in accordance with accepted views to say, that
there was an "alternating e. m. f," acting between the plates. This term is quite proper and applicable in all cases
where there is evidence of at least a possibility of an essential inter-dependence of the electric state of the plates, or
electric action in their neighborhood. But if the plates were removed to an infinite distance, or if at a finite
distance, there is no probability or necessity whatever for such dependence. I prefer to use the term "electrostatic
force," and to say that such a force is acting around each plate or electrified insulated body in general. There is an
inconvenience in using this expression as the term incidentally means a steady electric condition; but a proper
nomenclature will eventually settle this difficulty.
I now return to the experiment to which I have already alluded, and with which I desire to illustrate a striking
effect produced by a rapidly varying electrostatic force. I attach to the end of the wire, l (Fig. 171), which is in
connection with one of the terminals of the secondary of the induction coil, an exhausted bulb b. This bulb
contains a thin carbon filament f, which is fastened to a platinum wire w, sealed in the glass and leading outside of
the bulb, where it connects to the wire l. The bulb may be exhausted to any degree attainable with ordinary
apparatus. Just a moment before, you have witnessed the breaking down of the air between the charged brass
plates. You know that a plate of glass, or any other insulating material, would break down in like manner. Had I
therefore a metallic coating attached to the outside of the bulb, or placed near the same, and were this coating
connected to the other terminal of the coil, you would be prepared to see the glass give way if the strain were
sufficiently increased. Even were the coating not connected to the other terminal, but to an insulated plate, still, if
you have followed recent developments, you would naturally expect a rupture of the glass.

Fig. 171. Fig. 172a. Fig. 172b.

But it will certainly surprise you to note that under the action of the varying electrostatic force, the glass gives
way when all other bodies are removed from the bulb. In fact, all the surrounding bodies we perceive might be
removed to an infinite distance without affecting the result in the slightest. When the coil is set to work, the glass is
invariably broken through at the seal, or other narrow channel, and the vacuum is quickly impaired. Such a
damaging break would not occur with a steady force, even if the same were many times greater. The break is due
to the agitation of the molecules of the gas within the bulb, and outside of the same. This agitation, which is
generally most violent in the narrow pointed channel near the seal, causes a heating and rupture of the glass. This
rupture, would, however, not occur, not even with a varying force, if the medium filling the inside of the bulb, and
that surrounding it, were perfectly homogeneous. The break occurs much quicker if the top of the bulb is drawn
out into a fine fibre. In bulbs used with these coils such narrow, pointed channels must therefore be avoided.
When a conducting body is immersed in air, or similar insulating medium, consisting of, or containing, small
freely movable particles capable of being electrified, and when the electrification of the body is made to undergo a
very rapid change—which is equivalent to saying that the electrostatic force acting around the body is varying in
intensity,—the small particles are attracted and repelled, and their violent impacts against the body may cause a
mechanical motion of the latter. Phenomena of this kind are noteworthy, inasmuch as they have not been observed
before with apparatus such as has been commonly in use. If a very light conducting sphere be suspended on an
exceedingly fine wire, and charged to a steady potential, however high, the sphere will remain at rest. Even if the
potential would be rapidly varying, provided that the small particles of matter, molecules or atoms, are evenly
distributed, no motion of the sphere should result. But if one side of the conducting sphere is covered with a thick
insulating layer, the impacts of the particles will cause the sphere to move about, generally in irregular curves, Fig.
172a. In like manner, as I have shown on a previous occasion, a fan of sheet metal, Fig. 172b, covered partially
with insulating material as indicated, and placed upon the terminal of the coil so as to turn freely on it, is spun
around.
All these phenomena you have witnessed and others which will be shown later, are due to the presence of a
medium like air, and would not occur in a continuous medium. The action of the air may be illustrated still better
by the following experiment. I take a glass tube t, Fig. 173, of about an inch in diameter, which has a platinum
wire w sealed in the lower end, and to which is attached a thin lamp filament f. I connect the wire with the terminal
of the coil and set the coil to work. The platinum wire is now electrified positively and negatively in rapid
succession and the wire and air inside of the tube is rapidly heated by the impacts of the particles, which may be so
violent as to render the filament incandescent. But if I pour oil in the tube, just as soon as the wire is covered with
the oil, all action apparently ceases and there is no marked evidence of heating. The reason of this is that the oil is
a practically continuous medium. The displacements in such a continuous medium are, with these frequencies, to
all appearance incomparably smaller than in air, hence the work performed in such a medium is insignificant. But
oil would behave very differently with frequencies many times as great, for even though the displacements be
small, if the frequency were much greater, considerable work might be performed in the oil.

Fig. 173. Fig. 174.

The electrostatic attractions and repulsions between bodies of measurable dimensions are, of all the
manifestations of this force, the first so-called electrical phenomena noted. But though they have been known to us
for many centuries, the precise nature of the mechanism concerned in these actions is still unknown to us, and has
not been even quite satisfactorily explained. What kind of mechanism must that be? We cannot help wondering
when we observe two magnets attracting and repelling each other with a force of hundreds of pounds with
apparently nothing between them. We have in our commercial dynamos magnets capable of sustaining in mid-air
tons of weight. But what are even these forces acting between magnets when compared with the tremendous
attractions and repulsions produced by electrostatic force, to which there is apparently no limit as to intensity. In
lightning discharges bodies are often charged to so high a potential that they are thrown away with inconceivable
force and torn asunder or shattered into fragments. Still even such effects cannot compare with the attractions and
repulsions which exist between charged molecules or atoms, and which are sufficient to project them with speeds
of many kilometres a second, so that under their violent impact bodies are rendered highly incandescent and are
volatilized. It is of special interest for the thinker who inquires into the nature of these forces to note that whereas
the actions between individual molecules or atoms occur seemingly under any conditions, the attractions and
repulsions of bodies of measurable dimensions imply a medium possessing insulating properties. So, if air, either
by being rarefied or heated, is rendered more or less conducting, these actions between two electrified bodies
practically cease, while the actions between the individual atoms continue to manifest themselves.
An experiment may serve as an illustration and as a means of bringing out other features of interest. Some time
ago I showed that a lamp filament or wire mounted in a bulb and connected to one of the terminals of a high
tension secondary coil is set spinning, the top of the filament generally describing a circle. This vibration was very
energetic when the air in the bulb was at ordinary pressure and became less energetic when the air in the bulb was
strongly compressed. It ceased altogether when the air was exhausted so as to become comparatively good
conducting. I found at that time that no vibration took place when the bulb was very highly exhausted. But I
conjectured that the vibration which I ascribed to the electrostatic action between the walls of the bulb and the
filament should take place also in a highly exhausted bulb. To test this under conditions which were more
favorable, a bulb like the one in Fig. 174, was constructed. It comprised a globe b, in the neck of which was sealed
a platinum wire w carrying a thin lamp filament f. In the lower part of the globe a tube t was sealed so as to
surround the filament. The exhaustion was carried as far as it was practicable with the apparatus employed.