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Title: Cyclopedia of Telephony and Telegraphy, Vol. 1

Author: American School of Correspondence

Release date: April 14, 2005 [eBook #15617]
Most recently updated: December 14, 2020

Language: English

Other information and formats: www.gutenberg.org/ebooks/15617

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*** START OF THE PROJECT GUTENBERG EBOOK CYCLOPEDIA
OF TELEPHONY AND TELEGRAPHY, VOL. 1 ***

Cyclopedia
of

Telephony and Telegraphy
A General Reference Work on

TELEPHONY, SUBSTATIONS, PARTY-LINE SYSTEMS,
PROTECTION, MANUAL
SWITCHBOARDS, AUTOMATIC SYSTEMS, POWER PLANTS,

SPECIAL SERVICE FEATURES, CONSTRUCTION, ENGINEERING,
OPERATION, MAINTENANCE, TELEGRAPHY, WIRELESS
TELEGRAPHY AND TELEPHONY, ETC.

Prepared by a Corps of

TELEPHONE AND TELEGRAPH EXPERTS, AND ELECTRICAL
ENGINEERS OF
THE HIGHEST PROFESSIONAL STANDING

Illustrated with over Two Thousand Engravings

FOUR VOLUMES

CHICAGO
AMERICAN SCHOOL OF CORRESPONDENCE
1919

ToC

Authors and Collaborators

KEMPSTER B. MILLER. M.E.

Consulting Engineer and Telephone Expert
Of the Firm of McMeen and Miller, Electrical Engineers and Patent
Experts, Chicago
American Institute of Electrical Engineers
Western Society of Engineers

GEORGE W. PATTERSON, S.B., Ph.D.

Head, Department of Electrical Engineering, University of Michigan

CHARLES THOM

Chief of Quadruplex Department, Western Union Main Office, New York
City

ROBERT ANDREWS MILLIKAN, Ph.D.

Associate Professor of Physics, University of Chicago
Member, Executive Council, American Physical Society

SAMUEL G. McMEEN

Consulting Engineer and Telephone Expert
Of the Firm of McMeen and Miller, Electrical Engineers and Patent
Experts, Chicago
American Institute of Electrical Engineers
Western Society of Engineers

LAWRENCE K. SAGER, S.B., M.P.L.

Patent Attorney and Electrical Expert
Formerly Assistant Examiner, U.S. Patent Office

GLENN M. HOBBS, Ph.D.

Secretary, American School of Correspondence
Formerly Instructor in Physics, University of Chicago
American Physical Society

CHARLES G. ASHLEY

Electrical Engineer and Expert in Wireless Telegraphy and Telephony

A. FREDERICK COLLINS

Editor, Collins Wireless Bulletin
Author of "Wireless Telegraphy, Its History, Theory, and Practice"

FRANCIS B. CROCKER, E.M., Ph.D.

Head, Department of Electrical Engineering, Columbia University
Past-President, American Institute of Electrical Engineers

MORTON ARENDT, E.E.

Instructor in Electrical Engineering, Columbia University, New York

EDWARD B. WAITE

Head, Instruction Department, American School of Correspondence
American Society of Mechanical Engineers
Western Society of Engineers

DAVID P. MORETON, B.S., E.E.

Associate Professor of Electrical Engineering, Armour Institute of
Technology
American Institute of Electrical Engineers

LEIGH S. KEITH, B.S.

Managing Engineer with McMeen and Miller, Electrical Engineers and
Patent Experts Chicago
Associate Member, American Institute of Electrical Engineers

JESSIE M. SHEPHERD, A.B.

Associate Editor, Textbook Department, American School of
Correspondence

ERNEST L. WALLACE, B.S.

Assistant Examiner, United States Patent Office, Washington, D. C.

GEORGE R. METCALFE, M.E.

Editor, American Institute of Electrical Engineers
Formerly Head of Publication Department, Westinghouse Elec. & Mfg. Co.

J.P. SCHROETER

Graduate, Munich Technical School
Instructor in Electrical Engineering, American School of Correspondence

JAMES DIXON, E.E.

American Institute of Electrical Engineers

HARRIS C. TROW, S.B., Managing Editor

Editor-in-Chief, Textbook Department, American School of
Correspondence

Authorities Consulted
The editors have freely consulted the standard technical literature of
America and Europe in the preparation of these volumes. They desire to
express their indebtedness particularly to the following eminent authorities,
whose well-known works should be in the library of every telephone and
telegraph engineer.

Grateful acknowledgment is here made also for the invaluable co-
operation of the foremost engineering firms and manufacturers in making
these volumes thoroughly representative of the very best and latest practice
in the transmission of intelligence, also for the valuable drawings, data,
suggestions, criticisms, and other courtesies.

ARTHUR E. KENNELY, D.Sc.

Professor of Electrical Engineering, Harvard University.
Joint Author of "The Electric Telephone." "The Electric Telegraph,"
"Alternating
Currents," "Arc Lighting," "Electric Heating," "Electric Motors," "Electric
Railways,"
"Incandescent Lighting," etc.

HENRY SMITH CARHART, A.M., LL.D.

Professor of Physics and Director of the Physical Laboratory, University of
Michigan.
Author of "Primary Batteries," "Elements of Physics," "University Physics,"
"Electrical
Measurements," "High School Physics," etc.

FRANCIS B. CROCKER, M.E., Ph.D.

Head of Department of Electrical Engineering, Columbia University, New
York; Past-President,
American Institute of Electrical Engineers.
Author of "Electric Lighting;" Joint Author of "Management of Electrical
Machinery."

HORATIO A. FOSTER

Consulting Engineer; Member of American Institute of Electrical
Engineers; Member
of American Society of Mechanical Engineers.
Author of "Electrical Engineer's Pocket-Book."

WILLIAM S. FRANKLIN, M.S., D.Sc.

Professor of Physics, Lehigh University.
Joint Author of "The Elements of Electrical Engineering," "The Elements of
Alternating Currents."

LAMAR LYNDON, B.E., M.E.

Consulting Electrical Engineer; Associate Member of American Institute of
Electrical
Engineers; Member, American Electro-Chemical Society.
Author of "Storage Battery Engineering."

ROBERT ANDREWS MILLIKAN, Ph.D.

Professor of Physics, University of Chicago.
Joint Author of "A First Course in Physics," "Electricity, Sound and Light,"
etc.

KEMPSTER B. MILLER, M.E.

Consulting Engineer and Telephone Expert; of the Firm of McMeen and
Miller,
Electrical Engineers and Patent Experts, Chicago.
Author of "American Telephone Practice."

WILLIAM H. PREECE

Chief of the British Postal Telegraph.
Joint Author of "Telegraphy," "A Manual of Telephony," etc.—

LOUIS BELL, Ph.D.

Consulting Electrical Engineer; Lecturer on Power Transmission,
Massachusetts Institute of Technology.
Author of "Electric Power Transmission," "Power Distribution for Electric
Railways,"
"The Art of Illumination," "Wireless Telephony," etc.

OLIVER HEAVISIDE, F.R.S.

Author of "Electro-Magnetic Theory," "Electrical Papers," etc.

SILVANUS P. THOMPSON, D.Sc, B.A., F.R.S., F.R.A.S.

Principal and Professor of Physics in the City and Guilds of London
Technical College.
Author of "Electricity and Magnetism," "Dynamo-Electric Machinery,"
"Polyphase Electric Currents and Alternate-Current Motors," "The
Electromagnet," etc.

ANDREW GRAY, M.A., F.R.S.E.

Author of "Absolute Measurements in Electricity and Magnetism."

ALBERT CUSHING CREHORE, A.B., Ph.D.

Electrical Engineer; Assistant Professor of Physics, Dartmouth College;
Formerly instructor in Physics, Cornell University.
Author of "Synchronous and Other Multiple Telegraphs;" Joint Author of
"Alternating Currents."

J. J. THOMSON, D.Sc, LL.D., Ph.D., F.R.S.

Fellow of Trinity College, Cambridge University; Cavendish Professor of
Experimental Physics, Cambridge University.

Author of "The Conduction of Electricity through Gases," "Electricity and
Matter."

FREDERICK BEDELL, Ph. D.

Professor of Applied Electricity, Cornell University.
Author of "The Principles of the Transformer;" Joint Author of "Alternating
Currents."

DUGALD C. JACKSON, C.E.

Head of Department of Electrical Engineering, Massachusetts Institute of
Technology;
Member, American Institute of Electrical Engineers, etc.
Author of "A Textbook on Electromagnetism and the Construction of
Dynamos;"
Joint Author of "Alternating Currents and Alternating-Current Machinery."

MICHAEL IDVORSKY PUPIN, A.B., Sc.D., Ph.D.

Professor of Electro-Mechanics, Columbia University, New York.
Author of "Propagation of Long Electric Waves," and "Wave-Transmission
over Non-Uniform Cables and Long-Distance Air Lines."

FRANK BALDWIN JEWETT, A.B., Ph.D.

Transmission and Protection Engineer, with American Telephone &
Telegraph Co.

Author of "Modern Telephone Cable," "Effect of Pressure on Insulation
Resistance."

ARTHUR CROTCH

Formerly Lecturer on Telegraphy and Telephony at the Municipal Technical
Schools, Norwich, Eng.
Author of "Telegraphy and Telephony."

JAMES ERSKINE-MURRAY, D.Sc.

Fellow of the Royal Society of Edinburgh; Member of the Institution of
Electrical Engineers.
Author of "A Handbook of Wireless Telegraphy."

A.H. MCMILLAN, A.B., LL.B.

Author of "Telephone Law, A Manual on the Organization and Operation of
Telephone Companies."

WILLIAM ESTY, S.B., M.A.

Head of Department of Electrical Engineering, Lehigh University.
Joint Author of "The Elements of Electrical Engineering."

GEORGE W. WILDER, Ph.D.

Formerly Professor of Telephone Engineering, Armour Institute of
Technology.
Author of "Telephone Principles and Practice," "Simultaneous Telegraphy
and Telephony," etc.

WILLIAM L. HOOPER, Ph.D.

Head of Department of Electrical Engineering, Tufts College.
Joint Author of "Electrical Problems for Engineering Students."

DAVID S. HULFISH

Technical Editor, The Nickelodeon; Telephone and Motion-Picture Expert;
Solicitor of Patents.
Author of "How to Read Telephone Circuit Diagrams."

J.A. FLEMING, M.A., D.Sc. (Lond.), F.R.S.

Professor of Electrical Engineering in University College, London;
Late Fellow and Scholar of St. John's College, Cambridge; Fellow of
University College, London.
Author of "The Alternate-Current Transformer," "Radiotelegraphy and
Radiotelephony,"
"Principles of Electric Wave Telegraphy," "Cantor Lectures on Electrical
Oscillations and Electric Waves," "Hertzian Wave Wireless Telegraphy,"
etc.

F.A.C. PERRINE, A.M., D.Sc.

Consulting Engineer: Formerly President, Stanley Electric Manufacturing
Company;
Formerly Professor of Electrical Engineering, Leland Stanford, Jr.
University.
Author of "Conductors for Electrical Distribution."

A. FREDERICK COLLINS

Editor, Collins Wireless Bulletin.
Author of "Wireless Telegraphy, Its History, Theory and Practice," "Manual
of Wireless Telegraphy," "Design and Construction of Induction Coils," etc.

SCHUYLER S. WHEELER, D.Sc.

President, Crocker-Wheeler Co.; Past-President, American Institute of
Electrical Engineers.
Joint Author of "Management of Electrical Machinery."

CHARLES PROTEUS STEINMETZ

Consulting Engineer, with the General Electric Co.; Professor of Electrical
Engineering, Union College.
Author of "The Theory and Calculation of Alternating-Current
Phenomena," "Theoretical Elements of Electrical Engineering", etc.

GEORGE W. PATTERSON, S.B., Ph.D.

Head of Department of Electrical Engineering, University of Michigan.
Joint Author of "Electrical Measurements."

WILLIAM MAVER, JR.

Ex-Electrician Baltimore and Ohio Telegraph Company; Member of the
American Institute of Electrical Engineers.
Author of "American Telegraphy and Encyclopedia of the Telegraph,"
"Wireless Telegraphy."

JOHN PRICE JACKSON, M.E.

Professor of Electrical Engineering, Pennsylvania State College.
Joint Author of "Alternating Currents and Alternating-Current Machinery."

AUGUSTUS TREADWELL, JR., E.E.

Associate Member, American Institute of Electrical Engineers.
Author of "The Storage Battery, A Practical Treatise on Secondary
Batteries."

EDWIN J. HOUSTON, Ph.D.

Professor of Physics, Franklin Institute, Pennsylvania; Joint Inventor of
Thomson-Houston System of Arc Lighting; Electrical Expert and
Consulting Engineer.
Joint Author of "The Electric Telephone," "The Electric Telegraph,"

"Alternating Currents," "Arc Lighting," "Electric Heating," "Electric
Motors," "Electric Railways," "Incandescent Lighting," etc.

WILLIAM J. HOPKINS

Professor of Physics in the Drexel Institute of Art, Science, and Industry,
Philadelphia.
Author of "Telephone Lines and their Properties."

ToC

Foreword
The present day development of the "talking wire" has annihilated
both time and space, and has enabled men thousands of miles apart to get
into almost instant communication. The user of the telephone and the
telegraph forgets the tremendousness of the feat in the simplicity of its
accomplishment; but the man who has made the feat possible knows that its
very simplicity is due to the complexity of the principles and appliances
involved; and he realizes his need of a practical, working understanding of
each principle and its application. The Cyclopedia of Telephony and
Telegraphy presents a comprehensive and authoritative treatment of the
whole art of the electrical transmission of intelligence.

The communication engineer—if so he may be called—requires a
knowledge both of the mechanism of his instruments and of the vagaries of
the current that makes them talk. He requires as well a knowledge of plants
and buildings, of office equipment, of poles and wires and conduits, of
office system and time-saving methods, for the transmission of intelligence
is a business as well as an art. And to each of these subjects, and to all
others pertinent, the Cyclopedia gives proper space and treatment.

The sections on Telephony cover the installation, maintenance, and
operation of all standard types of telephone systems; they present without
prejudice the respective merits of manual and automatic exchanges; and
they give special attention to the prevention and handling of operating
"troubles." The sections on Telegraphy cover both commercial service and
train dispatching. Practical methods of wireless communication—both by
telephone and by telegraph—are thoroughly treated.

The drawings, diagrams, and photographs incorporated into the
Cyclopedia have been prepared especially for this work; and their
instructive value is as great as that of the text itself. They have been used to
illustrate and illuminate the text, and not as a medium around which to
build the text. Both drawings and diagrams have been simplified so far as is
compatible with their correctness, with the result that they tell their own
story and always in the same language.

The Cyclopedia is a compilation of many of the most valuable
Instruction Papers of the American School of Correspondence, and the
method adopted in its preparation is that which this School has developed
and employed so successfully for many years. This method is not an
experiment, but has stood the severest of all tests—that of practical use—
which has demonstrated it to be the best yet devised for the education of the
busy, practical man.

In conclusion, grateful acknowledgment is due to the staff of authors
and collaborators, without whose hearty co-operation this work would have
been impossible.

Table of Contents

VOLUME I

Fundamental Principles By K. B. Miller and S. G. McMeen [A]
Page 11

CHAPTER I—Acoustics—Characteristics of Sound—Loudness—Pitch—
Vibration of Diaphragms—Timbre—Human Voice—Human Ear

CHAPTER II—Speech—Magneto Telephones—Loose-Contact Principle
—Induction Coils

CHAPTER III—Simple Telephone Circuit—Capacity—Telephone
Currents—Audible and Visible Signals

CHAPTER IV—Telephone Lines—Conductors—Inductance—Insulation

Substation Equipment By K. B. Miller and S. G. McMeen Page 63

CHAPTER V—Transmitters—Variable Resistance—Materials—Single
and Multiple Electrodes—Solid-Back Transmitter—Types of Transmitters
—Electrodes—Packing—Acousticon Transmitter—Switchboard
Transmitter

CHAPTER VI—Receivers—Types of Receivers—Operator's Receiver

CHAPTER VII—Primary Cells—Series and Multiple Connections—
Types of Primary Cells

CHAPTER VIII—Magneto Signaling Apparatus—Battery Bell—Magneto
Bell—Magneto Generator—Armature—Automatic Shunt—Polarized
Ringer

CHAPTER IX—Hook Switch

CHAPTER X—Electromagnets—Impedance, Induction, and Repeating
Coils

CHAPTER XI—Non-Inductive Resistance Devices—Differentially-
Wound Unit

CHAPTER XII—Condensers—Materials

CHAPTER XIII—Current Supply to Transmitters—Local Battery—
Common Battery—Diagrams of Common-Battery Systems

CHAPTER XIV—Telephone Sets: Magneto, Series and Bridging,
Common-Battery

Party-Line Systems By K. B. Miller and S. G. McMeen Page 227

CHAPTER XV—Non-Selective Party-Line Systems—Series and Bridging
—Signal Code

CHAPTER XVI—Selective Party-Line Systems: Polarity, Harmonic, Step-
by-Step, and Broken-Line

CHAPTER XVII—Lock-Out Party-Line Systems: Poole, Step-by-Step,
and Broken-Line

Protection By K. B. Miller and S. G. McMeen Page 287

CHAPTER XVII—Electrical Hazards

CHAPTER XIX—High Potentials—Air-Gap Arrester—Discharge across
Gaps—Types of Arrester—Vacuum Arrester—Strong Currents—Fuses—
Sneak Currents—Line Protection—Central-Office and Subscribers' Station
Protectors—City Exchange Requirements—Electrolysis

Manual Switchboards By K. B. Miller and S. G. McMeen Page
317

CHAPTER XX—The Telephone Exchange—Subscribers', Trunk, and Toll
Lines—Districts—Switchboards

CHAPTER XXI—Simple Magneto Switchboard—Operation—
Commercial Types of Drops and Jacks—Manual vs. Automatic Restoration
—Switchboard Plugs and Cords—Ringing and Listening Keys—Operator's
Telephone Equipment—Circuits of Complete Switchboard—Night-Alarm
Circuits—Grounded and Metallic Circuit Line—Cord Circuit—
Switchboard Assembly

Review Questions Page 387

Index Page 401

[A] For professional standing of authors, see list of Authors
and Collaborators at front of volume.

List of Photographs
A TYPICAL SMALL MAGNETO SWITCHBOARD INSTALLATION

A TYPICAL CENTRAL OFFICE FOR RURAL EXCHANGE
Line Protectors on Wall at Left.

OLD BRANCH-TERMINAL MULTIPLE BOARD, PARIS, FRANCE

No. 10 SERIES MULTIPLE SWITCHBOARD
Monarch Telephone Mfg. Co.

OPERATOR'S EQUIPMENT
Clement Automanual System

MAIN ENTRANCE AND PUBLIC OFFICE, SAN FRANCISCO HOME TELEPHONE
COMPANY
Contract Department on Left. Accounting Department on Right.

MAIN OFFICE BUILDING, BERKELEY, CALIFORNIA
Containing Automatic Equipment, Forming Part of Larger System Operating in San Francisco
and Vicinity. Bay Cities Home Telephone Company.

GRANT AVENUE OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO, CAL.
A Type of Central-Office Buildings in Down-Town Districts of Large Cities.

INTERIOR OF WAREHOUSE FOR TELEPHONE CONSTRUCTION MATERIAL

HOWARD OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO
An All-Concrete Building Serving the District South of Market Street.

WEST OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO
Serving the General Western Business and Residence Districts.

COMPRESSED AIR WAGON FOR PNEUMATIC DRILLING AND CHIPPING IN MANHOLES

SOUTH OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO

THE OPERATING ROOM OF THE EXCHANGE AT WEBB CITY, MISSOURI

A TYPICAL MEDIUM-SIZED MULTIPLE SWITCHBOARD EQUIPMENT

MAIN OFFICE, KEYSTONE TELEPHONE COMPANY, PHILADELPHIA, PA.

MAIN OFFICE, KANSAS CITY HOME TELEPHONE CO., KANSAS CITY, MO.

VENTILATING PLANT FOR LARGE TELEPHONE OFFICE BUILDING

ONE WING OF OPERATING ROOM, BERLIN, GERMANY
Ultimate Capacity 24,000 Subscribers' Lines and 2,100 Trunk Lines. Siemens-Halske
Equipment. Note Horizontal Disposal of Multiple

OPERATING ROOM AT TOKYO, JAPAN

ONE WING OF OPERATING ROOM, BERLIN, GERMANY
Ultimate Capacity 24,000 Subscribers' Lines and 2,100 Trunk Lines. Siemens-Halske
Equipment. Note Horizontal Disposal of Multiple Jack Field.

VIEW OF A LARGE FOREIGN MULTIPLE SWITCHBOARD

OLD SWITCHBOARD OF BELL EXCHANGE SERVING CHINATOWN, SAN FRANCISCO,
CALIFORNIA

ONE OF THE FOUR WINGS OF THE OLD KELLOGG DIVIDED MULTIPLE BOARD OF THE
CUYAHOGA TELEPHONE COMPANY, CLEVELAND, OHIO
Ultimate Capacity, 24,000 Lines. One of the Two Examples in the United States of a Multiple
Switchboard Having an Ultimate Capacity over 18,000 Lines. Replaced Recently by a Kellogg
Straight Multiple Board Having an Ultimate Capacity of 18,000 Lines and a Present Capacity of
10,000 Lines.

MAIN EXCHANGE, CLEVELAND, OHIO.
Largest Four-Party Selective Ringing Switchboard in the World. Kellogg Switchboard and
Supply Co.

A SPECIALLY FORMED CABLE FOR KEY SHELF OF MONARCH SWITCHBOARD

TELEPHONY
INTRODUCTION

The telephone was invented in 1875 by Alexander Graham Bell, a
resident of the United States, a native of Scotland, and by profession a
teacher of deaf mutes in the art of vocal speech. In that year, Professor Bell
was engaged in the experimental development of a system of multiplex
telegraphy, based on the use of rapidly varying currents. During those
experiments, he observed an iron reed to vibrate before an electromagnet as
a result of another iron reed vibrating before a distant electromagnet
connected to the nearer one by wires.

The telephone resulted from this observation with great promptness. In
the instrument first made, sound vibrated a membrane diaphragm
supporting a bit of iron near an electromagnet; a line joined this simple
device of three elements to another like it; a battery in the line magnetized
both electromagnet cores; the vibration of the iron in the sending device
caused the current in the line to undulate and to vary the magnetism of the
receiving device. The diaphragm of the latter was vibrated in consequence
of the varying pull upon its bit of iron, and these vibrations reproduced the
sound that vibrated the sending diaphragm.

The first public use of the electric telephone was at the Centennial
Exposition in Philadelphia in 1876. It was there tested by many interested
observers, among them Sir William Thomson, later Lord Kelvin, the
eminent Scotch authority on matters of electrical communication. It was he
who contributed so largely to the success of the early telegraph cable
system between England and America. Two of his comments which are
characteristic are as follows:
To-day I have seen that which yesterday I should have deemed impossible. Soon lovers
will whisper their secrets over an electric wire.

Who can but admire the hardihood of invention which devised such slight means to
realize the mathematical conception that if electricity is to convey all the delicacies of
sound which distinguish articulate speech, the strength of its current must vary
continuously as nearly as may be in simple proportion to the velocity of a particle of the air
engaged in constituting the sound.

Contrary to usual methods of improving a new art, the earliest
improvement of the telephone simplified it. The diaphragms became thin
iron disks, instead of membranes carrying iron; the electromagnet cores
were made of permanently magnetized steel instead of temporarily
magnetized soft iron, and the battery was omitted from the line. The
undulatory current in a system of two such telephones joined by a line is
produced in the sending telephone by the vibration of the iron diaphragm.
The vibration of the diaphragm in the receiving telephone is produced by
the undulatory current. Sound is produced by the vibration of the diaphragm
of the receiving telephone.

Such a telephone is at once the simplest known form of electric
generator or motor for alternating currents. It is capable of translating
motion into current or current into motion through a wide range of
frequencies. It is not known that there is any frequency of alternating
current which it is not capable of producing and translating. It can produce
and translate currents of greater complexity than any other existing
electrical machine.

Though possessing these admirable qualities as an electrical machine,
the simple electromagnetic telephone had not the ability to transmit speech
loudly enough for all practical uses. Transmitters producing stronger
telephonic currents were developed soon after the fundamental invention.
Some forms of these were invented by Professor Bell himself. Other
inventors contributed devices embodying the use of carbon as a resistance
to be varied by the motions of the diaphragm. This general form of
transmitting telephone has prevailed and at present is the standard type.

It is interesting to note that the earliest incandescent lamps, as invented
by Mr. Edison, had a resistance material composed of carbon, and that such
a lamp retained its position as the most efficient small electric illuminant
until the recent introduction of metal filament lamps. It is possible that

some form of metal may be introduced as the resistance medium for
telephone transmitters, and that such a change as has taken place in
incandescent lamps may increase the efficiency of telephone transmitting
devices.

At the time of the invention of the telephone, there were in existence two
distinct types of telegraph, working in regular commercial service. In the
more general type, many telegraph stations were connected to a line and
whatever was telegraphed between two stations could be read by all the
stations of that line. In the other and less general type, many lines, each
having a single telegraph station, were centered in an office or "exchange,"
and at the desire of a user his line could be connected to another and later
disconnected from it.

Both of these types of telegraph service were imitated at once in
telephone practice. Lines carrying many telephones each, were established
with great rapidity. Telephones actually displaced telegraphic apparatus in
the exchange method of working in America. The fundamental principle on
which telegraph or telephone exchanges operate, being that of placing any
line in communication with any other in the system, gave to each line an
ultimate scope so great as to make this form of communication more
popular than any arrangement of telephones on a single line. Beginning in
1877, telephone exchanges were developed with great rapidity in all of the
larger communities of the United States. Telegraph switching devices were
utilized at the outset or were modified in such minor particulars as were
necessary to fit them to the new task.

In its simplest form, a telephone system is, of course, a single line
permanently joining two telephones. In its next simplest form, it is a line
permanently joining more than two telephones. In its most useful form, it is
a line joining a telephone to some means of connecting it at will to another.

A telephone exchange central office contains means for connecting lines
at will in that useful way. The least complicated machine for that purpose is
a switchboard to be operated by hand, having some way of letting the
operator know that a connection is wished and a way of making it. The
customary way of connecting the lines always has been by means of
flexible conductors fitted with plugs to be inserted in sockets. If the

switchboard be small enough so that all the lines are within arm's reach of
the operator, the whole process is individual, and may be said to be at its
best and simplest. There are but few communities, however, in which the
number of lines to be served and calls to be answered is small enough so
that the entire traffic of the exchange can be handled by a single person. An
obvious way, therefore, is to provide as many operators in a central office as
may be required by the number of calls to be answered, and to terminate
before each of the operators enough of the lines to bring enough work to
keep that operator economically occupied. This presents the additional
problem, how to connect a line terminating before one operator to a line
normally terminating before another operator. The obvious answer is to
provide lines from each operator's place of work to each other operator's
place, connecting a calling line to some one of these lines which are local
within the central office, and, in turn, connecting that chosen local line to
the line which is called.

Such lines between operators have come to be known as trunk lines,
because of the obvious analogy to trunk lines of railways between common
centers, and such a system of telephone lines may be called a trunking
system. Very good service has been given and can be given by such an
arrangement of local trunks, but the growth in lines and in traffic has
developed in most instances certain weaknesses which make it advisable to
find speedier, more accurate, and more reliable means.

For the serving of a large traffic from a large number of lines, as is
required in practically every city of the world, a very great contribution to
the practical art was made by the development of the multiple switchboard.
Such a switchboard is merely such a device as has been described for the
simpler cases, with the further refinement that within reach of each operator
in the central office appears every line which enters that office, and this
without regard to what point in the switchboard the lines may terminate for
the answering of calls. In other words, while each operator answers a
certain subordinate group of the total number of lines, each operator may
reach, for calling purposes, every line which enters that office. It is probable
that the invention and development of the multiple switchboard was the first
great impetus toward the wide-spread use of telephone service.

Coincident with the development of the multiple switchboard for
manually operated, central-office mechanisms was the beginning of the
development of automatic apparatus under the control of the calling
subscriber for finding and connecting with a called line. It is interesting to
note the general trend of the early development of automatic apparatus in
comparison with the development, to that time, of manual telephone
apparatus.

While the manual apparatus on the one hand attempted to meet its
problem by providing local trunks between the various operators of a
central office, and failing of success in that, finally developed a means
which placed all the lines of a central office within connecting reach of each
operator, automatic telephony, beginning at that point, failed of success in
attempting to bring each line in the central office within connecting reach of
each connecting mechanism.

In other terms, the first automatic switching equipment consisted of a
machine for each line, which machine was so organized as to be able to find
and connect its calling line with any called line of the entire central-office
group. It may be said that an attempt to develop this plan was the
fundamental reason for the repeated failure of automatic apparatus to solve
the problem it attacked. All that the earlier automatic system did was to
prove more or less successfully that automatic apparatus had a right to exist,
and that to demand of the subscriber that he manipulate from his station a
distant machine to make the connection without human aid was not
fallacious. When it had been recognized that the entire multiple switchboard
idea could not be carried into automatic telephony with success, the first
dawn of hope in that art may be said to have come.

Success in automatic telephony did come by the re-adoption of the
trunking method. As adopted for automatic telephony, the method
contemplates that the calling line shall be extended, link by link, until it
finds itself lengthened and directed so as to be able to seize the called line
in a very much smaller multiple than the total group of one office of the
exchange.

A similar curious reversion has taken place in the development of
telephone lines. The earliest telephone lines were merely telegraph lines

equipped with telephone instruments, and the earliest telegraph lines were
planned by Professor Morse to be insulated wires laid in the earth. A lack of
skill in preparing the wires for putting in the earth caused these early
underground lines to be failures. At the urging of one of his associates,
Professor Morse consented to place his earliest telegraph lines on poles in
the air. Each such line originally consisted of two wires, one for the going
and one for the returning current, as was then considered the action. Upon
its being discovered that a single wire, using the earth as a return, would
serve as a satisfactory telegraph line, such practice became universal. Upon
the arrival of the telephone, all lines obviously were built in the same way,
and until force of newer circumstances compelled it, the present metallic
circuit without an earth connection did not come into general use.

The extraordinary growth of the number of telephone lines in a
community and the development of other methods of electrical utilization,
as well as the growth in the knowledge of telephony itself, ultimately forced
the wires underground again. At the same time and for the same causes, a
telephone line became one of two wires, so that it becomes again the
counterpart of the earliest telegraph line of Professor Morse.

Another curious and interesting example of this reversion to type exists
in the simple telephone receiver. An early improvement in telephone
receivers after Professor Bell's original invention was to provide the
necessary magnetism of the receiver core by making it of steel and
permanently magnetizing it, whereas Professor Bell's instrument provided
its magnetism by means of direct current flowing in the line. In later days
the telephone receiver has returned almost to the original form in which
Professor Bell produced it and this change has simplified other elements of
telephone-exchange apparatus in a very interesting and gratifying way.

By reason of improvements in methods of line construction and
apparatus arrangement, the radius of communication steadily has increased.
Commercial speech now is possible between points several thousand miles
apart, and there is no theoretical reason why communication might not be
established between any two points on the earth's surface. The practical
reasons of demand and cost may prevent so great an accomplishment as
talking half around the earth. So far as science is concerned there would

seem to be no reason why this might not be done today, by the careful
application of what already is known.

In the United States, telephone service from its beginning has been
supplied to users by private enterprise. In other countries, it is supplied by
means of governmentally-owned equipment. In general, it may be said that
the adequacy and the amount, as well as the quality of telephone service, is
best in countries where the service is provided by private enterprise.

Telephone systems in the United States were under the control of the
Bell Telephone Company from the invention of the device in 1876 until
1893. The fundamental telephone patent expired in 1893. This opened the
telephone art to the general public, because it no longer was necessary to
secure telephones solely from the patent-holding company nor to pay
royalty for the right to use them, if secured at all. Manufacturers of
electrical apparatus generally then began to make and sell telephones and
telephone apparatus, and operating companies, also independent of the Bell
organization, began to install and use telephones. At the end of seventeen
years of patent monopoly in the United States, there were in operation a
little over 250,000 telephones. In the seventeen years since the expiration of
the fundamental patent, independent telephone companies throughout the
United States have installed and now have in daily successful use over
3,911,400 telephones. In other words, since its first beginnings, independent
telephony has brought into continuous daily use nearly sixteen times as
many telephones as were brought into use in the equal time of the complete
monopoly of the Bell organization.

At the beginning of 1910, there were in service by the Bell organization
about 3,633,900 telephones. These with the 3,911,400 independent
telephones, make a total of 7,545,300, or about one-twelfth as many
telephones as there are inhabitants of the United States. The influence of
this development upon the lives of the people has been profound. Whether
the influence has been wholly for good may not be so conclusively
apparent. Lord Bacon has declared that, excepting only the alphabet and the
art of printing, those inventions abridging distance are of the greatest
service to mankind. If this be true, it may be said that the invention of
telephony deserves high place among the civilizing influences.

There is no industrial art in which the advancement of the times has been
followed more closely by practical application than in telephony.
Commercial speech by telephone is possible by means of currents which so
far are practically unmeasurable. In other words, it is possible to speak
clearly and satisfactorily over a line by means of currents which cannot be
read, with certainty as to their amount, by any electrical measuring device
so far known. In this regard, telephony is less well fortified than are any of
the arts utilizing electrical power in larger quantities. The real wonder is
that with so little knowledge of what takes place, particularly as to amount,
those working in the art have been able to do as well as they have. When an
exact knowledge of quantity is easily obtainable, very striking advances
may be looked for.

The student of these phases of physical science and industrial art will do
well to combine three processes: study of the words of others; personal
experimentation; and digestive thought. The last mentioned is the process of
profoundest value. On it finally depends mastery. It is not of so much
importance how soon the concept shall finally be gained as that it is gained.
A statement by another may seem lifeless and inert and the meaning of an
observation may be obscure. Digestive thought is the only assimilative
process. The whole art of telephony hangs on taking thought of things.
Judge R.F. Taylor of Indiana said of Professor Bell, "It has been said that no
man by taking thought may add a cubit to his stature, yet here is a man who,
by taking thought, has added not cubits but miles to the lengths of men's
tongues and ears."

In observations of many students, it is found that the notion of each must
pass through a certain period of incubation before his private and personal
knowledge of Ohm's law is hatched. Once hatched, however, it is his. By
just such a process must come each principal addition to his stock of
concepts. The periods may vary and practice in the uses of the mind may
train it in alertness in its work. If time is required, time should be given, the
object always being to keep thinking or re-reading or re-trying until the
thought is wholly encompassed and possessed.

ToC

CHAPTER I
ACOUSTICS

Telephony is the art of reproducing at a distant point, usually by the
agency of electricity, sounds produced at a sending point. In this art the
elements of two general divisions of physical science are concerned, sound
and electricity.

Sound is the effect of vibrations of matter upon the ear. The vibrations
may be those of air or other matter. Various forms of matter transmit sound
vibrations in varying degrees, at different specific speeds, and with different
effects upon the vibrations. Any form of matter may serve as a transmitting
medium for sound vibrations. Sound itself is an effect of sound vibrations
upon the ear.

Propagation of Sound. Since human beings communicate with each
other by means of speech and hearing through the air, it is with air that the
acoustics of telephony principally is concerned. In air, sound vibrations
consist of successive condensations and rarefactions tending to proceed
outwardly from the source in all directions. The source is the center of a
sphere of sound vibrations. Whatever may be the nature of the sounds or of
the medium transmitting them, they consist of waves emitted by the source
and observed by the ear. A sound wave is one complete condensation and
rarefaction of the transmitting medium. It is produced by one complete
vibration of the sound-producing thing.

Sound waves in air travel at a rate of about 1,090 feet per second. The
rate of propagation of sound waves in other materials varies with the
density of the material. For example, the speed of transmission is much
greater in water than in air, and is much less in highly rarefied air than in air
at ordinary density. The propagation of sound waves in a vacuum may be
said not to take place at all.

Characteristics of Sound. Three qualities distinguish sound: loudness,
pitch, and timbre.

Loudness. Loudness depends upon the violence of the effect upon the
ear; sounds may be alike in their other qualities and differ in loudness, the
louder sounds being produced by the stronger vibrations of the air or other
medium at the ear. Other things being equal, the louder sound is produced
by the source radiating the greater energy and so producing the greater
degree of condensation and rarefaction of the medium.

Pitch. Pitch depends upon the frequency at which the sound waves strike
the ear. Pitches are referred to as high or low as the frequency of waves
reaching the ear are greater or fewer. Familiar low pitches are the left-hand
strings of a piano; the larger ones of stringed instruments generally; bass
voices; and large bells. Familiar high pitches are right-hand piano strings;
smaller ones of other stringed instruments; soprano voices; small bells; and
the voices of most birds and insects.

Doppler's Principle:—As pitch depends upon the frequency at which
sound waves strike the ear, an object may emit sound waves at a constant
frequency, yet may produce different pitches in ears differently situated.
Such a case is not usual, but an example of it will serve a useful purpose in
fixing certain facts as to pitch. Conceive two railroad trains to pass each
other, running in opposite directions, the engine bells of both trains ringing.
Passengers on each train will hear the bell of the other, first as a rising
pitch, then as a falling one. Passengers on each train will hear the bell of
their own train at a constant pitch.

The difference in the observations in such a case is due to relative
positions between the ear and the source of the sound. As to the bell of their
own train, the passengers are a fixed distance from it, whether the train
moves or stands; as to the bell of the other train, the passengers first rapidly
approach it, then pass it, then recede from it. The distances at which it is
heard vary as the secants of a circle, the radius in this case being a length
which is the closest approach of the ear to the bell.

If the bell have a constant intrinsic fundamental pitch of 200 waves per
second (a wave-length of about 5.5 feet), it first will be heard at a pitch of

about 200 waves per second. But this pitch rises rapidly, as if the bell were
changing its own pitch, which bells do not do. The rising pitch is heard
because the ear is rushing down the wave-train, every instant nearer to the
source. At a speed of 45 miles an hour, the pitch rises rapidly, about 12
vibrations per second. If the rate of approach between the ear and the bell
were constant, the pitch of the bell would be heard at 212 waves per second.
But suddenly the ear passes the bell, hears the pitch stop rising and begin to
fall; and the tone drops 12 waves per second as it had risen. Such a
circumflex is an excellent example of the bearing of wavelengths and
frequencies upon pitch.

Vibration of Diaphragms:—Sound waves in air have the power to move
other diaphragms than that of the ear. Sound waves constantly vibrate such
diaphragms as panes of windows and the walls of houses. The recording
diaphragm of a phonograph is a window pane bearing a stylus adapted to
engrave a groove in a record blank. In the cylinder form of record, the
groove varies in depth with the vibrations of the diaphragm. In the disk type
of phonograph, the groove varies sidewise from its normal true spiral.

If the disk record be dusted with talcum powder, wiped, and examined
with a magnifying glass, the waving spiral line may be seen. Its variations
are the result of the blows struck upon the diaphragm by a train of sound
waves.

In reproducing a phonograph record, increasing the speed of the record
rotation causes the pitch to rise, because the blows upon the air are
increased in frequency and the wave-lengths shortened. A transitory
decrease in speed in recording will cause a transitory rise in pitch when that
record is reproduced at uniform speed.

Timbre. Character of sound denotes that difference of effect produced
upon the ear by sounds otherwise alike in pitch and loudness. This
characteristic is called timbre. It is extraordinarily useful in human affairs,
human voices being distinguished from each other by it, and a great part of
the joy of music lying in it.

A bell, a stretched string, a reed, or other sound-producing body, emits a
certain lowest possible tone when vibrated. This is called its fundamental

tone. The pitch, loudness, and timbre of this tone depend upon various
controlling causes. Usually this fundamental tone is accompanied by a
number of others of higher pitch, blending with it to form the general tone
of that object. These higher tones are called harmonics. The Germans call
them overtones. They are always of a frequency which is some multiple of
the fundamental frequency. That is, the rate of vibration of a harmonic is 2,
3, 4, 5, or some other integral number, times as great as the fundamental
itself. A tone having no harmonics is rare in nature and is not an attractive
one. The tones of the human voice are rich in harmonics.

In any tone having a fundamental and harmonics (multiples), the wave-
train consists of a complex series of condensations and rarefactions of the
air or other transmitting medium. In the case of mere noises the train of
vibrations is irregular and follows no definite order. This is the difference
between vowel sounds and other musical tones on the one hand and all
unmusical sounds (or noises) on the other.

Human Voice. Human beings communicate with each other in various
ways. The chief method is by speech. Voice is sound vibration produced by
the vocal cords, these being two ligaments in the larynx. The vocal cords in
man are actuated by the air from the lungs. The size and tension of the vocal
cords and the volume and the velocity of the air from the lungs control the
tones of the voice. The more tightly the vocal cords be drawn, other things
being equal, the higher will be the pitch of the sound; that is, the higher the
frequency of vibration produced by the voice. The pitches of the human
voice lie, in general, between the frequencies of 87 and 768 per second.
These are the extremes of pitch, and it is not to be understood that any such
range of pitch is utilized in ordinary speech. An average man speaks mostly
between the fundamental frequencies of 85 and 160 per second. Many
female speaking voices use fundamental frequencies between 150 and 320
vibrations per second. It is obvious from what has been said that in all cases
these speaking fundamentals are accompanied by their multiples, giving
complexity to the resulting wave-trains and character to the speaking voice.

Speech-sounds result from shocks given to the air by the organs of
speech; these organs are principally the mouth cavity, the tongue, and the
teeth. The vocal cords are voice-organs; that is, man only truly speaks, yet

the lower animals have voice. Speech may be whispered, using no voice.
Note the distinction between speech and voice, and the organs of both.

The speech of adults has a mean pitch lower than that of children; of
adult males, lower than that of females. There is no close analogue for the
voice-organ in artificial mechanism, but the use of the lips in playing a
bugle, trumpet, cornet, or trombone is a fairly close one. Here the lips, in
contact with each other, are stretched across one end of a tube (the
mouthpiece) while the air is blown between the lips by the lungs. A musical
tone results; if the instrument be a bugle or a trumpet of fixed tube length,
the pitch will be some one of several certain tones, depending on the
tension on the lips. The loudness depends on the force of the blast of air; the
character depends largely on the bugle.

Human Ear. The human ear, the organ of hearing in man, is a complex
mechanism of three general parts, relative to sound waves: a wave-
collecting part; a wave-observing part, and a wave-interpreting part.

The outer ear collects and reflects the waves inwardly to beat upon the
tympanum, or ear drum, a membrane diaphragm. The uses of the rolls or
convolutions of the outer ear are not conclusively known, but it is observed
that when they are filled up evenly with a wax or its equivalent, the sense of
direction of sound is impaired, and usually of loudness also.

The diaphragm of the ear vibrates when struck by sound waves, as does
any other diaphragm. By means of bone and nerve mechanism, the
vibration of the diaphragm finally is made known to the brain and is
interpretable therein.

The human ear can appreciate and interpret sound waves at frequencies
from 32 to about 32,000 vibrations per second. Below the lesser-number,
the tendency is to appreciate the separate vibrations as separate sounds.
Above the higher number, the vibrations are inaudible to the human ear.
The most acute perception of sound differences lies at about 3,000
vibrations per second. It may be that the range of hearing of organisms
other than man lies far above the range with which human beings are
familiar. Some trained musicians are able to discriminate between two
sounds as differing one from the other when the difference in frequency is

less than one-thousandth of either number. Other ears are unable to detect a
difference in two sounds when they differ by as much as one full step of the
chromatic scale. Whatever faculty an individual may possess as to tone
discrimination, it can be improved by training and practice.

ToC

CHAPTER II
ELECTRICAL REPRODUCTION OF SPEECH

The art of telephony in its present form has for its problem so to relate
two diaphragms and an electrical system that one diaphragm will respond to
all the fundamental and harmonic vibrations beating upon it and cause the
other to vibrate in exact consonance, producing just such vibrations, which
beat upon an ear.

The art does not do all this today; it falls short of it in every phase. Many
of the harmonics are lost in one or another stage of the process; new
harmonics are inserted by the operations of the system itself and much of
the volume originally available fails to reappear. The art, however, has been
able to change commercial and social affairs in a profound degree.

Conversion from Sound Waves to Vibration of Diaphragm. However
produced, by the voice or otherwise, sounds to be transmitted by telephone
consist of vibrations of the air. These vibrations, upon reaching a
diaphragm, cause it to move. The greatest amplitude of motion of a
diaphragm is, or is wished to be, at its center, and its edge ordinarily is
fixed. The diaphragm thus serves as a translating device, changing the
energy carried by the molecules of the air into localized oscillations of the
matter of the diaphragm. The waves of sound in the air advance; the
vibrations of the molecules are localized. The agency of the air as a medium
for sound transmission should be understood to be one in which its general
volume has no need to move from place to place. What occurs is that the
vibrations of the sound-producer cause alternate condensations and
rarefactions of the air. Each molecule of the air concerned merely oscillates
through a small amplitude, producing, by joint action, shells of waves, each
traveling outward from the sound-producing center like rapidly growing
coverings of a ball.

Conversion from Vibration to Voice Currents. Fig. 1 illustrates a
simple machine adapted to translate motion of a diaphragm into an

alternating electrical current. The device is merely one form of magneto
telephone chosen to illustrate the point of immediate conversion. 1 is a
diaphragm adapted to vibrate in response to the sounds reaching it. 2 is a
permanent magnet and 3 is its armature. The armature is in contact with one
pole of the permanent magnet and nearly in contact with the other. The
effort of the armature to touch the pole it nearly touches places the
diaphragm under tension. The free arm of the magnet is surrounded by a
coil 4, whose ends extend to form the line.

Fig. 1. Type of Magneto Telephone
View full size illustration.

When sound vibrates the diaphragm, it vibrates the armature also,
increasing and decreasing the distance from the free pole of the magnet.
The lines of force threading the coil 4 are varied as the gap between the
magnet and the armature is varied.

The result of varying the lines of force through the turns of the coil is to
produce an electromotive force in them, and if a closed path is provided by
the line, a current will flow. This current is an alternating one having a
frequency the same as the sound causing it. As in speech the frequencies
vary constantly, many pitches constituting even a single spoken word, so
the alternating voice currents are of great varying complexity, and every
fundamental frequency has its harmonics superposed.

Conversion from Voice Currents to Vibration. The best knowledge of
the action of such a telephone as is shown in Fig. 1 leads to the conclusion

that a half-cycle of alternating current is produced by an inward stroke of
the diaphragm and a second half-cycle of alternating current by the
succeeding outward stroke, these half-cycles flowing in opposite directions.
Assume one complete cycle of current to pass through the line and also
through another such device as in Fig. 1 and that the first half-cycle is of
such direction as to increase the permanent magnetism of the core. The
effort of this increase is to narrow the gap between the armature and pole
piece. The diaphragm will throb inward during the half-cycle of current.
The succeeding half-cycle being of opposite direction will tend to oppose
the magnetism of the core. In practice, the flow of opposing current never
would be great enough wholly to nullify and reverse the magnetism of the
core, so that the opposition results in a mere decrease, causing the
armature's gap to increase and the diaphragm to respond by an outward
blow.

Complete Cycle of Conversion. The cycle of actions thus is complete;
one complete sound-wave in air has produced a cycle of motion in a
diaphragm, a cycle of current in a line, a cycle of magnetic change in a
core, a cycle of motion in another diaphragm, and a resulting wave of
sound. It is to be observed that the chain of operation involves the
expenditure of energy only by the speaker, the only function of any of the
parts being that of translating this energy from one form to another. In
every stage of these translations, there are losses; the devising of means of
limiting these losses as greatly as possible is a problem of telephone
engineering.

Fig. 2. Magneto Telephones and Line
View full size illustration.

Magneto Telephones. The device in Fig. 1 is a practical magneto
receiver and transmitter. It is chosen as best picturing the idea to be
proposed. Fig. 2 illustrates a pair of magneto telephones of the early Bell
type; 1-1 are diaphragms; 2-2 are permanent magnets with a free end of

each brought as near as possible, without touching, to the diaphragm. Each
magnet bears on its end nearest the diaphragm a winding of fine wire, the
two ends of each of these windings being joined by means of a two-wire
line. All that has been said concerning Fig. 1 is true also of the electrical
and magnetic actions of the devices of Fig. 2. In the latter, the flux which
threads the fine wire winding is disturbed by motions of the transmitting
diaphragm. This disturbance of the flux creates electromotive forces in
those windings. Similarly, a variation of the electromotive forces in the
windings varies the pull of the permanent magnet of the receiving
instrument upon its diaphragm.

Fig. 3. Magneto Telephones without Permanent Magnets
View full size illustration.

Fig. 3 illustrates a similar arrangement, but it is to be understood that the
cores about which the windings are carried in this case are of soft iron and
not of hard magnetized steel. The necessary magnetism which constantly
enables the cores to exert a pull upon the diaphragm is provided by the
battery which is inserted serially in the line. Such an arrangement in action
differs in no particular from that of Fig. 2, for the reason that it matters not
at all whether the magnetism of the core be produced by electromagnetic or
by permanently magnetic conditions. The arrangement of Fig. 3 is a
fundamental counterpart of the original telephone of Professor Bell, and it
is of particular interest in the present stage of the art for the reason that a
tendency lately is shown to revert to the early type, abandoning the use of
the permanent magnet.

The modifications which have been made in the original magneto
telephone, practically as shown in Fig. 2, have been many. Thirty-five years'
experimentation upon and daily use of the instrument has resulted in its
refinement to a point where it is a most successful receiver and a most
unsuccessful transmitter. Its use for the latter purpose may be said to be

nothing. As a receiver, it is not only wholly satisfactory for commercial use
in its regular function, but it is, in addition, one of the most sensitive
electrical detecting devices known to the art.

Loose Contact Principle. Early experimenters upon Bell's device, all
using in their first work the arrangement utilizing current from a battery in
series with the line, noticed that sound was given out by disturbing loose
contacts in the line circuit. This observation led to the arrangement of
circuits in such a way that some imperfect contacts could be shaken by
means of the diaphragm, and the resistance of the line circuit varied in this
manner. An early and interesting form of such imperfect contact transmitter
device consisted merely of metal conductors laid loosely in contact. A
simple example is that of three wire nails, the third lying across the other
two, the two loose contacts thus formed being arranged in series with a
battery, the line, and the receiving instrument. Such a device when slightly
jarred, by the voice or other means, causes abrupt variation in the resistance
of the line, and will transmit speech.

Early Conceptions. The conception of the possibility and desirability of
transmitting speech by electricity may have occurred to many, long prior to
its accomplishment. It is certain that one person, at least, had a clear idea of
the general problem. In 1854, Charles Bourseul, a Frenchman, wrote: "I
have asked myself, for example, if the spoken word itself could not be
transmitted by electricity; in a word, if what was spoken in Vienna might
not be heard in Paris? The thing is practicable in this way:

Fig. 4. Reis Transmitter
View full size illustration.

"Suppose that a man speaks near a movable disk sufficiently flexible to
lose none of the vibrations of the voice; that this disk alternately makes and
breaks the connection from a battery; you may have at a distance another
disk which will simultaneously execute the same vibrations." The idea so

expressed is weak in only one particular. This particular is shown by the
words italicized by ourselves. It is impossible to transmit a complex series
of waves by any simple series of makes and breaks. Philipp Reis, a German,
devised the arrangement shown in Fig. 4 for the transmission of sound,
letting the make and break of the contact between the diaphragm 1 and the
point 2 interrupt the line circuit. His receiver took several forms, all
electromagnetic. His success was limited to the transmission of musical
sounds, and it is not believed that articulate speech ever was transmitted by
such an arrangement.

It must be remembered that the art of telegraphy, particularly in America,
was well established long before the invention of the telephone, and that an
arrangement of keys, relays, and a battery, as shown in Fig. 5, was well
known to a great many persons. Attaching the armatures of the relays of
such a line to diaphragms, as in Fig. 6, at any time after 1838, would have
produced the telephone. "The hardihood of invention" to conceive such a
change was the quality required.

Fig. 5. Typical Telegraph Line
View full size illustration.

Limitations of Magneto Transmitter. For reasons not finally
established, the ability of the magneto telephone to produce large currents
from large sounds is not equal to its ability to produce large sounds from
large currents. As a receiving device, it is unexcelled, and but slight
improvement has been made since its first invention. It is inadequate as a
transmitter, and as early as 1876, Professor Bell exhibited other means than
electromagnetic action for producing the varying currents as a consequence
of diaphragm motion. Much other inventive effort was addressed to this
problem, the aim of all being to send out more robust voice currents.

Fig. 6. Telegraph Equipment Converted into Telephone Equipment
View full size illustration.

Other Methods of Producing Voice Currents. Some of these means
are the variation of resistance in the path of direct current, variation in the
pressure of the source of that current, and variation in the electrostatic
capacity of some part of the circuit.

Electrostatic Telephone. The latter method is principally that of Dolbear
and Edison. Dolbear's thought is illustrated in Fig. 7. Two conducting plates
are brought close together. One is free to vibrate as a diaphragm, while the
other is fixed. The element 1 in Fig. 7 is merely a stud to hold rigid the plate
it bears against. Each of two instruments connected by a line contains such
a pair of plates, and a battery in the line keeps them charged to its potential.
The two diaphragms of each instrument are kept drawn towards each other
because their unlike charges attract each other. The vibration of one of the
diaphragms changes the potential of the other pair; the degree of attraction
thus is varied, so that vibration of the diaphragm and sound waves result.

Examples of this method of telephone transmission are more familiar to
later practice in the form of condenser receivers. A condenser, in usual
present practice, being a pair of closely adjacent conductors of considerable
surface insulated from each other, a rapidly varying current actually may
move one or both of the conductors. Ordinarily these are of thin sheet metal
(foil) interleaved with an insulating material, such as paper or mica. Voice
currents can vibrate the metal sheets in a degree to cause the condenser to
speak. These condenser methods of telephony have not become
commercial.

Fig. 7. Electrostatic Telephone
View full size illustration.

Variation of Electrical Pressure. Variation of the pressure of the source
is a conceivable way of transmitting speech. To utilize it, would require that
the vibrations of the diaphragm cause the electromotive force of a battery or
machine to vary in harmony with the sound waves. So far as we are
informed this method never has come into practical use.

Variation of Resistance. Variation of resistance proportional to the
vibrations of the diaphragm is the method which has produced the present
prevailing form of transmission. Professor Bell's Centennial exhibit
contained a water-resistance transmitter. Dr. Elisha Gray also devised one.
In both, the diaphragm acted to increase and diminish the distance between
two conductors immersed in water, lowering and raising the resistance of
the line. It later was discovered by Edison that carbon possesses a peculiarly
great property of varying its resistance under pressure. Professor David E.
Hughes discovered that two conducting bodies, preferably of rather poor
conductivity, when laid together so as to form a loose contact between
them, possessed, in remarkable degree, the ability to vary the resistance of
the path through them when subject to such vibrations as would alter the
intimacy of contact. He thus discovered and formulated the principles of
loose contact upon which the operation of all modern transmitters rests.
Hughes' device was named by him a "microphone," indicating a
magnification of sound or an ability to respond to and make audible minute
sounds. It is shown in Fig. 8. Firmly attached to a board are two carbon
blocks, shown in section in the figure. A rod of carbon with cone-shaped
ends is supported loosely between the two blocks, conical depressions in
the blocks receiving the ends of the rod. A battery and magneto receiver are
connected in series with the device. Under certain conditions of contact, the
arrangement is extraordinarily sensitive to small sounds and approaches an
ability indicated by its name. Its practical usefulness has been not as a
serviceable speech transmitter, but as a stimulus to the devising of

transmitters using carbon in other ways. Variation of the resistance of metal
conductors and of contact between metals has served to transmit voice
currents, but no material approaches carbon in this property.

Fig. 8. Hughes' Microphone
View full size illustration.

Carbon. Adaptability. The application of carbon to use in transmitters
has taken many forms. They may be classified as those having a single
contact and those having a plurality of contacts; in all cases, the intimacy of
contact is varied by the diaphragm excursions. An example of the single-
contact type is the Blake transmitter, long familiar in America. An example
of the multiple-contact type is the loose-carbon type universal now. Other
types popular at other times and in particular places use solid rods or blocks
of carbon having many points of contact, though not in a powdered or
granular form. Fig. 9 shows an example of each of the general forms of
transmitters.

The use of granular carbon as a transmitter material has extended greatly
the radius of speech, and has been a principal contributing cause for the
great spread of the telephone industry.

Fig. 9. General Types of Transmitters
View full size illustration.

Superiority. The superiority of carbon over other resistance-varying
materials for transmitters is well recognized, but the reason for it is not well
known. Various theories have been proposed to explain why, for example,
the resistance of a mass of carbon granules varies with the vibrations or
compressions to which they are subjected.

Four principal theories respectively allege:
First, that change in pressure actually changes the specific resistance of carbon.

Second, that upon the surface of carbon bodies exists some gas in some form of
attachment or combination, variations of pressure causing variations of resistance merely
by reducing the thickness of this intervening gas.

Third, that the change of resistance is caused by variations in the length of electrical
arcs between the particles.

Fourth, that change of pressure changes the area of contact, as is true of solids generally.

One may take his choice. A solid carbon block or rod is not found to
decrease its resistance by being subjected to pressure. The gas theory lacks
experimental proof also. The existence of arcs between the granules never
has been seen or otherwise observed under normal working conditions of a
transmitter; when arcs surely are experimentally established between the
granules the usefulness of the transmitter ceases. The final theory, that
change of pressure changes area of surface contact, does not explain why

other conductors than carbon are not good materials for transmitters. This, it
may be noticed, is just what the theories set out to make clear.

There are many who feel that more experimental data is required before
a conclusive and satisfactory theory can be set up. There is need of one, for
a proper theory often points the way for effective advance in practice.

Carbon and magneto transmitters differ wholly in their methods of
action. The magneto transmitter produces current; the carbon transmitter
controls current. The former is an alternating-current generator; the latter is
a rheostat. The magneto transmitter produces alternating current without
input of any electricity at all; the carbon transmitter merely controls a direct
current, supplied by an external source, and varies its amount without
changing its direction.

The carbon transmitter, however, may be associated with other devices
in a circuit in such a way as to transform direct currents into alternating
ones, or it may be used merely to change constant direct currents into
undulating ones, which never reverse direction, as alternating currents
always do. These distinctions are important.

Fig. 10. Battery in Line Circuit
View full size illustration.

Limitations. A carbon transmitter being merely a resistance-varying
device, its usefulness depends on how much its resistance can vary in
response to motions of air molecules. A granular-carbon transmitter may
vary between resistances of 5 to 50 ohms while transmitting a particular
tone, having the lower resistance when its diaphragm is driven inward.
Conceive this transmitter to be in a line as shown in Fig. 10, the line, distant
receiver, and battery together having a resistance of 1,000 ohms. The
minimum resistance then is 1,005 ohms and the maximum 1,050 ohms. The
variation is limited to about 4.5 per cent. The greater the resistance of the

line and other elements than the transmitter, the less relative change the
transmitter can produce, and the less loudly the distant receiver can speak.

Fig. 11. Battery in Local Circuit
View full size illustration.

Induction Coil. Mr. Edison realized this limitation to the use of the
carbon transmitter direct in the line, and contributed the means of removing
it. His method is to introduce an induction coil between the line and the
transmitter, its function being to translate the variation of the direct current
controlled by the transmitter into true alternating currents.

An induction coil is merely a transformer, and for the use under
discussion consists of two insulated wires wound around an iron core.
Change in the current carried by one of the windings produces a current in
the other. If direct current be flowing in one of the windings, and remains
constant, no current whatever is produced in the other. It is important to
note that it is change, and change only, which produces that alternating
current.

Fig. 11 shows an induction coil related to a carbon transmitter, a battery,
and a receiver. Fig. 12 shows exactly the same arrangement, using
conventional signs. The winding of the induction coil which is in series
with the transmitter and the battery is called the primary winding; the other
is called the secondary winding. In the arrangement of Figs. 11 and 12 the
battery has no metallic connection with the line, so that it is called a local
battery. The circuit containing the battery, transmitter, and primary winding
of the induction coil is called the local circuit.

Let us observe what is the advantage of this arrangement over the case
of Fig. 10. Using the same values of resistance in the transmitter and line,
assume the local circuit apart from the transmitter to have a fixed resistance

of 5 ohms. The limits of variations in the local circuit, therefore, are 10 and
55 ohms, thus making the maximum 5.5 times the minimum, or an increase
of 450 per cent as against 4.5 per cent in the case of Fig. 10. The changes,
therefore, are 100 times as great.

Fig. 12. Conventional Diagram of Talking Circuit
View full size illustration.

The relation between the windings of the induction coil in this practice
are such that the secondary winding contains many more turns than the
primary winding. Changes in the circuit of the primary winding produce
potentials in the secondary winding correspondingly higher than the
potentials producing them. These secondary potentials depend upon the
ratio of turns in the two windings and therefore, within close limits, may be
chosen as wished. High potentials in the secondary winding are admirably
adapted to transmit currents in a high-resistance line, for exactly the same
reason that long-distance power transmission meets with but one-quarter of
one kind of loss when the sending potential is doubled, one-hundredth of
that loss when it is raised tenfold, and similarly. The induction coil,
therefore, serves the double purpose of a step-up transformer to limit line
losses and a device for vastly increasing the range of change in the
transmitter circuit.

Fig. 13 is offered to remind the student of the action of an induction coil
or transformer in whose primary circuit a direct current is increased and
decreased. An increase of current in the local winding produces an impulse
of opposite direction in the turns of the secondary winding; a decrease of
current in the local winding produces an impulse of the same direction in
the turns of the secondary winding. The key of Fig. 13 being closed, current
flows upward in the primary winding as drawn in the figure, inducing a
downward impulse of current in the secondary winding and its circuit as
noted at the right of the figure. On the key being opened, current ceases in

the primary circuit, inducing an upward impulse of current in the secondary
winding and circuit as shown. During other than instants of opening and
closing (changing) the local circuit, no current whatever flows in the
secondary circuit.

Fig. 13. Induction-Coil Action
View full size illustration.

It is by these means that telephone transmitters draw direct current from
primary batteries and send high-potential alternating currents over lines; the
same process produces what in Therapeutics are called "Faradic currents,"
and enables also a simple vibrating contact-maker to produce alternating
currents for operating polarized ringers of telephone sets.

Detrimental Effects of Capacity. Electrostatic capacity plays an
important part in the transmission of speech. Its presence between the wires
of a line and between them and the earth causes one of the losses from
which long-distance telephony suffers. Its presence in condensers assists in
the solution of many circuit and apparatus problems.

A condenser is a device composed of two or more conductors insulated
from each other by a medium called the dielectric. A pair of metal plates
separated by glass, a pair of wires separated by air, or a pair of sheets of foil
separated by paper or mica may constitute a condenser. The use of
condensers as pieces of apparatus and the problems presented by
electrostatic capacity in lines are discussed in other chapters.

Measurements of Telephone Currents. It has been recognized in all
branches of engineering that a definite advance is possible only when
quantitative data exists. The lack of reliable means of measuring telephone
currents has been a principal cause of the difficulty in solving many of its
problems. It is only in very recent times that accurate and reliable means
have been worked out for measuring the small currents which flow in

telephone lines. These ways are of two general kinds: by thermal and by
electromagnetic means.

Thermal Method. The thermal methods simply measure, in some way,
the amount of heat which is produced by a received telephone current.
When this current is allowed to pass through a conductor the effect of the
heat generated in that conductor, is observed in one of three ways: by the
expansion of the conductor, by its change in resistance, or by the production
of an electromotive force in a thermo-electric couple heated by the
conductor. Any one of these three ways can be used to get some idea of the
amount of current which is received. None of them gives an accurate
knowledge of the forms of the waves which cause the reproduction of
speech in the telephone receiver.

Fig. 14. Oscillogram of Telephone Currents
View full size illustration.

Electromagnetic Method. An electromagnetic device adapted to tell
something of the magnitude of the telephone current and also something of
its form, i.e., something of its various increases and decreases and also of its
changes in direction is the oscillograph. An oscillograph is composed of a
magnetic field formed by direct currents or by a permanent magnet, a turn
of wire under mechanical tension in that field, and a mirror borne by the

turn of wire, adapted to reflect a beam of light to a photographic film or to a
rotating mirror.

When a current is to be measured by the oscillograph, it is passed
through the turn of wire in the magnetic field. While no current is passing,
the wire does not move in the magnetic field and its mirror reflects a
stationary beam of light. A photographic film moved in a direction normal
to the axis of the turn of wire will have drawn upon it a straight line by the
beam of light. If the beam of light, however, is moved by a current, from
side to side at right angles to this axis, it will draw a wavy line on the
photographic film and this wavy line will picture the alternations of that
current and the oscillations of the molecules of air which carried the
originating sound. Fig. 14 is a photograph of nine different vowel sounds
which have caused the oscillograph to take their pictures. They are copies
of records made by Mr. Bela Gati, assisted by Mr. Tolnai. The measuring
instrument consisted of an oscillograph of the type described, the
transmitter being of the carbon type actuated by a 2-volt battery. The
primary current was transformed by an induction coil of the ordinary type
and the transformed current was sent through a non-inductive resistance of
3,000 ohms. No condensers were placed in the circuit. It will be seen that
the integral values of the curves, starting from zero, are variable. The
positive and the negative portions of the curves are not equal, so that the
solution of the individual harmonic motion is difficult and laborious.

These photographs point out several facts very clearly. One is that the
alternations of currents in the telephone line, like the motions of the
molecules of air of the original sound, are highly complex and are not, as
musical tones are, regular recurrences of equal vibrations. They show also
that any vowel sound may be considered to be a regular recurrence of
certain groups of vibrations of different amplitudes and of different
frequencies.

ToC

CHAPTER III
ELECTRICAL SIGNALS

Electric calls or signals are of two kinds: audible and visible.

Fig. 15. Telegraph Sounder and Key
View full size illustration.

Fig. 16. Vibrating Bell
View full size illustration.

Audible Signals. Telegraph Sounder. The earliest electric signal was an
audible one, being the telegraph sounder, or the Morse register considered
apart from its registering function. Each telegraph sounder serves as an
audible electric signal and is capable of signifying more than that the call is
being made. Such a signal is operated by the making and breaking of
current from a battery. An arrangement of this kind is shown in Fig. 15, in
which pressure upon the key causes the current from the battery to energize
the sounder and give one sharp audible rap of the lever upon the striking
post.

Vibrating Bell. The vibrating bell, so widely used as a door bell, is a
device consequent to the telegraph. Its action is to give a series of blows on
its gong when its key or push button closes the battery circuit. At the risk of
describing a trite though not trivial thing, it may be said that when the
contact 1 of Fig. 16 is closed, current from the battery energizes the
armature 2, causing the latter to strike a blow on the gong and to break the
line circuit as well, by opening the contact back of the armature. So de-
energized, the armature falls back and the cycle is repeated until the button
contact is released. A comparison of this action with that of the polarized
ringer (to be described later) will be found of interest.

Fig. 17. Elemental Magneto-Generator
View full size illustration.

Magneto-Bell. The magneto-bell came into wide use with the spread of
telephone service. Its two fundamental parts are an alternating-current
generator and a polarized bell-ringing device. Each had its counterpart long
before the invention of the telephone, though made familiar by the latter.
The alternating-current generator of the magneto-bell consists of a rotatable
armature composed of a coil of insulated wire and usually a core of soft
iron, its rotation taking place in a magnetic field. This field is usually
provided by a permanent magnet, hence the name "magneto-generator." The
purist in terms may well say, however, that every form whatever of the
dynamo-electric generator is a magneto-generator, as magnetism is one link
in every such conversion of mechanical power into electricity. The terms
magneto-electric, magneto-generator, etc., involving the term "magneto,"
have come to imply the presence of permanently magnetized steel as an
element of the construction.

In its early form, the magneto-generator consisted of the arrangement
shown in Fig. 17, wherein a permanent magnet can rotate on an axis before
an electromagnet having soft iron cores and a winding. Reversals of
magnetism produce current in alternately reversing half-cycles, one
complete rotation of the magnet producing one such cycle. Obviously the

result would be the same if the magnet were stationary and the coils should
rotate, which is the construction of more modern devices. The turning of the
crank of a magneto-bell rotates the armature in the magnetic field by some
form of gearing at a rate usually of the order of twenty turns per second,
producing an alternating current of that frequency. This current is caused by
an effective electromotive force which may be as great as 100 volts,
produced immediately by the energy of the user. In an equipment using a
magneto-telephone as both receiver and transmitter and a magneto-bell as
its signal-sending machine, as was usual in 1877, it is interesting to note
that the entire motive power for signals and speech transmission was
supplied by the muscular tissues of the user—a case of working one's
passage.

Fig. 18. Extension of a Permanent Magnet
View full size illustration.

The alternating current from the generator is received and converted into
sound by means of the polarized ringer, a device which is interesting as
depending upon several of the electrical, mechanical, and magnetic actions
which are the foundations of telephone engineering.

Fig. 19. Extension of a Permanent Magnet
View full size illustration.

"Why the ringer rings" may be gathered from a study of Figs. 18 to 21. A
permanent magnet will impart temporary magnetism to pieces of iron near
it. In Fig. 18 two pieces of iron are so energized. The ends of these pieces
which are nearest to the permanent magnet 1 are of the opposite polarity to

the end they approach, the free ends being of opposite polarity. In the
figure, these free ends are marked N, meaning they are of a polarity to point
north if free to point at all. English-speaking persons call this north
polarity. Similarly, as in Fig. 19, any arrangement of iron near a permanent
magnet always will have free poles of the same polarity as the end of the
permanent magnet nearest them.

A permanent magnet so related to iron forms part of a polarized ringer.
So does an electromagnet composed of windings and iron cores. Fig. 20
reminds us of the law of electromagnets. If current flows from the plus
towards the minus side, with the windings as drawn, polarities will be
induced as marked.

Fig. 20. Electromagnet
View full size illustration.

Fig. 21. Polarized Ringer

View full size illustration.

If, now, such an electromagnet, a permanent magnet, and a pivoted
armature be related to a pair of gongs as shown in Fig. 21, a polarized
ringer results. It should be noted that a permanent magnet has both its poles
presented (though one of the poles is not actually attached) to two parts of
the iron of the electro-magnet. The result is that the ends of the armature are
of south polarity and those of the core are of north polarity. All the
markings of Fig. 21 relate to the polarity produced by the permanent
magnet. If, now, a current flow in the ringer winding from plus to minus,
obviously the right-hand pole will be additively magnetized, the current
tending to produce north magnetism there; also the left-hand pole will be
subtractively magnetized, the current tending to produce south magnetism
there. If the current be of a certain strength, relative to the certain ringer
under study, magnetism in the left pole will be neutralized and that in the
right pole doubled. Hence the armature will be attracted more by the right
pole than by the left and will strike the right-hand gong. A reversal of
current produces an opposite action, the left-hand gong being struck. The
current ceasing, the armature remains where last thrown.

It is important to note that the strength of action depends upon the
strength of the current up to a certain point only. That depends upon the
strength of the permanent magnet. Whenever the current is great enough
just to neutralize the normal magnetism of one pole and to double that of
the other, no increase in current will cause the device to ring any louder.
This makes obvious the importance of a proper permanent magnetism and
displays the fallacy of some effort to increase the output of various devices
depending upon these principles. This discussion of magneto-electric
signaling is introduced here because of a belief in its being fundamental.
Chapter VIII treats of such a signaling in further detail.

Telephone Receiver. The telephone receiver itself serves a useful purpose
as an audible signal. An interrupted or alternating current of proper
frequency and amount will produce in it a musical tone which can be heard
throughout a large room. This fact enables a telephone central office to
signal a subscriber who has left his receiver off the switch hook, so that
normal conditions may be restored.

Visible Signals. Electromagnetic Signal. Practical visual signals are of
two general kinds: electromagnetic devices for moving a target or pointer,
and incandescent lamps. The earliest and most widely used visible signal in
telephone practice was the annunciator, having a shutter adapted to fall
when the magnet is energized. Fig. 22 is such a signal. Shutter 1 is held by
the catch 2 from dropping to the right by its own gravity. The name
"gravity-drop" is thus obvious. Current energizing the core attracts the
armature 3, lifts the catch 2, and the shutter falls. A simple modification of
the gravity-drop produces the visible signal shown in Fig. 23. Energizing
the core lifts a target so as to render it visible through an opening in the
plate 1. A contrast of color between the plate and the target heightens the
effect.

Fig. 22. Gravity-Drop
View full size illustration.

The gravity-drop is principally adapted to the magneto-bell system of
signaling, where an alternating current is sent over the line to a central
office by the operation of a bell crank at the subscriber's station, this
current, lasting only as long as the crank is turned, energizes the drop,
which may be restored by hand or otherwise and will remain latched. The
visible signal is better adapted to lines in which the signaling is done by
means of direct current, as, for example, in systems where the removal of
the receiver from the hook at the subscriber's station closes the line circuit,
causing current to flow through the winding of the visible signal and so
displaying it until the receiver has been hung upon the hook or the circuit
opened by some operation at the central office. Visible signals of the
magnetic type of Fig. 23 have been widely used in connection with
common-battery systems, both for line signals and for supervisory
purposes, indicating the state and the progress of the connection and
conversation.

Fig. 23. Electromagnetic Visible Signal
View full size illustration.

Fig. 24. Lamp Signal and Lens
View full size illustration.

Electric-Lamp Signal. Incandescent electric lamps appeared in telephony
as a considerable element about 1890. They are better than either form of
mechanical visible signals because of three principal qualities: simplicity
and ease of restoring them to normal as compared with drops; their
compactness; and their greater prominence when displayed. Of the latter
quality, one may say that they are more insistent, as they give out light
instead of reflecting it, as do all other visible signals. In its best form, the
lamp signal is mounted behind a hemispherical lens, either slightly clouded
or cut in facets. This lens serves to distribute the rays of light from the
lamp, with the result that the signal may be seen from a wide angle with the
axis of the lens, as shown in Fig. 24. This is of particular advantage in
connection with manual-switchboard connecting cords, as it enables the
signals to be mounted close to and even among the cords, their great visible
prominence when shining saving them from being hidden.

The influence of the lamp signal was one of the potent ones in the
development of the type of multiple switchboard which is now universal as
the mechanism of large manual exchanges. The first large trial of such an
equipment was in 1896 in Worcester, Mass. No large and successful
multiple switchboard with any other type of signal has been built since that
time.

Any electric signal has upper and lower limits of current between which
it is to be actuated. It must receive current enough to operate but not enough
to become damaged by overheating. The magnetic types of visible signals
have a wider range between these limits than have lamp signals. If current
in a lamp is too little, its filament either will not glow at all or merely at a
dull red, insufficient for a proper signal. If the current is too great, the
filament is heated beyond its strength and parts at the weakest place.

This range between current limits in magnetic visible signals is great
enough to enable them to be used direct in telephone lines, the operating
current through the line and signal in series with a fixed voltage at the
central office being not harmfully great when the entire line resistance is
shunted out at or near the central office. The increase of current may be as
great as ten times without damage to the winding of such a signal. In lamps,
the safe margin is much less. The current which just gives a sufficient
lighting of the signal may be about doubled with safety to the filament of
the lamp. Consequently it is not feasible to place the lamp directly in series
with long exposed lines. A short circuit of such a line near the central office
will burn it out.

Fig. 25. Lamp Signal Controlled by Relay
View full size illustration.

The qualities of electromagnets and lamps in these respects are used to
advantage by the lamp signal arrangement shown in Fig. 25. A relay is in
series with the line and provides a large range of sensibility. It is able to

carry any current the central-office current source can pass through it. The
local circuit of the relay includes the lamp. Energizing the relay lights the
lamp, and the reverse; the lamp is thus isolated from danger and receives
the current best adapted to its needs.

All lines are not long and when enclosed in cable or in well-insulated
interior wire, may be only remotely in danger of being short-circuited. Such
conditions exist in private-branch exchanges, which are groups of
telephones, usually local to limited premises, connected to a switchboard on
those premises. Such a situation permits the omission of the line relay, the
lamp being directly in the line. Fig. 26 shows the extreme simplicity of the
arrangement, containing no moving parts or costly elements. Lamps for
such service have improved greatly since the demand began to grow. The
small bulk permitted by the need of compactness, the high filament
resistance required for simplicity of the general power scheme of the
system, and the need of considerable sturdiness in the completed thing have
made the task a hard one. The practical result, however, is a signal lamp
which is highly satisfactory.

Fig. 26. Lamp Signal Directly in Line
View full size illustration.

Fig. 27. Lamp Signal and Ballast
View full size illustration.

The nature of carbon and certain earths being that their conductivity
rises with the temperature and that of metals being that their conductivity
falls with the temperature, has enabled the Nernst lamp to be successful.
The same relation of properties has enabled incandescent-lamp signals to be
connected direct to lines without relays, but compensated against too great a
current by causing the resistance in series with the lamp to be increased
inversely as the resistance of the filament. Employment of a "ballast"
resistance in this way is referred to in Chapter XI. In Fig. 27 is shown its
relation to a signal lamp directly in the line. 1 is the carbon-filament lamp; 2
is the ballast. The latter's conductor is fine iron wire in a vacuum. The
resistance of the lamp falls as that of the ballast rises. Within certain limits,
these changes balance each other, widening the range of allowable change
in the total resistance of the line.

ToC

CHAPTER IV
TELEPHONE LINES

The line is a path over which the telephone current passes from
telephone to telephone. The term "telephone line circuit" is equivalent.
"Line" and "line circuit" mean slightly different things to some persons,
"line" meaning the out-of-doors portion of the line and "line circuit"
meaning the indoor portion, composed of apparatus and associated wiring.
Such shades of meaning are inevitable and serve useful purposes. The
opening definition hereof is accurate.

A telephone line consists of two conductors. One of these conductors
may be the earth; the other always is some conducting material other than
the earth—almost universally it is of metal and in the form of a wire. A line
using one wire and the earth as its pair of conductors has several defects, to
be discussed later herein. Both conductors of a line may be wires, the earth
serving as no part of the circuit, and this is the best practice. A line
composed of one wire and the earth is called a grounded line; a line
composed of two wires not needing the earth as a conductor is called a
metallic circuit.

In the earliest telephone practice, all lines were grounded ones. The
wires were of iron, supported by poles and insulated from them by glass,
earthenware, or rubber insulators. For certain uses, such lines still represent
good practice. For telegraph service, they represent the present standard
practice.

Copper is a better conductor than iron, does not rust, and when drawn
into wire in such a way as to have a sufficient tensile strength to support
itself is the best available conductor for telephone lines. Only one metal
surpasses it in any quality for the purpose: silver is a better conductor by 1
or 2 per cent. Copper is better than silver in strength and price.

In the open country, telephone lines consist of bare wires of copper, of
iron, of steel, or of copper-covered steel supported on insulators borne by
poles. If the wires on the poles be many, cross-arms carry four to ten wires
each and the insulators are mounted on pins in the cross-arms. If the wires
on the poles be few, the insulators are mounted on brackets nailed to the
poles. Wires so carried are called open wires.

In towns and cities where many wires are to be carried along the same
route, the wires are reduced in size, insulated by a covering over each, and
assembled into a group. Such a bundle of insulated wires is called a cable.
It may be drawn into a duct in the earth and be called an underground
cable; it may be laid on the bottom of the sea or other water and be called a
submarine cable; or it may be suspended on poles and be called an aërial
cable. In the most general practice each wire is insulated from all others by
a wrapping of paper ribbon, which covering is only adequate when very
dry. Cables formed of paper-insulated wires, therefore, are covered by a
seamless, continuous lead sheath, no part of the paper insulation of the
wires being exposed to the atmosphere during the cable's entire life in
service. Telephone cables for certain uses are formed of wires insulated
with such materials as soft rubber, gutta-percha, and cotton or jute saturated
with mineral compounds. When insulated with rubber or gutta-percha, no
continuous lead sheath is essential for insulation, as those materials, if
continuous upon the wire, insulate even when the cable is immersed in
water. Sheaths and other armors can assist in protecting these insulating
materials from mechanical injury, and often are used for that purpose. The
uses to which such cables are suitable in telephony are not many, as will be
shown.

A wire supported on poles requires that it be large enough to support its
own weight. The smaller the wire, the weaker it is, and with poles a given
distance apart, the strength of the wire must be above a certain minimum. In
regions where freezing occurs, wires in the open air can collect ice in winter
and everywhere open wires are subject to wind pressure; for these reasons
additional strength is required. Speaking generally, the practical and
economical spacing of poles requires that wires, to be strong enough to
meet the above conditions, shall have a diameter not less than .08 inch, if of
hard-drawn copper, and .064 inch, if of iron or steel. The honor of

developing ways of drawing copper wire with sufficient tensile strength for
open-air uses belongs to Mr. Thomas B. Doolittle of Massachusetts.

Lines whose lengths are limited to a few miles do not require a
conductivity as great as that of copper wire of .08-inch diameter. A wire of
that size weighs approximately 100 pounds per mile. Less than 100 pounds
of copper per mile of wire will not give strength enough for use on poles;
but as little as 10 pounds per mile of wire gives the necessary conductivity
for the lines of the thousands of telephone stations in towns and cities.

Open wires, being exposed to the elements, suffer damage from storms;
their insulation is injured by contact with trees; they may make contact with
electric power circuits, perhaps injuring apparatus, themselves, and persons;
they endanger life and property by the possibility of falling; they and their
cross-arm supports are less sightly than a more compact arrangement.

Grouping small wires of telephone lines into cables has, therefore, the
advantage of allowing less copper to be used, of reducing the space
required, of improving appearance, and of increasing safety. On the other
hand, this same grouping introduces negative advantages as well as the
foregoing positive ones. It is not possible to talk as far or as well over a line
in an ordinary cable as over a line of two open wires. Long-distance
telephone circuits, therefore, have not yet been placed in cables for lengths
greater than 200 or 300 miles, and special treatment of cable circuits is
required to talk through them for even 100 miles. One may talk 2,000 miles
over open wires. The reasons for the superiority of the open wires have to
do with position rather than material. Obviously it is possible to insulate
and bury any wire which can be carried in the air. The differences in the
properties of lines whose wires are differently situated with reference to
each other and surrounding things are interesting and important.

A telephone line composed of two conductors always possesses four
principal properties in some amount: (1) conductivity of the conductors; (2)
electrostatic capacity between the conductors; (3) inductance of the circuit;
(4) insulation of each conductor from other things.

Conductivity of Conductors. The conductivity of a wire depends upon
its material, its cross-section, its length, and its temperature. Conductivity

of a copper wire, for example, increases in direct ratio to its weight, in
inverse ratio to its length, and its conductivity falls as the temperature rises.
Resistance is the reciprocal of conductivity and the properties, conductivity
and resistance, are more often expressed in terms of resistance. The unit of
the latter is the ohm; of the former the mho. A conductor having a resistance
of 100 ohms has a conductivity of .01 mho. The exact correlative terms are
resistance and conductance, resistivity and conductivity. The use of the
terms as in the foregoing is in accordance with colloquial practice.

Current in a circuit having resistance only, varies inversely as the
resistance. Electromotive force being a cause, and resistance a state, current
is the result. The formula of this relation, Ohm's law, is

C=E÷R

C being the current which results from E, the electromotive force, acting
upon R, the resistance. The units are: of current, the ampere; of
electromotive force, the volt; of resistance, the ohm.

As the conductivity or resistance of a line is the property of controlling
importance in telegraphy, a similar relation was expected in early telephony.
As the current in the telephone line varies rapidly, certain other properties
of the line assume an importance they do not have in telegraphy in any such
degree.

The importance that these properties assume is, that if they did not act
and the resistance of the conductors alone limited speech, transmission
would be possible direct from Europe to America over a pair of wires
weighing 200 pounds per mile of wire, which is less than half the weight of
the wire of the best long-distance land lines now in service. The distance
from Europe to America is about twice as great as the present commercial
radius by land lines of 435-pound wire. In other words, good speech is
possible through a mere resistance twenty times greater than the resistance
of the longest actual open-wire line it is possible to talk through. The
talking ratio between a mere resistance and the resistance of a regular
telephone cable is still greater.

Electrostatic Capacity. It is the possession of electrostatic capacity
which enables the condenser, of which the Leyden jar is a good example, to
be useful in a telephone line. The simplest form of a condenser is illustrated
in Fig. 28, in which two conducting surfaces are separated by an insulating
material. The larger the surfaces, the closer they are together; and the higher
the specific inductive capacity of the insulator, the greater the capacity of
the device. An insulator used in this relation to two conducting surfaces is
called the dielectric.

Fig. 28. Simple Condenser
View full size illustration.

Fig. 29. Condenser Symbols
View full size illustration.

Two conventional signs are used to illustrate condensers, the upper one
of Fig. 29 growing out of the original condenser of two metal plates, the
lower one suggesting the thought of interleaved conductors of tin foil, as for
many years was the practice in condenser construction.

With relation to this property, a telephone line is just as truly a condenser
as is any other arrangement of conductors and insulators. Assume such a
line to be open at the distant end and its wires to be well insulated from
each other and the earth. Telegraphy through such a line by ordinary means
would be impossible. All that the battery or other source could do would be
to cause current to flow into the line for an infinitesimal time, raising the
wires to its potential, after which no current would flow. But, by virtue of
electrostatic capacity, the condition is much as shown in Fig. 30. The
condensers which that figure shows bridged across the line from wire to

wire are intended merely to fix in the mind that there is a path for the
transfer of electrical energy from wire to wire.

Fig. 30. Line with Shunt Capacity
View full size illustration.

A simple test will enable two of the results of a short-circuiting capacity
to be appreciated. Conceive a very short line of two wires to connect two
local battery telephones. Such a line possesses negligible resistance,
inductance, and shunt capacity. Its insulation is practically infinite. Let
condensers be bridged across the line, one by one, while conversation goes
on. The listening observer will notice that the sounds reaching his ear
steadily grow less loud as the capacity across the line increases. The
speaking observer will notice that the sounds he hears through the receiver
in series with the line steadily grow louder as the capacity across the line
increases. Fig. 31 illustrates the test.

The speaker's observation in this test shows that increasing the capacity
across the line increased the amount of current entering it. The hearer's
observation in this test shows that increasing the capacity across the line
decreased the amount of energy turned into sound at his receiver.

Fig. 31. Test of Line with Varying Shunt Capacity
View full size illustration.

The unit of electrostatic capacity is the farad. As this unit is
inconveniently large, for practical applications the unit microfarad—
millionth of a farad—is employed. If quantities are known in microfarads
and are to be used in calculations in which the values of the capacity require
to be farads, care should be taken to introduce the proper corrective factor.

The electrostatic capacity between the conductors of a telephone line
depends upon their surface area, their length, their position, and the nature
of the materials separating them from each other and from other things. For
instance, in an open wire line of two wires, the electrostatic capacity
depends upon the diameter of the wires, upon the length of the line, upon
their distance apart, upon their distance above the earth, and upon the
specific inductive capacity of the air. Air being so common an insulating
medium, it is taken as a convenient material whose specific inductive
capacity may be used as a basis of reference. Therefore, the specific
inductive capacity of air is taken as unity. All solid matter has higher
specific inductive capacity than air.

The electrostatic capacity of two open wires .165 inch diameter, 1 ft.
apart, and 30 ft. above the earth, is of the order of .009 microfarads per
mile. This quantity would be higher if the wires were closer together; or
nearer the earth; or if they were surrounded by a gas other than the air or
hydrogen; or if the wires were insulated not by a gas but by any solid
covering. As another example, a line composed of two wires of a diameter
of .036 inch, if wrapped with paper and twisted into a pair as a part of a
telephone-cable, has a mutual electrostatic capacity of approximately .08
microfarads per mile, this quantity being greater if the cable be more tightly
compressed.

The use of paper as an insulator for wires in telephone cables is due to
its low specific inductive capacity. This is because the insulation of the
wires is so largely dry air. Rubber and similar insulating materials give
capacities as great as twice that of dry paper.

The condenser or other capacity acts as an effective barrier to the steady
flow of direct currents. Applying a fixed potential causes a mere rush of
current to charge its surface to a definite degree, dependent upon the
particular conditions. The condenser does not act as such a barrier to
alternating currents, for it is possible to talk through a condenser by means
of the alternating voice currents of telephony, or to pass through it
alternating currents of much lower frequency. A condenser is used in series
with a polarized ringer for the purpose of letting through alternating current
for ringing the bell, and of preventing the flow of direct current.

The degree to which the condenser allows alternating currents to pass
while stopping direct currents, depends on the capacity of the condenser
and on the frequencies of alternating current. The larger the condenser
capacity or the higher the frequency of the alternations, the greater will be
the current passing through the circuit. The degree to which the current is
opposed by the capacity is the reactance of that capacity for that frequency.
The formula is

Capacity reactance = 1 ÷ C ω

wherein C is the capacity in farads and ω is 2 π n, or twice 3.1416 times
the frequency.

All the foregoing leads to the generalization that the higher the
frequency, the less the opposition of a capacity to an alternating current. If
the frequency be zero, the reactance is infinite, i.e., the circuit is open to
direct current. If the frequency be infinite, the reactance is zero, i.e., the
circuit is as if the condenser were replaced by a solid conductor of no
resistance. Compare this statement with the correlative generalization which
follows the next thought upon inductance.

Inductance of the Circuit. Inductance is the property of a circuit by
which change of current in it tends to produce in itself and other conductors
an electromotive force other than that which causes the current. Its unit is
the henry. The inductance of a circuit is one henry when a change of one
ampere per second produces an electromotive force of one volt. Induction
between circuits occurs because the circuits possess inductance; it is called
mutual induction. Induction within a circuit occurs because the circuit
possesses inductance; it is called self-induction. Lenz' law says: In all cases
of electromagnetic induction, the induced currents have such a direction
that their reaction tends to stop the motion which produced them.

Fig. 32. Spiral of Wire
View full size illustration.

Fig. 33. Spiral of Wire Around Iron Core
View full size illustration.

All conductors possess inductance, but straight wires used in lines have
negligible inductance in most actual cases. All wires which are wound into
coils, such as electromagnets, possess inductance in a greatly increased
degree. A wire wound into a spiral, as indicated in Fig. 32, possesses much
greater inductance than when drawn out straight. If iron be inserted into the
spiral, as shown in Fig. 33, the inductance is still further increased. It is for
the purpose of eliminating inductance that resistance coils are wound with
double wires, so that current passing through such coils turns in one
direction half the way and in the other direction the other half.

A simple test will enable the results of a series inductance in a line to be
appreciated. Conceive a very short line of two wires to connect two local
battery telephones. Such a line possesses negligible resistance, inductance,
and shunt capacity. Its insulation is practically infinite. Let inductive coils
such as electromagnets be inserted serially in the wires of the line one by
one, while conversation goes on. The listening observer will notice that the
sounds reaching his ear steadily grow faint as the inductance in the line
increases and the speaking observer will notice the same thing through the
receiver in series with the line.

Both observations in this test show that the amount of current entering
and emerging from the line decreased as the inductance increased. Compare
this with the test with bridged capacity and the loading of lines described
later herein, observing the curious beneficial result when both hurtful
properties are present in a line. The test is illustrated in Fig. 34.

The degree in which any current is opposed by inductance is termed the
reactance of that inductance. Its formula is

Inductive reactance = L ω

wherein L is the inductance in henrys and ω is 2 π n, or twice 3.1416
times the frequency. To distinguish the two kinds of reactance, that due to

the capacity is called capacity reactance and that due to inductance is called
inductive reactance.

All the foregoing leads to the generalization that the higher the
frequency, the greater the opposition of an inductance to an alternating
current. If the frequency be zero, the reactance is zero, i.e., the circuit
conducts direct current as mere resistance. If the frequency be infinite, the
reactance is infinite, i.e., the circuit is "open" to the alternating current and
that current cannot pass through it. Compare this with the correlative
generalization following the preceding thought upon capacity.

Fig. 34. Test of Line with Varying Serial Inductance
View full size illustration.

Capacity and inductance depend only on states of matter. Their
reactances depend on states of matter and actions of energy.

In circuits having both resistance and capacity or resistance and
inductance, both properties affect the passage of current. The joint reaction
is expressed in ohms and is called impedance. Its value is the square root of
the sum of the squares of the resistance and reactance, or, Z being
impedance,

and

the symbols meaning as before.

In words, these formulas mean that, knowing the frequency of the
current and the capacity of a condenser, or the frequency of the current and
the inductance of a circuit (a line or piece of apparatus), and in either case
the resistance of the circuit, one may learn the impedance by calculation.

Insulation of Conductors. The fourth property of telephone lines,
insulation of the conductors, usually is expressed in ohms as an insulation
resistance. In practice, this property needs to be intrinsically high, and
usually is measured by millions of ohms resistance from the wire of a line
to its mate or to the earth. It is a convenience to employ a large unit. A
million ohms, therefore, is called a megohm. In telephone cables, an
insulation resistance of 500 megohms per mile at 60° Fahrenheit is the
usual specification. So high an insulation resistance in a paper-insulated
conductor is only attained by applying the lead sheath to the cable when its
core is made practically anhydrous and kept so during the splicing and
terminating of the cable.

Insulation resistance varies inversely as the length of the conductor. If a
piece of cable 528 feet long has an insulation resistance of 6,750 megohms,
a mile (ten times as much) of such cable, will have an insulation resistance
of 675 megohms, or one-tenth as great.

Inductance vs. Capacity. The mutual capacity of a telephone line is
greater as its wires are closer together. The self-induction of a telephone
line is smaller as its wires are closer together. The electromotive force
induced by the capacity of a line leads the impressed electromotive force by
90 degrees. The inductive electromotive force lags 90 degrees behind the
impressed electromotive force. And so, in general, the natures of these two
properties are opposite. In a cable, the wires are so close together that their
induction is negligible, while their capacity is so great as to limit
commercial transmission through a cable having .06 microfarads per mile
capacity and 94 ohms loop resistance per mile, to a distance of about 30
miles. In the case of open wires spaced 12 inches apart, the limit of
commercial transmission is greater, not only because the wires are larger,
but because the capacity is lower and the inductance higher.

Table I shows-the practical limiting conversation distance over uniform
lines with present standard telephone apparatus.

TABLE I

Limiting Transmission Distances

Size and Gauge of Wire Limiting Distance
No. 8 B. W. G. copper 900 miles
10 B. W. G. copper 700 miles
10 B. & S. copper 400 miles
12 N. B. S. copper 400 miles
12 B. & S. copper 240 miles
14 N. B. S. copper 240 miles
8 B. W. G. iron 135 miles
10 B. W. G. iron 120 miles
12 B. W. G. iron 90 miles
16 B. & S. cable, copper 40 miles
19 B. & S. cable, copper 30 miles
22 B. & S. cable, copper 20 miles

In 1893, Oliver Heaviside proposed that the inductance of telephone lines
be increased above the amount natural for the inter-axial spacing, with a
view to counteracting the hurtful effects of the capacity. His meaning was
that the increased inductance—a harmful quality in a circuit not having also
a harmfully great capacity—would act oppositely to the capacity, and if
properly chosen and applied, should decrease or eliminate distortion by
making the line's effect on fundamentals and harmonics more nearly
uniform, and as well should reduce the attenuation by neutralizing the
action of the capacity in dissipating energy.

There are two ways in which inductance might be introduced into a
telephone line. As the capacity whose effects are to be neutralized is
distributed uniformly throughout the line, the counteracting inductance
must also be distributed throughout the line. Mere increase of distance
between two wires of the line very happily acts both to increase the
inductance and to lower the capacity; unhappily for practical results, the
increase of separation to bring the qualities into useful neutralizing relation

is beyond practical limits. The wires would need to be so far above the earth
and so far apart as to make the arrangement commercially impossible.

Practical results have been secured in increasing the distributed
inductance by wrapping fine iron wire over each conductor of the line. Such
a treatment increases the inductance and improves transmission.

The most marked success has come as a result of the studies of Professor
Michael Idvorsky Pupin. He inserts inductances in series with the wires of
the line, so adapting them to the constants of the circuit that attenuation and
distortion are diminished in a gratifying degree. This method of
counteracting the effects of a distributed capacity by the insertion of
localized inductance requires not only that the requisite total amount of
inductance be known, but that the proper subdivision and spacing of the
local portions of that inductance be known. Professor Pupin's method is
described in a paper entitled "Wave Transmission Over Non-uniform Cables
and Long-Distance Air Lines," read by him at a meeting of the American
Institute of Electrical Engineers in Philadelphia, May 19, 1900.
NOTE. United States Letters Patent were issued to Professor Pupin on June 19, 1900,
upon his practical method of reducing attenuation of electrical waves. A paper upon
"Propagation of Long Electric Waves" was read by Professor Pupin before the American
Institute of Electrical Engineers on March 22, 1899, and appears in Vol. 15 of the
Transactions of that society. The student will find these documents useful in his studies on
the subject. He is referred also to "Electrical Papers" and "Electromagnetic Theory" of
Oliver Heaviside.

Professor Pupin likens the transmission of electric waves over long-
distance circuits to the transmission of mechanical waves over a string.
Conceive an ordinary light string to be fixed at one end and shaken by the
hand at the other; waves will pass over the string from the shaken to the
fixed end. Certain reflections will occur from the fixed end. The amount of
energy which can be sent in this case from the shaken to the fixed point is
small, but if the string be loaded by attaching bullets to it, uniformly
throughout its length, it now may transmit much more energy to the fixed
end.

The addition of inductance to a telephone line is analogous to the
addition of bullets to the string, so that a telephone line is said to be loaded

when inductances are inserted in it, and the inductances themselves are
known as loading coils.

Fig. 35 shows the general relation of Pupin loading coils to the capacity
of the line. The condensers of the figure are merely conventionals to
represent the condenser which the line itself forms. The inductances of the
figure are the actual loading coils.

Fig. 35. Loaded Line
View full size illustration.

The loading of open wires is not as successful in practice as is that of
cables. The fundamental reason lies in the fact that two of the properties of
open wires—insulation and capacity—vary with atmospheric change. The
inserted inductance remaining constant, its benefits may become detriments
when the other two "constants" change.

The loading of cable circuits is not subject to these defects. Such loading
improves transmission; saves copper; permits the use of longer underground
cables than are usable when not loaded; lowers maintenance costs by
placing interurban cables underground; and permits submarine telephone
cables to join places not otherwise able to speak with each other.

Underground long-distance lines now join or are joining Boston and
New York, Philadelphia and New York, Milwaukee and Chicago. England
and France are connected by a loaded submarine cable. There is no
theoretical reason why Europe and America should not speak to each other.

The student wishing to determine for himself what are the effects of the
properties of lines upon open or cable circuits will find most of the subject
in the following equation. It tells the value of a in terms of the four
properties, a being the attenuation constant of the line. That is, the larger a
is, the more the voice current is reduced in passing over the line. The
equation is

The quantities are

R = Resistance in ohms
L = Inductance in henrys
C = Mutual (shunt) capacity in farads
ω = 2π n = 6.2832 times the frequency
S = Shunt leakage in mhos

The quantity S is a measure of the combined direct-current conductance
(reciprocal of insulation resistance) and the apparent conductance due to
dielectric hysteresis.
NOTE. An excellent paper, assisting such study, and of immediate practical value as
helping the understanding of cables and their reasons, is that of Mr. Frank B. Jewett,
presented at the Thousand Islands Convention of the American Institute of Electrical
Engineers, July 1, 1909.

Chapter 43 treats cables in further detail. They form a most important part of telephone
wire-plant practice, and their uses are becoming wider and more valuable.

Possible Ways of Improving Transmission. Practical ways of
improving telephone transmission are of two kinds: to improve the lines and
to improve the apparatus. The foregoing shows what are the qualities of
lines and the ways they require to be treated. Apparatus treatment, in the
present state of the art, is addressed largely to the reduction of losses.
Theoretical considerations seem to show, however, that great advance in
apparatus effectiveness still is possible. More powerful transmitters—and
more faithful ones—more sensitive and accurate receivers, and more
efficient translating devices surely are possible. Discovery may need to
intervene, to enable invention to restimulate.

In both telegraphy and telephony, the longer the line the weaker the
current which is received at the distant end. In both telegraphy and
telephony, there is a length of line with a given kind and size of wire and
method of construction over which it is just possible to send intelligible
speech or intelligible signals. A repeater, in telegraphy, is a device in the
form of a relay which is adapted to receive these highly attenuated signal

impulses and to re-transmit them with fresh power over a new length of
line. An arrangement of two such relays makes it possible to telegraph both
ways over a pair of lines united by such a repeater. It is practically possible
to join up several such links of lines to repeating devices and, if need be,
even submarine cables can be joined to land lines within practical limits. If
it were necessary, it probably would be possible to telegraph around the
world in this way.

If it were possible to imitate the telegraph repeater in telephony,
attenuated voice currents might be caused to actuate it so as to send on
those voice currents with renewed power over a length of line, section by
section. Such a device has been sought for many years, and it once was
quoted in the public press that a reward of one million dollars had been
offered by Charles J. Glidden for a successful device of that kind. The
records of the patent offices of the world show what effort has been made in
that direction and many more devices have been invented than have been
patented in all the countries together.

Like some other problems in telephony, this one seems simpler at first
sight than it proves to be after more exhaustive study. It is possible for any
amateur to produce at once a repeating device which will relay telephone
circuits in one direction. It is required, however, that in practice the voice
currents be relayed in both directions, and further, that the relay actually
augment the energy which passes through it; that is, that it will send on a
more powerful current than it receives. Most of the devices so far invented
fail in one or the other of these particulars. Several ways have been shown
of assembling repeating devices which will talk both ways, but not many
assembling repeating devices have been shown that will talk both ways and
augment in both directions.

Fig. 36. Shreeve Repeater and Circuit

View full size illustration.

Practical repeaters have been produced, however, and at least one type is
in daily successful use. It is not conclusively shown even of it that it
augments in the same degree all of the voice waves which reach it, or even
that it augments some of them at all. Its action, however, is distinctly an
improvement in commercial practice. It is the invention of Mr. Herbert E.
Shreeve and is shown in Fig. 39. Primarily it consists of a telephone
receiver, of a particular type devised by Gundlach, associated with a
granular carbon transmitter button. It is further associated with an
arrangement of induction coils or repeating coils, the object of these being
to accomplish the two-way action, that is, of speaking in both directions and
of preventing reactive interference between the receiving and transmitting
elements. The battery 1 energizes the field of the receiving element; the
received line current varies that field; the resulting motion varies the
resistance of the carbon button and transforms current from battery 2 into a
new alternating line current.

By reactive interference is meant action whereby the transmitter
element, in emitting a wave, affects its own controlling receiver element,
thus setting up an action similar to that which occurs when the receiver of a
telephone is held close to its transmitter and humming or singing ensues.
No repeater is successful unless it is free from this reactive interference.

Fig. 37. Mercury-Arc Telephone Relay
View full size illustration.

Enough has been accomplished by practical tests of the Shreeve device
and others like it to show that the search for a method of relaying telephone
voice currents is not looking for a pot of gold at the end of the rainbow. The
most remarkable truth established by the success of repeaters of the Shreeve
type is that a device embodying so large inertia of moving parts can succeed
at all. If this mean anything, it is that a device in which inertia is absolutely
eliminated might do very much better. Many of the methods already
proposed by inventors attack the problem in this way and one of the most
recent and most promising ways is that of Mr. J.B. Taylor, the circuit of
whose telephone-relay patent is shown in Fig. 37. In it, 1 is an
electromagnet energized by voice currents; its varying field varies an arc
between the electrodes 2-2 and 3 in a vacuum tube. These fluctuations are
transformed into line currents by the coil 4.

ToC

CHAPTER V
TRANSMITTERS

Variable Resistance. As already pointed out in Chapter II, the variable-
resistance method of producing current waves, corresponding to sound
waves for telephonic transmission, is the one that lends itself most readily
to practical purposes. Practically all telephone transmitters of today employ
this variable-resistance principle. The reason for the adoption of this
method instead of the other possible ones is that the devices acting on this
principle are capable, with great simplicity of construction, of producing
much more powerful results than the others. Their simplicity is such as to
make them capable of being manufactured at low cost and of being used
successfully by unskilled persons.

Materials. Of all the materials available for the variable-resistance
element in telephone transmitters, carbon is by far the most suitable, and its
use is well nigh universal. Sometimes one of the rarer metals, such as
platinum or gold, is to be found in commercial transmitters as part of the
resistance-varying device, but, even when this is so, it is always used in
combination with carbon in some form or other. Most of the transmitters in
use, however, depend solely upon carbon as the conductive material of the
variable-resistance element.

Arrangement of Electrodes. Following the principles pointed out by
Hughes, the transmitters of today always employ as their variable-resistance
elements one or more loose contacts between one or more pairs of
electrodes, which electrodes, as just stated, are usually of carbon. Always
the arrangement is such that the sound waves will vary the intimacy of
contact between the electrodes and, therefore, the resistance of the path
through the electrodes.

A multitude of arrangements have been proposed and tried. Sometimes a
single pair of electrodes has been employed having a single point of loose
contact between them. These may be termed single-contact transmitters.

Sometimes the variable-resistance element has included a greater number of
electrodes arranged in multiple, or in series, or in series-multiple, and these
have been termed multiple-electrode transmitters, signifying a plurality of
electrodes. A later development, an outgrowth of the multiple-electrode
transmitter, makes use of a pair of principal electrodes, between which is
included a mass of finely divided carbon in the form of granules or small
spheres or pellets. These, regardless of the exact form of the carbon
particles, are called granular-carbon transmitters.

Fig. 38. Blake Transmitter
View full size illustration.

Single Electrode. Blake. The most notable example of the single-contact
transmitter is the once familiar Blake instrument. At one time this formed a
part of the standard equipment of almost every telephone in the United
States, and it was also largely used abroad. Probably no transmitter has ever
exceeded it in clearness of articulation, but it was decidedly deficient in
power in comparison with the modern transmitters. In this instrument,
which is shown in Fig. 38, the variable-resistance contact was that between
a carbon and a platinum electrode. The diaphragm 1 was of sheet iron
mounted, as usual in later transmitters, in a soft rubber gasket 2. The whole
diaphragm was mounted in a cast-iron ring 3, supported on the inside of the
box containing the entire instrument. The front electrode 4 was mounted on
a light spring 5, the upper end of which was supported by a movable bar or
lever 6, flexibly supported on a spring 7 secured to the casting which

supported the diaphragm. The tension of this spring 5 was such as to cause
the platinum point to press lightly away from the center of the diaphragm.
The rear electrode was of carbon in the form of a small block 9, secured in a
heavy brass button 10. The entire rear electrode structure was supported on
a heavier spring 11 carried on the same lever as the spring 5. The tension of
this latter spring was such as to press against the front electrode and, by its
greater strength, press this against the center of the diaphragm. The
adjustment of the instrument was secured by means of the screw 12, carried
in a lug extending rearwardly from the diaphragm supporting casting, this
screw, by its position, determining the strength with which the rear
electrode pressed against the front electrode and that against the diaphragm.
This instrument was ordinarily mounted in a wooden box together with the
induction coil, which is shown in the upper portion of the figure.

The Blake transmitter has passed almost entirely out of use in this
country, being superseded by the various forms of granular instruments,
which, while much more powerful, are not perhaps capable of producing
quite such clear and distinct articulation.

The great trouble with the single-contact transmitters, such as the Blake,
was that it was impossible to pass enough current through the single point
of contact to secure the desired power of transmission without overheating
the contact. If too much current is sent through such transmitters, an undue
amount of heat is generated at the point of contact and a vibration is set up
which causes a peculiar humming or squealing sound which interferes with
the transmission of other sounds.

Multiple Electrode. To remedy this difficulty the so-called multiple-
electrode transmitter was brought out. This took a very great number of
forms, of which the one shown in Fig. 39 is typical. The diaphragm shown
at 1, in this particular form, was made of thin pine wood. On the rear side of
this, suspended from a rod 3 carried in a bracket 4, were a number of carbon
rods or pendants 5, loosely resting against a rod 2, carried on a bracket 6
also mounted on the rear of the diaphragm. The pivotal rod 3 and the rod 2,
against which the pendants rested, were sometimes, like the pendant rods,
made of carbon and sometimes of metal, such as brass. When the
diaphragm vibrated, the intimacy of contact between the pendant rod 5 and

the rod 2 was altered, and thus the resistance of the path through all of the
pendant rods in multiple was changed.

Fig. 39. Multiple-Electrode Transmitter
View full size illustration.

A multitude of forms of such transmitters came into use in the early
eighties, and while they in some measure remedied the difficulty
encountered with the Blake transmitter, i.e., of not being able to carry a
sufficiently large current, they were all subject to the effects of extreme
sensitiveness, and would rattle or break when called upon to transmit
sounds of more than ordinary loudness. Furthermore, the presence of such
large masses of material, which it was necessary to throw into vibration by
the sound waves, was distinctly against this form of transmitter. The inertia
of the moving parts was so great that clearness of articulation was interfered
with.

Granular Carbon. The idea of employing a mass of granular carbon,
supported between two electrodes, one of which vibrated with the sound
waves and the other was stationary, was proposed by Henry Hunnings in the
early eighties. While this idea forms the basis of all modern telephone
transmitters, yet it did not prevent the almost universal adoption of the
single-contact form of instrument during the next decade.

Western Electric Solid-Back Transmitter. In the early nineties,
however, the granular-carbon transmitter came into its own with the advent
and wide adoption of the transmitter designed by Anthony C. White, known
as the White, or solid-back, transmitter. This has for many years been the
standard instrument of the Bell companies operating throughout the United
States, and has found large use abroad. A horizontal cross-section of this
instrument is shown in Fig. 40, and a rear view of the working parts in Fig.

41. The working parts are all mounted on the front casting 1. This is
supported in a cup 2, in turn supported on the lug 3, which is pivoted on the
transmitter arm or other support. The front and rear electrodes of this
instrument are formed of thin carbon disks shown in solid black. The rear
electrode, the larger one of these disks, is securely attached by solder to the
face of a brass disk having a rearwardly projecting screw-threaded shank,
which serves to hold it and the rear electrode in place in the bottom of a
heavy brass cup 4. The front electrode is mounted on the rear face of a stud.
Clamped against the head of this stud, by a screw-threaded clamping ring 7,
is a mica washer, or disk 6. The center portion of this mica washer is
therefore rigid with respect to the front electrode and partakes of its
movements. The outer edge of this mica washer is similarly clamped
against the front edge of the cup 4, a screw-threaded ring 9 serving to hold
the edge of the mica rigidly against the front of the cup. The outer edge of
this washer is, therefore, rigid with respect to the rear electrode, which is
fixed. Whatever relative movement there is between the two electrodes
must, therefore, be permitted by the flexing of the mica washer. This mica
washer not only serves to maintain the electrodes in their normal relative
positions, but also serves to close the chamber which contains the
electrodes, and, therefore, to prevent the granular carbon, with which the
space between the electrodes is filled, from falling out.

Fig. 40. White Solid-Back Transmitter
View full size illustration.

The cup 4, containing the electrode chamber, is rigidly fastened with
respect to the body of the transmitter by a rearwardly projecting shank held

in a bridge piece 8 which is secured at its ends to the front block. The
needed rigidity of the rear electrode is thus obtained and this is probably the
reason for calling the instrument the solid-back. The front electrode, on the
other hand, is fastened to the center of the diaphragm by means of a shank
on the stud, which passes through a hole in the diaphragm and is clamped
thereto by two small nuts. Against the rear face of the diaphragm of this
transmitter there rest two damping springs. These are not shown in Fig. 40
but are in Fig. 41. They are secured at one end to the rear flange of the front
casting 1, and bear with their other or free ends against the rear face of the
diaphragm. The damping springs are prevented from coming into actual
contact with the diaphragm by small insulating pads. The purpose of the
damping springs is to reduce the sensitiveness of the diaphragm to
extraneous sounds. As a result, the White transmitter does not pick up all of
the sounds in its vicinity as readily as do the more sensitive transmitters,
and thus the transmission is not interfered with by extraneous noises. On the
other hand, the provision of these heavy damping springs makes it
necessary that this transmitter shall be spoken into directly by the user.

Fig. 41. White Solid-Back Transmitter
View full size illustration.

The action of this transmitter is as follows: Sound waves are
concentrated against the center of the diaphragm by the mouth-piece, which
is of the familiar form. These waves impinge against the diaphragm,
causing it to vibrate, and this, in turn, produces similar vibrations in the

front electrode. The vibrations of the front electrode are permitted by the
elasticity of the mica washer 6. The rear electrode is, however, held
stationary within the heavy chambered block 4 and which in turn is held
immovable by its rigid mounting. As a result, the front electrode approaches
and recedes from the rear electrode, thus compressing and decompressing
the mass of granular carbon between them. As a result, the intimacy of
contact between the electrode plates and the granules and also between the
granules themselves is altered, and the resistance of the path from one
electrode to the other through the mass of granules is varied.

New Western Electric Transmitter. The White transmitter was the
prototype of a large number of others embodying the same features of
having the rear electrode mounted in a stationary cup or chamber and the
front electrode movable with the diaphragm, a washer of mica or other
flexible insulating material serving to close the front of the electrode
chamber and at the same time to permit the necessary vibration of the front
electrode with the diaphragm.

Fig. 42. New Western Electric Transmitter
View full size illustration.

One of these transmitters, embodying these same features but with
modified details, is shown in Fig. 42, this being the new transmitter
manufactured by the Western Electric Company. In this the bridge of the
original White transmitter is dispensed with, the electrode chamber being
supported by a pressed metal cup 1, which supports the chamber as a whole.
The electrode cup, instead of being made of a solid block as in the White
instrument, is composed of two portions, a cylindrical or tubular portion 2
and a back 3. The cylindrical portion is externally screw-threaded so as to
engage an internal screw thread in a flanged opening in the center of the
cup 1. By this means the electrode chamber is held in place in the cup 1,
and by the same means the mica washer 4 is clamped between the flange in
this opening and the tubular portion 2 of the electrode chamber. The front
electrode is carried, as in the White transmitter, on the mica washer and is
rigidly attached to the center of the diaphragm so as to partake of the
movement thereof. It will be seen, therefore, that this is essentially a White
transmitter, but with a modified mounting for the electrode chamber.

A feature in this transmitter that is not found in the White transmitter is
that both the front and the rear electrodes, in fact, the entire working
portions of the transmitter, are insulated from the exposed metal parts of the
instrument. This is accomplished by insulating the diaphragm and the
supporting cup 1 from the transmitter front. The terminal 5 on the cup 1
forms the electrical connection for the rear electrode, while the terminal 6,
which is mounted on but insulated from the cup 1 and is connected with the
front electrode by a thin flexible connecting strip, forms the electrical
connection for the front electrode.

Kellogg Transmitter. The transmitter of the Kellogg Switchboard and
Supply Company, originally developed by Mr. W.W. Dean and modified by
his successors in the Kellogg Company, is shown in Fig. 43. In this, the
electrode chamber, instead of being mounted in a stationary and rigid
position, as in the case of the White instrument, is mounted on, and, in fact,
forms a part of the diaphragm. The electrode which is associated with the
mica washer instead of moving with the diaphragm, as in the White
instrument, is rigidly connected to a bridge so as to be as free as possible
from all vibrations.

Referring to Fig. 43, which is a horizontal cross-section of the
instrument, 1 indicates the diaphragm. This is of aluminum and it has in its
center a forwardly deflected portion forming a chamber for the electrodes.
The front electrode 2 of carbon is backed by a disk of brass and rigidly
secured in the front of this chamber, as clearly indicated. The rear electrode
3, also of carbon, is backed by a disk of brass, and is clamped against the
central portion of a mica disk by means of the enlarged head of stud 6. A
nut 7, engaging the end of a screw-threaded shank from the back of the rear
electrode, serves to bind these two parts together securely, clamping the
mica washer between them. The outer edge of the mica washer is clamped
to the main diaphragm 1 by an aluminum ring and rivets, as clearly
indicated. It is seen, therefore, that the diaphragm itself contains the
electrode chamber as an integral part thereof. The entire structure of the
diaphragm, the front and back electrodes, and the granular carbon within
are permanently assembled in the factory and cannot be dissociated without
destroying some of the parts. The rear electrode is held rigidly in place by
the bridge 5 and the stud 6, this stud passing through a block 9 mounted on
the bridge but insulated from it. The stud 6 is clamped in the block 9 by
means of the set screw 8, so as to hold the rear electrode in proper position
after this position has been determined.

Fig. 43. Kellogg Transmitter

View full size illustration.

In this transmitter, as in the transmitter shown in Fig. 42, all of the
working parts are insulated from the exposed metal casing. The diaphragm
is insulated from the front of the instrument by means of a washer 4 of
impregnated cloth, as indicated. The rear electrode is insulated from the
other portions of the instrument by means of the mica washer and by means
of the insulation between the block 9 and the bridge 5. The terminal for the
rear electrode is mounted on the block 9, while the terminal for the front
electrode, shown at 10, is mounted on, but insulated from, the bridge. This
terminal 10 is connected with the diaphragm and therefore with the front
electrode by means of a thin, flexible metallic connection. This transmitter
is provided with damping springs similar to those of the White instrument.

It is claimed by advocates of this type of instrument that, in addition to
the ordinary action due to the compression and decompression of the
granular carbon between the electrodes, there exists another action due to
the agitation of the granules as the chamber is caused to vibrate by the
sound waves. In other words, in addition to the ordinary action, which may
be termed the piston action between the electrodes, it is claimed that the
general shaking-up effect of the granules when the chamber vibrates
produces an added effect. Certain it is, however, that transmitters of this
general type are very efficient and have proven their capability of giving
satisfactory service through long periods of time.

Another interesting feature of this instrument as it is now manufactured
is the use of a transmitter front that is struck up from sheet metal rather than
the employment of a casting as has ordinarily been the practice. The
formation of the supporting lug for the transmitter from the sheet metal
which forms the rear casing or shell of the instrument is also an interesting
feature.

Automatic Electric Company Transmitter. The transmitter of the
Automatic Electric Company, of Chicago, shown in Fig. 44, is of the same
general type as the one just discussed, in that the electrode chamber is
mounted on and vibrates with the diaphragm instead of being rigidly
supported on the bridge as in the case of the White or solid-back type of
instrument. In this instrument the transmitter front 1 is struck up from sheet

metal and contains a rearwardly projecting flange, carrying an internal
screw thread. A heavy inner cup 2, together with the diaphragm 3, form an
enclosure containing the electrode chamber. The diaphragm is, in this case,
permanently secured at its edge to the periphery of the inner cup 2 by a
band of metal 4 so formed as to embrace the edges of both the cup and the
diaphragm and permanently lock them together. This inner chamber is held
in place in the transmitter front 1 by means of a lock ring 5 externally
screw-threaded to engage the internal screw-thread on the flange on the
front. The electrode chamber proper is made in the form of a cup, rigidly
secured to the diaphragm so as to move therewith, as clearly indicated. The
rear electrode is mounted on a screw-threaded stud carried in a block which
is fitted to a close central opening in the cup 2.

This transmitter does not make use of a mica washer or diaphragm, but
employs a felt washer which surrounds the shank of the rear electrode and
serves to close and seal the carbon containing cup. By this means the
granular carbon is retained in the chamber and the necessary flexibility or
freedom of motion is permitted between the front and the rear electrodes.
As in the Kellogg and the later Bell instruments, the entire working parts of
this transmitter are insulated from the metal containing case, the inner
chamber, formed by the cup 2 and the diaphragm 3, being insulated from
the transmitter front and its locking ring by means of insulating washers, as
shown.

Fig. 44. Automatic Electric Company Transmitter
View full size illustration.

Monarch Transmitter. The transmitter of the Monarch Telephone
Manufacturing Company, shown in Fig. 45, differs from both the
stationary-cup and the vibrating-cup types, although it has the
characteristics of both. It might be said that it differs from each of these two
types of transmitters in that it has the characteristics of both.

This transmitter, it will be seen, has two flexible mica washers between
the electrodes and the walls of the electrode cup. The front and the back
electrodes are attached to the diaphragm and the bridge, respectively, by a
method similar to that employed in the solid-back transmitters, while the
carbon chamber itself is free to vibrate with the diaphragm as is
characteristic of the Kellogg transmitter.

Fig. 45. Monarch Transmitter
View full size illustration.

An aluminum diaphragm is employed, the circumferential edge of which
is forwardly deflected to form a seat. The edge of the diaphragm rests
against and is separated from the brass front by means of a one-piece gasket
of specially treated linen. This forms an insulator which is not affected by
heat or moisture. As in the transmitters previously described, the electrodes
are firmly soldered to brass disks which have solid studs extending from
their centers. In the case of both the front and the rear electrodes, a mica
disk is placed over the supporting stud and held in place by a brass hub
which has a base of the same size as the electrode. The carbon-chamber
wall consists of a brass ring to which are fastened the mica disks of the
front and the back electrodes by means of brass collars clamped over the
edge of the mica and around the rim of the brass ring forming the chamber.

Electrodes. The electrode plates of nearly all modern transmitters are of
specially treated carbon. These are first copper-plated and soldered to their
brass supporting disks. After this they are turned and ground so as to be
truly circular in form and to present absolutely flat faces toward each other.
These faces are then highly polished and the utmost effort is made to keep
them absolutely clean. Great pains are taken to remove from the pores of
the carbon, as well as from the surface, all of the acids or other chemicals

that may have entered them during the process of electroplating them or of
soldering them to the brass supporting disk. That the two electrodes, when
mounted in a transmitter, should be parallel with each other, is an item of
great importance as will be pointed out later.

In a few cases, as previously stated, gold or platinum has been
substituted for the carbon electrodes in transmitters. These are capable of
giving good results when used in connection with the proper form of
granular carbon, but, on the whole, the tendency has been to abandon all
forms of electrode material except carbon, and its use is now well nigh
universal.

Preparation of Carbon. The granular carbon is prepared from carefully
selected anthracite coal, which is specially treated by roasting or "re-
carbonizing" and is then crushed to approximately the proper fineness. The
crushed carbon is then screened with extreme care to eliminate all dust and
to retain only granules of uniform size.

Packing. In the earlier forms of granular-carbon transmitters a great deal
of trouble was experienced due to the so-called packing of the instrument.
This, as the term indicates, was a trouble due to the tendency of the carbon
granules to settle into a compact mass and thus not respond to the variable
pressure. This was sometimes due to the presence of moisture in the
electrode chamber; sometimes to the employment of granules of varying
sizes, so that they would finally arrange themselves under the vibration of
the diaphragm into a fairly compact mass; or sometimes, and more
frequently, to the granules in some way wedging the two electrodes apart
and holding them at a greater distance from each other than their normal
distance. The trouble due to moisture has been entirely eliminated by so
sealing the granule chambers as to prevent the entrance of moisture. The
trouble due to the lack of uniformity in size of the granules has been
entirely eliminated by making them all of one size and by making them of
sufficient hardness so that they would not break up into granules of smaller
size. The trouble due to the settling of the granules and wedging the
electrodes apart has been practically eliminated in well-designed
instruments, by great mechanical nicety in manufacture.

Almost any transmitter may be packed by drawing the diaphragm
forward so as to widely separate the electrodes. This allows the granules to
settle to a lower level than they normally occupy and when the diaphragm is
released and attempts to resume its normal position it is prevented from
doing so by the mass of granules between. Transmitters of the early types
could be packed by placing the lips against the mouthpiece and drawing in
the breath. The slots now provided at the base of standard mouthpieces
effectually prevent this.

In general it may be said that the packing difficulty has been almost
entirely eliminated, not by the employment of remedial devices, such as
those often proposed for stirring up the carbon, but by preventing the
trouble by the design and manufacture of the instruments in such forms that
they will not be subject to the evil.

Carrying Capacity. Obviously, the power of a transmitter is dependent
on the amount of current that it may carry, as well as on the amount of
variation that it may make in the resistance of the path through it. Granular
carbon transmitters are capable of carrying much heavier current than the
old Blake or other single or multiple electrode types. If forced to carry too
much current, however, the same frying or sizzling sound is noticeable as in
the earlier types. This is due to the heating of the electrodes and to small
arcs that occur between the electrodes and the granules.

One way to increase the current-carrying capacity of a transmitter is to
increase the area of its electrodes, but a limit is soon reached in this
direction owing to the increased inertia of the moving electrode, which
necessarily comes with its larger size.

The carrying capacity of transmitters may also be increased by providing
special means for carrying away the heat generated in the variable-
resistance medium. Several schemes have been proposed for this. One is to
employ unusually heavy metal for the electrode chamber, and this practice
is best exemplified in the White solid-back instrument. It has also been
proposed by others to water-jacket the electrode chamber, and also to keep
it cool by placing it in close proximity to the relatively cool joints of a
thermopile. Neither of these two latter schemes seems to be warranted in
ordinary commercial practice.

Sensitiveness. In all the transmitters so far discussed damping springs of
one form or another have been employed to reduce the sensitiveness of the
instrument. For ordinary commercial use too great a degree of sensitiveness
is a fault, as has already been pointed out. There are, however, certain
adaptations of the telephone transmitter which make a maximum degree of
sensitiveness desirable. One of these adaptations is found in the telephone
equipments for assisting partially deaf people to hear. In these the
transmitter is carried on some portion of the body of the deaf person, the
receiver is strapped or otherwise held at his ear, and a battery for furnishing
the current is carried in his pocket. It is not feasible, for this sort of use, that
the sound which this transmitter is to reproduce shall always occur
immediately in front of the transmitter. It more often occurs at a distance of
several feet. For this reason the transmitter is made as sensitive as possible,
and yet is so constructed that it will not be caused to produce too loud or
unduly harsh sounds in response to a loud sound taking place immediately
in front of it. Another adaptation of such highly sensitive transmitters is
found in the special intercommunicating telephone systems for use between
the various departments or desks in business offices. In these it is desirable
that the transmitter shall be able to respond adequately to sounds occurring
anywhere in a small-sized room, for instance.

Acousticon Transmitter. In Fig. 46 is shown a transmitter adapted for
such use. This has been termed by its makers the acousticon transmitter.
Like all the transmitters previously discussed, this is of the variable-
resistance type, but it differs from them all in that it has no damping
springs; in that carbon balls are substituted for carbon granules; and in that
the diaphragm itself serves as the front electrode.

This transmitter consists of a cup 1, into which is set a cylindrical block
2, in one face of which are a number of hemispherical recesses. The
diaphragm 3 is made of thin carbon and is so placed in the transmitter as to
cover the openings of the recesses in the carbon block, and lie close enough
to the carbon block, without engaging it, to prevent the carbon particles
from falling out. The diaphragm thus serves as the front electrode and the
carbon block as the rear electrode. The recesses in the carbon block are
about two-thirds filled with small carbon balls, which are about the size of
fine sand. The front piece 4 of the transmitter is of sheet metal and serves to

hold the diaphragm in place. To admit the sound waves it is provided with a
circular opening opposite to and about the size of the rear electrode block.
On this front piece are mounted the two terminals of the transmitter,
connected respectively to the two electrodes, terminal 5 being insulated
from the front piece and connected by a thin metal strip with the diaphragm,
while terminal 6 is mounted directly on the front piece and connected
through the cup 1 with the carbon block 2, or back electrode of the
transmitter.

Fig. 46. Acousticon Transmitter
View full size illustration.

When this transmitter is used in connection with outfits for the deaf, it is
placed in a hard rubber containing case, consisting of a hollow cylindrical
piece 7, which has fastened to it a cover 8. This cover has a circular row of
openings or holes near its outer edge, as shown at 9, through which the
sound waves may pass to the chamber within, and thence find their way
through the round hole in the center of the front plate 4 to the diaphragm 3.
It is probable also that the front face of the cover 8 of the outer case
vibrates, and in this way also causes sound waves to impinge against the
diaphragm. This arrangement provides a large receiving surface for the
sound waves, but, owing to the fact that the openings in the containing case
are not opposite the opening in the transmitter proper, the sound waves do
not impinge directly against the diaphragm. This peculiar arrangement is
probably the result of an endeavor to prevent the transmitter from being too

strongly actuated by violent sounds close to it. Instruments of this kind are
very sensitive and under proper conditions are readily responsive to words
spoken in an ordinary tone ten feet away.

Fig. 47. Switchboard Transmitter
View full size illustration.

Switchboard Transmitter. Another special adaptation of the telephone
transmitter is that for use of telephone operators at central-office
switchboards. The requirements in this case are such that the operator must
always be able to speak into the transmitter while seated before the
switchboard, and yet allow both of her hands to be free for use. This was
formerly accomplished by suspending an ordinary granular-carbon
transmitter in front of the operator, but a later development has resulted in
the adoption of the so-called breast transmitter, shown in Fig. 47. This is
merely an ordinary granular-carbon transmitter mounted on a plate which is
strapped on the breast of the operator, the transmitter being provided with a
long curved mouthpiece which projects in such a manner as to lie just in
front of the operator's lips. This device has the advantage of automatically
following the operator in her movements. The breast transmitter shown in
Fig. 47, is that of the Dean Electric Company.

Fig. 48. Transmitter Symbols
View full size illustration.

Conventional Diagram. There are several common ways of illustrating
transmitters in diagrams of circuits in which they are employed. The three
most common ways are shown in Fig. 48. The one at the left is supposed to
be a side view of an ordinary instrument, the one in the center a front view,
and the one at the right to be merely a suggestive arrangement of the
diaphragm and the rear electrode. The one at the right is best and perhaps
most common; the center one is the poorest and least used.

ToC

CHAPTER VI
RECEIVERS

The telephone receiver is the device which translates the energy of the
voice currents into the energy of corresponding sound waves. All telephone
receivers today are of the electromagnetic type, the voice currents causing a
varying magnetic pull on an armature or diaphragm, which in turn produces
the sound waves corresponding to the undulations of the voice currents.

Early Receivers. The early forms of telephone receivers were of the
single-pole type; that is, the type wherein but one pole of the electromagnet
was presented to the diaphragm. The single-pole receiver that formed the
companion piece to the old Blake transmitter and that was the standard of
the Bell companies for many years, is shown in Fig. 49. While this has
almost completely passed out of use, it may be profitably studied in order
that a comparison may be made between certain features of its construction
and those of the later forms of receivers.

The coil of this receiver was wound on a round iron core 2, flattened at
one end to afford means for attaching the permanent magnet. The
permanent magnet was of laminated construction, consisting of four hard
steel bars 1, extending nearly the entire length of the receiver shell. These
steel bars were all magnetized separately and placed with like poles
together so as to form a single bar magnet. They were laid together in pairs
so as to include between the pairs the flattened end of the pole piece 2 at
one end and the flattened portion of the tail piece 3 at the other end. This
whole magnet structure, including the core, the tail piece, and the
permanently magnetized steel bars, was clamped together by screws as
shown. The containing shell was of hard rubber consisting of three pieces,
the barrel 4, the ear-piece 5, and the tail cap 6. The barrel and the ear piece
engaged each other by means of a screw thread and served to clamp the
diaphragm between them. The compound bar magnet was held in place
within the shell by means of a screw 7 passing through the hard rubber tail

cap 6 and into the tail block 3 of the magnet. External binding posts
mounted on the tail cap, as shown, were connected by heavy leading-in
wires to the terminals of the electromagnet.

A casual consideration of the magnetic circuit of this instrument will
show that it was inefficient, since the return path for the lines of force set up
by the bar magnet was necessarily through a very long air path.
Notwithstanding this, these receivers were capable of giving excellent
articulation and were of marvelous delicacy of action. A very grave fault
was that the magnet was supported in the shell at the end farthest removed
from the diaphragm. As a result it was difficult to maintain a permanent
adjustment between the pole piece and the diaphragm. One reason for this
was that hard rubber and steel contract and expand under changes of
temperature at very different rates, and therefore the distance between the
pole piece and the diaphragm changed with changes of temperature.
Another grave defect, brought about by this tying together of the permanent
magnet and the shell which supported the diaphragm at the end farthest
from the diaphragm, was that any mechanical shocks were thus given a
good chance to alter the adjustment.

Fig. 49. Single-Pole Receiver
View full size illustration.

Modern Receivers. Receivers of today differ from this old single-pole
receiver in two radical respects. In the first place, the modern receiver is of
the bi-polar type, consisting essentially of a horseshoe magnet presenting
both of its poles to the diaphragm. In the second place, the modern practice
is to either support all of the working parts of the receiver, i.e., the magnet,
the coils, and the diaphragm, by an inner metallic frame entirely
independent of the shell; or, if the shell is used as a part of the structure, to
rigidly fasten the several parts close to the diaphragm rather than at the end
farthest removed from the diaphragm.

Western Electric Receiver. The standard bi-polar receiver of the
Western Electric Company, in use by practically all of the Bell operating
companies throughout this country and in large use abroad, is shown in Fig.
50. In this the shell is of three pieces, consisting of the barrel 1, the ear cap

2, and the tail cap 3. The tail cap and the barrel are permanently fastened
together to form substantially a single piece. Two permanently magnetized
bar magnets 4-4 are employed, these being clamped together at their upper
ends, as shown, so as to include the soft iron block 5 between them. The
north pole of one of these magnets is clamped to the south pole of the other,
so that in reality a horseshoe magnet is formed. At their lower ends, these
two permanent magnets are clamped against the soft iron pole pieces 6-6, a
threaded block 7 also being clamped rigidly between these pole pieces at
this point. On the ends of the pole pieces the bobbins are wound. The whole
magnet structure is secured within the shell 1 by means of a screw thread on
the block 7 which engages a corresponding internal screw thread in the
shell 1. As a result of this construction the whole magnet structure is bound
rigidly to the shell structure at a point close to the diaphragm,
comparatively speaking, and as a result of this close coupling, the relation
between the diaphragm and the pole piece is very much more rigid and
substantial than in the case where the magnet structure and the shell were
secured together at the end farthest removed from the diaphragm.

Fig. 50. Western Electric Receiver
View full size illustration.

Although this receiver shown in Fig. 50 is the standard in use by the Bell
companies throughout this country, its numbers running well into the
millions, it cannot be said to be a strictly modern receiver, because of at
least one rather antiquated feature. The binding posts, by which the circuit
conductors are led to the coils of this instrument, are mounted on the
outside of the receiver shell, as indicated, and are thus subject to danger of
mechanical injury and they are also exposed to the touch of the user, so that
he may, in case of the wires being charged to an abnormal potential, receive
a shock. Probably a more serious feature than either one of these is that the
terminals of the flexible cords which attach to these binding posts are
attached outside of the receiver shell, and are therefore exposed to the wear
and tear of use, rather than being protected as they should be within the
shell. Notwithstanding this undesirable feature, this receiver is a very
efficient one and is excellently constructed.

Fig. 51. Kellogg Receiver
View full size illustration.

Kellogg Receiver. In Fig. 51 is shown a bi-polar receiver with internal
or concealed binding posts. This particular receiver is typical of a large
number of similar kinds and is manufactured by the Kellogg Switchboard
and Supply Company. Two straight permanently magnetized bar magnets 1-
1 are clamped together at their opposite ends so as to form a horseshoe
magnet. At the end opposite the diaphragm these bars clamp between them
a cylindrical piece of iron 2, so as to complete the magnetic circuit at the
end. At the end nearest the diaphragm they clamp between them the ends of
the soft iron pole pieces 3-3, and also a block of composite metal 4 having a
large circular flange 4' which serves as a means for supporting the magnet
structure within the shell. The screws by means of which the disk 4' is
clamped to the shouldered seat in the shell do not enter the shell directly,
but rather enter screw-threaded brass blocks which are moulded into the
structure of the shell. It is seen from this construction that the diaphragm
and the pole pieces and the magnet structure itself are all rigidly secured
together through the medium of the shell at a point as close as possible to
the diaphragm.

Between the magnets 1-1 there is clamped an insulating block 5, to
which are fastened the terminal plates 6, one on each side of the receiver.
These terminal plates are thoroughly insulated from the magnets themselves
and from all other metallic parts by means of sheets of fiber, as indicated by
the heavy black lines. On these plates 6 are carried the binding posts for the
receiver cord terminals. A long tongue extends from each of the plates 6
through a hole in the disk 4', into the coil chamber of the receiver, at which
point the terminal of the magnet winding is secured to it. This tongue is
insulated from the disk 4', where it passes through it, by means of insulating
bushing, as shown. The other terminal of the magnet coils is brought out to
the other plate 6 by means of a similar tongue on the other side.

In order that the receiver terminals proper may not be subjected to any
strain in case the receiver is dropped and its weight caught on the receiver
cord, a strain loop is formed as a continuation of the braided covering of the
receiver cord, and this is tied to the permanent magnet structure, as shown.
By making this strain loop short, it is obvious that whatever pull the cord
receives will not be taken by the cord conductors leading to the binding
posts or by the binding posts or the cord terminals themselves.

A number of other manufacturers have gone even a step further than this
in securing permanency of adjustment between the receiver diaphragm and
pole pieces. They have done this by not depending at all on the hard rubber
shell as a part of the structure, but by enclosing the magnet coil in a cup of
metal upon which the diaphragm is mounted, so that the permanency of
relation between the diaphragm and the pole pieces is dependent only upon
the metallic structure and not at all upon the less durable shell.

Direct-Current Receiver. Until about the middle of the year 1909, it
was the universal practice to employ permanent magnets for giving the
initial polarization to the magnet cores of telephone receivers. This is still
done, and necessarily so, in receivers employed in connection with magneto
telephones. In common-battery systems, however, where the direct
transmitter current is fed from the central office to the local stations, it has
been found that this current which must flow at any rate through the line
may be made to serve the additional purpose of energizing the receiver
magnets so as to give them the necessary initial polarity. A type of receiver

has come into wide use as a result, which is commonly called the direct-
current receiver, deriving its name from the fact that it employs the direct
current that is flowing in the common-battery line to magnetize the receiver
cores. The Automatic Electric Company, of Chicago, was probably the first
company to adopt this form of receiver as its standard type. Their receiver is
shown in cross-section in Fig. 52, and a photograph of the same instrument
partially disassembled is given in Fig. 53. The most noticeable thing about
the construction of this receiver is the absence of permanent magnets. The
entire working parts are contained within the brass cup 1, which serves not
only as a container for the magnet, but also as a seat for the diaphragm. This
receiver is therefore illustrative of the type mentioned above, wherein the
relation between the diaphragm and the pole pieces is not dependent upon
any connection through the shell.

Fig. 52. Automatic Electric Company Direct-Current Receiver
View full size illustration.

Fig. 53. Automatic Electric Company Direct-Current Receiver
View full size illustration.

The coil of this instrument consists of a single cylindrical spool 2,
mounted on a cylindrical core. This bobbin lies within a soft iron-punching
3, the form of which is most clearly shown in Fig. 53, and this punching
affords a return path to the diaphragm for the lines of force set up in the
magnet core. Obviously a magnetizing current passing through the winding
of the coil will cause the end of the core toward the diaphragm to be
polarized, say positively, while the end of the enclosing shell will be
polarized in the other polarity, negatively. Both poles of the magnet are
therefore presented to the diaphragm and the only air gap in the magnetic
circuit is that between the diaphragm and these poles. The magnetic circuit
is therefore one of great efficiency, since it consists almost entirely of iron,
the only air gap being that across which the attraction of the diaphragm is to
take place.

The action of this receiver will be understood when it is stated that in
common-battery practice, as will be shown in later chapters, a steady
current flows over the line for energizing the transmitter. On this current is
superposed the incoming voice currents from a distant station. The steady
current flowing in the line will, in the case of this receiver, pass through the
magnet winding and establish a normal magnetic field in the same way as if
a permanent magnet were employed. The superposed incoming voice
currents will then be able to vary this magnetic field in exactly the same
way as in the ordinary receiver.

An astonishing feature of this recent development of the so-called direct-
current receiver is that it did not come into use until after about twenty
years of common-battery practice. There is nothing new in the principles

involved, as all of them were already understood and some of them were
employed by Bell in his original telephone; in fact, the idea had been
advanced time and again, and thrown aside as not being worth
consideration. This is an illustration of a frequent occurrence in the
development of almost any rapidly growing art. Ideas that are discarded as
worthless in the early stages of the art are finally picked up and made use
of. The reason for this is that in some cases the ideas come in advance of
the art, or they are proposed before the art is ready to use them. In other
cases the idea as originally proposed lacked some small but essential detail,
or, as is more often the case, the experimenter in the early days did not have
sufficient skill or knowledge to make it fit the requirements as he saw them.

Monarch Receiver. The receiver of the Automatic Electric Company
just discussed employs but a single electromagnet by which the initial
magnetization of the cores and also the variable magnetization necessary
for speech reproduction is secured. The problem of the direct-current
receiver has been attacked in another way by Ernest E. Yaxley, of the
Monarch Telephone Manufacturing Company, with the result shown in Fig.
54. The construction in this case is not unlike that of an ordinary
permanent-magnet receiver, except that in the place of the permanent
magnets two soft iron cores 1-1 are employed. On these are wound two
long bobbins of insulated wire so that the direct current flowing over the
telephone line will pass through these and magnetize the cores to the same
degree and for the same purpose as in the case of permanent magnets. These
soft iron magnet cores 1-1 continue to a point near the coil chamber, where
they join the two soft iron pole pieces 2-2, upon which the ordinary voice-
current coils are wound. The two long coils 4-4, which may be termed the
direct-current coils, are of somewhat lower resistance than the two voice-
current coils 3-3. They are, however, by virtue of their greater number of
turns and the greater amount of iron that is included in their cores, of much
higher impedance than the voice-current coils 3-3. These two sets of coils
4-4 and 3-3 are connected in multiple. As a result of their lower ohmic
resistance the coils 4-4 will take a greater amount of the steady current
which comes over the line, and therefore the greater proportion of the
steady current will be employed in magnetizing the bar magnets. On
account of their higher impedance to alternating currents, however, nearly
all of the voice currents which are superposed on the steady currents,

flowing in the line will pass through the voice-current coils 3-3, and, being
near the diaphragm, these currents will so vary the steady magnetism in the
cores 2-2 as to produce the necessary vibration of the diaphragm.

Fig. 54. Monarch Direct-Current Receiver
View full size illustration.

This receiver, like the one of the Automatic Electric Company, does not
rely on the shell in any respect to maintain the permanency of relation
between the pole pieces and the diaphragm. The cup 5, which is of pressed
brass, contains the voice-current coils and also acts as a seat for the
diaphragm. The entire working parts of this receiver may be removed by
merely unscrewing the ear piece from the hard rubber shell, thus permitting
the whole works to be withdrawn in an obvious manner.

Dean Receiver. Of such decided novelty as to be almost revolutionary in
character is the receiver recently put on the market by the Dean Electric
Company and shown in Fig. 55. This receiver is of the direct-current type
and employs but a single cylindrical bobbin of wire. The core of this bobbin
and the return path for the magnetic lines of force set up in it are composed
of soft iron punchings of substantially E shape. These punchings are laid

together so as to form a laminated soft-iron field, the limbs of which are
about square in cross-section. The coil is wound on the center portion of
this E as a core, the core being, as stated, approximately square in cross-
section. The general form of magnetic circuit in this instrument is therefore
similar to that of the Automatic Electric Company's receiver, shown in Figs.
52 and 53, but the core is laminated instead of being solid as in that
instrument.

Fig. 55. Dean Steel Shell Receiver
View full size illustration.

The most unusual feature of this Dean receiver is that the use of hard
rubber or composition does not enter into the formation of the shell, but
instead a shell composed entirely of steel stampings has been substituted
therefor. The main portion of this shell is the barrel 1. Great skill has
evidently been exercised in the forming of this by the cold-drawn process, it
presenting neither seams nor welds. The ear piece 2 is also formed of steel
of about the same gauge as the barrel 1. Instead of screw-threading the steel
parts, so that they would directly engage each other, the ingenious device
has been employed of swaging a brass ring 3 in the barrel portion and a
similar brass ring 4 in the ear cap portion, these two being slotted and

keyed, as shown at 8, so as to prevent their turning in their respective seats.
The ring 3 is provided with an external screw thread and the ring 4 with an
internal screw thread, so that the receiver cap is screwed on to the barrel in
the same way as in the ordinary rubber shell. By the employment of these
brass screw-threaded rings, the rusting together of the parts so that they
could not be separated when required—a difficulty heretofore encountered
in steel construction of similar parts—has been remedied.

Fig. 56. Working Parts of Dean Receiver
View full size illustration.

The entire working parts of this receiver are contained within the cup 5,
the edge of which is flanged outwardly to afford a seat for the diaphragm.
The diaphragm is locked in place on the shell by a screw-threaded ring 6, as
is clearly indicated. A ring 7 of insulating material is seated within the
enlarged portion of the barrel 1, and against this the flange of the cup 5 rests
and is held in place by the cap 2 when it is screwed home. The working
parts of this receiver partially disassembled are shown in Fig. 56, which
gives a clear idea of some of the features not clearly illustrated in Fig. 55.

It cannot be denied that one of the principal items of maintenance of
subscribers' station equipment has been due to the breakage of receiver
shells. The users frequently allow their receiver to fall and strike heavily
against the wall or floor, thus not only subjecting the cords to great strain,
but sometimes cracking or entirely breaking the receiver shell. The
innovation thus proposed by the Dean Company of making the entire
receiver shell of steel is of great interest. The shell, as will be seen, is
entirely insulated from the circuit of the receiver so that no contact exists by
which a user could receive a shock. The shell is enameled inside and out

with a heavy black insulating enamel baked on, and said to be of great
durability. How this enamel will wear remains to be seen. The insulation of
the interior portions of the receiver is further guarded by providing a lining
of fiber within the shell at all points where it seems possible that a cross
could occur between some of the working parts and the metal of the shell.
This type of receiver has not been on the market long enough to draw
definite conclusions, based on experience in use, as to what its permanent
performance will be.

Thus far in this chapter only those receivers which are commonly called
hand receivers have been discussed. These are the receivers that are
ordinarily employed by the general public.

Fig. 57. Operator's Receiver
View full size illustration.

Operator's Receiver. At the central office in telephone exchanges the
operators are provided with receivers in order that they may communicate
with the subscribers or with other operators. In order that they may have
both of their hands free to set up and take down the connections and to
perform all of the switching operations required, a special form of receiver
is employed for this purpose, which is worn as a part of a head-gear and is
commonly termed a head receiver. These are necessarily of very light
construction, in order not to be burdensome to the operators, and obviously
they must be efficient. They are ordinarily held in place at the ear by a
metallic head band fitting over the head of the operator.

Such a receiver is shown in cross-section in Fig. 57, and completely
assembled with its head band in Fig. 58. Referring to Fig. 57 the shell 1 of
the receiver is of aluminum and the magnets are formed of steel rings 2,
cross-magnetized so as to present a north pole on one side of the ring and a
south pole on the other. The two L-shaped pole pieces 3 are secured by
screws to the poles of these ring magnets, and these pole pieces carry the
magnet coils, as is clearly indicated. These poles are presented to a soft iron
diaphragm in exactly the same way as in the larger hand receivers, the
diaphragm being clamped in place by a hard rubber ear piece, as shown.
The head bands are frequently of steel covered with leather. They have
assumed numerous forms, but the general form shown in Fig. 58 is the one
commonly adopted.

Fig. 58. Operator's Receiver and Cord
View full size illustration.

Fig. 59. Receiver Symbols
View full size illustration.

Conventional Symbols. The usual diagrammatic symbols for hand and
head receivers are shown in Fig. 59. They are self-explanatory. The symbol
at the left in this figure, showing the general outline of the receiver, is the
one most commonly used where any sort of a receiver is to be indicated in a
circuit diagram, but where it becomes desirable to indicate in the diagram

the actual connections with the coil or coils of the receiver, the symbol
shown at the right is to be preferred, and obviously it may be modified as to
number of windings and form of core as desired.

ToC

CHAPTER VII
PRIMARY CELLS

Galvani, an Italian physician, discovered, in 1786, that a current of
electricity could be produced by chemical action. In 1800, Volta, a
physicist, also an Italian, threw further light on Galvani's discovery and
produced what we know as the voltaic, or galvanic, cell. In honor of these
two discoverers we have the words volt, galvanic, and the various words
and terms derived therefrom.

Simple Voltaic Cell. A very simple voltaic cell may be made by placing
two plates, one of copper and one of zinc, in a glass vessel partly filled with
dilute sulphuric acid, as shown in Fig. 60. When the two plates are not
connected by a wire or other conductor, experiment shows that the copper
plate bears a positive charge with respect to the zinc plate, and the zinc
plate bears a negative charge with respect to the copper. When the two
plates are connected by a wire, a current flows from the copper to the zinc
plate through the metallic path of the wire, just as is to be expected when
any conductor of relatively high electrical potential is joined to one of
relatively low electrical potential. Ordinarily, when one charged body is
connected to another of different potential, the resulting current is of but
momentary duration, due to the redistribution of the charges and consequent
equalization of potential. In the case of the simple cell, however, the current
is continuous, showing that some action is maintaining the charges on the
two plates and therefore maintaining the difference of potential between
them. The energy of this current is derived from the chemical action of the
acid on the zinc. The cell is in reality a sort of a zinc-burning furnace.

In the action of the cell, when the two plates are joined by a wire, it may
be noticed that the zinc plate is consumed and that bubbles of hydrogen gas
are formed on the surface of the copper plate.

Theory. Just why or how chemical action in a voltaic cell results in the
production of a negative charge on the consumed plate is not known.

Modern theory has it that when an acid is diluted in water the molecules of
the acid are split up or dissociated into two oppositely charged atoms, or
groups of atoms, one bearing a positive charge and the other a negative
charge of electricity. Such charged atoms or groups of atoms are called ions.
This separation of the molecules of a chemical compound into positively
and negatively charged ions is called dissociation.

Thus, in the simple cell under consideration the sulphuric acid, by
dissociation, splits up into hydrogen ions bearing positive charges, and SO4
ions bearing negative charges. The solution as a whole is neutral in
potential, having an equal number of equal and opposite charges.

Fig. 60. Simple Voltaic Cell
View full size illustration.

It is known that when a metal is being dissolved by an acid, each atom of
the metal which is torn off by the solution leaves the metal as a positively
charged ion. The carrying away of positive charges from a hitherto neutral
body leaves that body with a negative charge. Hence the zinc, or consumed
plate, becomes negatively charged.

In the chemical attack of the sulphuric acid on the zinc, the positive
hydrogen ions are liberated, due to the affinity of the negative SO4 ions for

the positive zinc ions, this resulting in the formation of zinc sulphate in the
solution. Now the solution itself becomes positively charged, due to the
positive charges leaving the zinc plate with the zinc ions, and the free
positively charged hydrogen ions liberated in the solution as just described
are repelled to the copper plate, carrying their positive charges thereto.
Hence the copper plate, or the unconsumed plate, becomes positively
charged and also coated with hydrogen bubbles.

The plates or electrodes of a voltaic cell need not consist of zinc and
copper, nor need the fluid, called the electrolyte, be of sulphuric acid; any
two dissimilar elements immersed in an electrolyte that attacks one of them
more readily than the other will form a voltaic cell. In every such cell it will
be found that one of the plates is consumed, and that on the other plate
some element is deposited, this element being sometimes a gas and
sometimes a solid. The plate which is consumed is always the negative
plate, and the one on which the element is deposited is always the positive,
the current through the connecting wire always being, therefore, from the
unconsumed to the consumed plate. Thus, in the simple copper-zinc cell
just considered, the zinc is consumed, the element hydrogen is deposited on
the copper, and the current flow through the external circuit is from the
copper to the zinc.

The positive charges, leaving the zinc, or consumed, plate, and passing
through the electrolyte to the copper, or unconsumed, plate, constitute in
effect a current of electricity flowing within the electrolyte. The current
within the cell passes, therefore, from the zinc plate to the copper plate. The
zinc is, therefore, said to be positive with respect to the copper.

Difference of Potential. The amount of electromotive force, that is
generated between two dissimilar elements immersed in an electrolyte is
different for different pairs of elements and for different electrolytes. For a
given electrolyte each element bears a certain relation to another; i.e., they
are either electro-positive or electro-negative relative to each other. In the
following list a group of elements are arranged with respect to the potentials
which they assume with respect to each other with dilute sulphuric acid as
the electrolyte. The most electro-positive elements are at the top and the
most electro-negative at the bottom.

+ Sodium Lead Copper
Magnesium Iron Silver
Zinc Nickel Gold
Cadmium Bismuth Platinum
Tin Antimony - Graphite (Carbon)

Any two elements selected from this list and immersed in dilute
sulphuric acid will form a voltaic cell, the amount of difference of potential,
or electromotive force, depending on the distance apart in this series of the
two elements chosen. The current within the cell will always flow from the
one nearest the top of the list to the one nearest the bottom, i.e., from the
most electro-positive to the most electro-negative; and, therefore, the
current in the wire joining the two plates will flow from the one lowest
down in the list to the one highest up.

From this series it is easy to see why zinc and copper, and also zinc and
carbon, are often chosen as elements of voltaic cells. They are widely
separated in the series and comparatively cheap.

This series may not be taken as correct for all electrolytes, for different
electrolytes alter somewhat the order of the elements in the series. Thus, if
two plates, one of iron and the other of copper, are immersed in dilute
sulphuric acid, a current is set up which proceeds through the liquid from
the iron to the copper; but, if the plates after being carefully washed are
placed in a solution of potassium sulphide, a current is produced in the
opposite direction. The copper is now the positive element.

Table II shows the electrical deportment of the principal metals in three
different liquids. It is arranged like the preceding one, each metal being
electro-positive to any one lower in the list.

TABLE II

Behavior of Metals in Different Electrolytes

Caustic Potash Hydrochloric Acid Potassium Sulphide

+ Zinc + Zinc + Zinc
Tin Cadmium Copper
Cadmium Tin Cadmium
Antimony Lead Tin
Lead Iron Silver
Bismuth Copper Antimony
Iron Bismuth Lead
Copper Nickel Bismuth
Nickel Silver Nickel
- Silver - Antimony - Iron

It is important to remember that in all cells, no matter what elements or
what electrolyte are used, the electrode which is consumed is the one that
becomes negatively charged and its terminal, therefore, becomes the
negative terminal or pole, while the electrode which is not consumed is the
one that becomes positively charged, and its terminal is, therefore, the
positive terminal or pole of the cell. However, because the current in the
electrolyte flows from the consumed plate to the unconsumed plate, the
consumed plate is called the positive plate and the unconsumed, the
negative. This is likely to become confusing, but if one remembers that the
active plate is the positive plate, because it sends forth positive ions in the
electrolyte, and, therefore, itself becomes negatively charged, one will have
the proper basis always to determine the direction of the current flow, which
is the important thing.

Polarization. If the simple cell already described have its terminals
connected by a wire for some time, it will be found that the current rapidly
weakens until it ceases to be manifest. This weakening results from two
causes: first, the hydrogen gas which is liberated in the action of the cell is
deposited in a layer on the copper plate, thereby covering the plate and
reducing the area of contact with the liquid. This increases the internal
resistance of the cell, since hydrogen is a non-conductor. Second, the plate
so covered becomes in effect a hydrogen electrode, and hydrogen stands
high as an electro-positive element. There is, therefore, actual reduction in
the electromotive force of the cell, as well as an increase in internal

resistance. This phenomenon is known as polarization, and in commercial
cells means must be taken to prevent such action as far as possible.

The means by which polarization of cells is prevented or reduced in
practice may be divided into three general classes:
First—mechanical means. If the hydrogen bubbles be simply brushed away from the
surface of the electrode the resistance and the counter polarity which they cause will be
diminished. The same result may be secured if air be blown into the solution through a
tube, or if the liquid be kept agitated. If the surface of the electrode be roughened or
covered with points, the bubbles collect more freely at the points and are more quickly
carried away to the surface of the liquid. These means are, however, hardly practical except
in cells for laboratory use.

Second—chemical means. If a highly oxidizing substance be added to the electrolyte, it
will destroy the hydrogen bubbles by combining with them while they are in a nascent
state, and this will prevent the increase in internal resistance and the opposing
electromotive force. Such substances are bichromate of potash, nitric acid, and chlorine,
and are largely used.

Third—electro-chemical means. Double cells, arranged to separate the elements and
liquids by means of porous partitions or by gravity, may be so arranged that solid copper is
liberated instead of hydrogen at a point where the current leaves the liquid, thereby entirely
obviating polarization. This method also is largely used.

Local Action. When a simple cell stands idle, i.e., with its circuit open,
small hydrogen bubbles may be noticed rising from the zinc electrode
instead of from copper, as is the case where the circuit is closed. This is due
to impurities in the zinc plate, such as particles of iron, tin, arsenic, carbon,
etc. Each of these particles acts with the surrounding zinc just as might be
expected of any pair of dissimilar elements opposed to each other in an
electrolyte; in other words, they constitute small voltaic cells. Local
currents, therefore, are generated, circulating between the two adjacent
metals, and, as a result, the zinc plate and the electrolyte are needlessly
wasted and the general condition of the cell is impaired. This is called local
action.

Amalgamated Zincs. Local action might be prevented by the use of
chemically pure zinc, but this, on account of its expense, cannot be
employed commercially. Local action, however, may be overcome to a
great extent by amalgamating the zinc, i.e., coating it with mercury. The
iron particles or other impurities do not dissolve in the mercury, as does the

zinc, but they float to the surface, whence the hydrogen bubbles which may
form speedily carry them off, and, in other cases, the impurities fall to the
bottom of the cell. As the zinc in the pasty amalgam dissolves in the acid,
the film of mercury unites with fresh zinc, and so always presents a clear,
bright, homogeneous surface to the action of the electrolyte.

The process of amalgamating the zinc may be performed by dipping it in
a solution composed of

Nitric Acid 1 lb.
Muriatic Acid 2 lbs.
Mercury 8 oz.

The acids should be first mixed and then the mercury slowly added until
dissolved. Clean the zinc with lye and then dip it in the solution for a
second or two. Rinse in clean water and rub with a brush.

Another method of amalgamating zincs is to clean them by dipping them
in dilute sulphuric acid and then in mercury, allowing the surplus to drain
off.

Commercial zincs, for use in voltaic cells as now manufactured, usually
have about 4 per cent of mercury added to the molten zinc before casting
into the form of plates or rods.

Series and Multiple Connections. When a number of voltaic cells are
joined in series, the positive pole of one being connected to the negative
pole of the next one, and so on throughout the series, the electromotive
forces of all the cells are added, and the electromotive force of the group,
therefore, becomes the sum of the electromotive forces of the component
cells. The currents through all the cells in this case will be equal to that of
one cell.

If the cells be joined in multiple, the positive poles all being connected
by one wire and the negative poles by another, then the currents of all the
cells will be added while the electromotive force of the combination
remains the same as that of a single cell, assuming all the cells to be alike in
electromotive force.

Obviously combinations of these two arrangements may be made, as by
forming strings of cells connected in series, and connecting the strings in
multiple or parallel.

The term battery is frequently applied to a single voltaic cell, but this
term is more properly used to designate a plurality of cells joined together
in series, or in multiple, or in series multiple so as to combine their actions
in causing current to flow through an external circuit. We may therefore
refer to a battery of so many cells. It has, however, become common,
though technically improper, to refer to a single cell as a battery, so that the
term battery, as indicating necessarily more than one cell, has largely lost its
significance.

Cells may be of two types, primary and secondary.

Primary cells are those consisting of electrodes of dissimilar elements
which, when placed in an electrolyte, become immediately ready for action.

Secondary cells, commonly called storage cells and accumulators,
consist always of two inert plates of metal, or metallic oxide, immersed in
an electrolyte which is incapable of acting on either of them until a current
has first been passed through the electrolyte from one plate to the other. On
the passage of a current in this way, the decomposition of the electrolyte is
effected and the composition of the plates is so changed that one of them
becomes electro-positive and the other electro-negative. The cell is then,
when the charging current ceases, capable of acting as a voltaic cell.

This chapter is devoted to the primary cell or battery alone.

Types of Primary Cells. Primary cells may be divided into two general
classes: first, those adapted to furnish constant current; and second, those
adapted to furnish only intermittent currents. The difference between cells
in this respect rests largely in the means employed for preventing or
lessening polarization. Obviously in a cell in which polarization is entirely
prevented the current may be allowed to flow constantly until the cell is
completely exhausted; that is, until the zinc is all eaten up or until the
hydrogen is exhausted from the electrolyte or both. On the other hand some
cells are so constituted that polarization takes place faster than the means

intended to prevent it can act. In other words, the polarization gradually
gains on the preventive means and so gradually reduces the current by
increasing the resistance of the cell and lowering its electromotive force. In
cells of this kind, however, the arrangement is such that if the cell is
allowed to rest, that is, if the external circuit is opened, the depolarizing
agency will gradually act to remove the hydrogen from the unattacked
electrode and thus place the cell in good condition for use again.

Of these two types of primary cells the intermittent-current cell is of far
greater use in telephony than the constant-current cell. This is because the
use of primary batteries in telephony is, in the great majority of cases,
intermittent, and for that reason a cell which will give a strong current for a
few minutes and which after such use will regain practically all of its initial
strength and be ready for use again, is more desirable than one which will
give a weaker current continuously throughout a long period of time.

Since the cells which are adapted to give constant current are commonly
used in connection with circuits that are continuously closed, they are called
closed-circuit cells. The other cells, which are better adapted for
intermittent current, are commonly used on circuits which stand open most
of the time and are closed only occasionally when their current is desired.
For this reason these are termed open-circuit cells.

Open-Circuit Cells. LeClanché Cell:—By far the most important
primary cell for telephone work is the so-called LeClanché cell. This
assumes a large variety of forms, but always employs zinc as the negatively
charged element, carbon as the positively charged element, and a solution
of sal ammoniac as the electrolyte. This cell employs a chemical method of
taking care of polarization, the depolarizing agent being peroxide of
manganese, which is closely associated with the carbon element.

The original form of the LeClanché cell, a form in which it was very
largely used up to within a short time ago, is shown in Fig. 61. In this the
carbon element is placed within a cylindrical jar of porous clay, the walls of
this jar being of such consistency as to allow moisture slowly to permeate
through it. Within this porous cup, as it is called, a plate or disk of carbon is
placed, and around this the depolarizing agent, consisting of black oxide of
manganese. This is usually mixed with, broken carbon, so as to increase the

effective area of the carbon element in contact with the depolarizing agent,
and also to reduce the total internal resistance of the cell. The zinc electrode
usually consisted merely in a rod of zinc, as shown, with a suitable terminal
at its upper end.

Fig. 61. LeClanché Cell
View full size illustration.

The chemical action taking place within the LeClanché cell is, briefly, as
follows: Sal ammoniac is chemically known as chloride of ammonium and
is a combination of chlorine and ammonia. In the action which is assumed
to accompany the passage of current in this cell, the sal ammoniac is
decomposed, the chlorine leaving the ammonia to unite with an atom of the
zinc plate, forming chloride of zinc and setting free ammonia and hydrogen.
The ammonia is immediately dissolved in the water of the cell, and the
hydrogen enters the porous cup and would speedily polarize the cell by
adhering to the carbon plate but for the fact that it encounters the peroxide
of manganese. This material is exceedingly rich in oxygen and it therefore
readily gives up a part of its oxygen, which forms water by combination
with the already liberated hydrogen and leaves what is termed a sesquioxide
of manganese. This absorption or combination of the hydrogen prevents
immediate polarization, but hydrogen is evolved during the operation of the
cell more rapidly than it can combine with the oxygen of the manganese,
thereby leading to polarization more rapidly than the depolarizer can

prevent it when the cell is heavily worked. When, however, the cell is left
with its external circuit open for a time, depolarization ensues by the
gradual combination of the hydrogen with the oxygen of the peroxide of
manganese, and as a result the cell recuperates and in a short time attains its
normal electromotive force.

The electromotive force of this cell when new is about 1.47 volts. The
internal resistance of the cell of the type shown in Fig. 61 is approximately
1 ohm, ordinarily less rather than more.

A more recent form of LeClanché cell is shown in cross-section in Fig.
62. This uses practically the same materials and has the same chemical
action as the old disk LeClanché cell shown in Fig. 61. It dispenses,
however, with the porous cup and instead employs a carbon electrode,
which in itself forms a cup for the depolarizing agent.

Fig. 62. Carbon Cylinder LeClanché Cell
View full size illustration.

The carbon electrode is in the form of a corrugated hollow cylinder which engages
by means of an internal screw thread a corresponding screw thread on the outer side of
the carbon cover. Within this cylinder is contained a mixture of broken carbon and
peroxide of manganese. The zinc electrode is in the form of a hollow cylinder almost
surrounding the carbon electrode and separated therefrom by means of heavy rubber
bands stretched around the carbon. The rod, forming the terminal of the zinc, passes
through a porcelain bushing on the cover plate to obviate short circuits. This type of
cell has an electromotive force of about 1.55 volts and recuperates very quickly after
severe use. It also has considerably lower internal resistance than the type of
LeClanché cell employing a porous cup, and, therefore, is capable of generating a
considerably larger current.

Cells of this general type have assumed a variety of forms. In some the carbon
electrode, together with the broken carbon and peroxide of manganese, were packed
into a canvas bag which was suspended in the electrolyte and usually surrounded by the
zinc electrode. In other forms the carbon electrode has moulded with it the manganese
depolarizer.

In order to prevent the salts within the cell from creeping over the edge of the
containing glass jar and also over the upper portion of the carbon electrode, it is
common practice to immerse the upper end of the carbon element and also the upper
edge of the glass jar in hot paraffin.

In setting up the LeClanché cell, place not more than four ounces of white sal
ammoniac in the jar, fill the jar one-third full of water, and stir until the sal ammoniac

is all dissolved. Then put the carbon and zinc elements in place. A little water poured in
the vent hole of the porous jar or carbon cylinder will tend to hasten the action.

An excess of sal ammoniac should not be used, as a saturated solution tends to
deposit crystals on the zinc; on the other hand, the solution should not be allowed to
become too weak, as in that case the chloride of zinc will form on the zinc. Both of
these causes materially increase the resistance of the cell.

A great advantage of the LeClanché cell is that when not in use there is but little
material waste. It contains no highly corrosive chemicals. Such cells require little
attention, and the addition of water now and then to replace the loss due to evaporation
is about all that is required until the elements become exhausted. They give a relatively
high electromotive force and have a moderately low internal resistance, so that they are
capable of giving rather large currents for short intervals of time. If properly made,
they recuperate quickly after polarization due to heavy use.

Dry Cell. All the forms of cells so far considered may be quite properly termed wet
cells because of the fact that a free liquid electrolyte is used. This term is employed in
contradistinction to the later developed cell, commonly termed the dry cell. This term
"dry cell" is in some respects a misnomer, since it is not dry and if it were dry it would
not work. It is essential to the operation of these cells that they shall be moist within,
and when such moisture is dissipated the cell is no longer usable, as there is no further
useful chemical action.

The dry cells are all of the LeClanché type, the liquid electrolyte of that type being
replaced by a semi-solid substance that is capable of retaining moisture for a
considerable period.

As in the ordinary wet LeClanché cell, the electrodes are of carbon and zinc, the
zinc element being in the form of a cylindrical cup and forming the retaining vessel of
the cell, while the carbon element is in the form of a rod or plate and occupies a central
position with regard to the zinc, being held out of contact with the zinc, however, at all
points.

A cross-section of an excellent form of dry cell is shown in Fig. 63. The outer casing
is of zinc, formed in the shape of a cylindrical cup, and serves not only as the retaining
vessel, but as the negatively charged electrode. The outer surface of the zinc is
completely covered on its sides and bottom with heavy pasteboard so as to insulate it
from bodies with which it may come in contact, and particularly from the zinc cups of
other cells used in the same battery. The positively charged electrode is a carbon rod
corrugated longitudinally, as shown, in order to obtain greater surface. This rod is held
in the center of the zinc cup out of contact therewith, and the intervening space is filled
with a mixture of peroxide of manganese, powdered carbon, and sal ammoniac. Several

thicknesses of blotting paper constitute a lining for the inner portion of the zinc
electrode and serve to prevent the manganese mixture from coming directly into
contact therewith. The cell is sealed with pitch, which is placed on a layer of sand and
sawdust mixed in about equal parts.

Fig. 63. Dry Cell
View full size illustration.

The electrolyte in such cells varies largely as to quantities and proportions of the
materials employed in various types of cells, and also varies in the method in which the
elements are introduced into the container.

The following list and approximate proportions of material will serve as a fair
example of the filling mixture in well-known types of cells.

Manganese dioxide 45 per cent
Carbon or graphite, or both 45 per cent
Sal ammoniac 7 per cent
Zinc chloride 3 per cent

Water is added to the above and a sufficient amount of mixture is taken for each cell
to fill the zinc cup about seven-eighths full when the carbon is in place. The most
suitable quantity of water depends upon the original dryness and fineness of material
and upon the quality of the paper lining.

In some forms of dry batteries, starch or other paste is added to improve the contact
of the electrolyte with the zinc and promote a more even distribution of action

throughout the electrolyte. Mercury, too, is often added to effect amalgamation of the
zinc.

As in the ordinary wet type of LeClanché cell, the purpose of the manganese is to
act as a depolarizer; the carbon or graphite being added to give conductivity to the
manganese and to form a large electrode surface. It is important that the sal ammoniac,
which is the active agent of the cell, should be free from lumps in order to mix properly
with the manganese and carbon.

A small local action takes place in the dry cell, caused by the dissimilar metals
necessarily employed in soldering up the zinc cup and in soldering the terminal rod of
zinc to the zinc cup proper. This action, however, is slight in the better grades of cells.
As a result of this, and also of the gradual drying out of the moisture within the cell,
these cells gradually deteriorate even when not in use—this is commonly called shelf-
wear. Shelf-wear is much more serious in the very small sizes of dry cells than in the
larger ones.

Dry cells are made in a large number of shapes and sizes. The most useful form,
however, is the ordinary cylindrical type. These are made in sizes varying from one and
one-half inches high and three-quarters inch in diameter to eight inches high and three
and three-quarters inches in diameter. The most used and standard size of dry cell is of
cylindrical form six inches high and two and three-quarters inches in diameter. The dry
cell when new and in good condition has an open-circuit voltage of from 1.5 to 1.6
volts. Perhaps 1.55 represents the usual average.

A cell of the two and three-quarters by six-inch size will give throughout its useful
life probably thirty ampere hours as a maximum, but this varies greatly with the
condition of use and the make of cell. Its effective voltage during its useful life
averages about one volt, and if during this life it gives a total discharge of thirty ampere
hours, the fair energy rating of the cell will be thirty watt-hours. This may not be taken
as an accurate figure, however, as the watt-hour capacity of a cell depends very largely,
not only on the make of the cell, but on the rate of its discharge.

An examination of Fig. 63 shows that the dry cell has all of the essential elements of
the LeClanché cell. The materials of which the electrodes are made are the same and
the porous cup of the disk LeClanché cell is represented in the dry cell by the blotting-
paper cylinder, which separates the zinc from the carbon electrode. The positively
charged electrode must not be considered as merely the carbon plate or rod alone, but
rather the carbon rod with its surrounding mixture of peroxide of manganese and
broken carbon. Such being the case, it is obvious that the separation between the
electrodes is very small, while the surface presented by both electrodes is very large.
As a result, the internal resistance of the cell is small and the current which it will give

on a short circuit is correspondingly large. A good cell of the two and three-quarters by
six-inch size will give eighteen or twenty amperes on short-circuit, when new.

As the action of the cell proceeds, zinc chloride and ammonia are formed, and there
being insufficient water to dissolve the ammonia, there results the formation of double
chlorides of zinc and ammonium. These double chlorides are less soluble than the
chlorides and finally occupy the pores of the paper lining between the electrolyte and
the zinc and greatly increase the internal resistance of the cell. This increase of
resistance is further contributed to by the gradual drying out of the cell as its age
increases.

Within the last few years dry batteries have been so perfected mechanically,
chemically, and electrically that they have far greater outputs and better recuperative
power than any of the other types of LeClanché batteries, while in point of convenience
and economy, resulting from their small size and non-breakable, non-spillable features
and low cost, they leave no room for comparison.

Closed-Circuit Cells. Gravity-Cell:—Coming now to the consideration of closed-
circuit or constant-current cells, the most important is the well-known gravity, or blue-
stone, cell, devised by Daniell. It is largely used in telegraphy, and often in telephony in
such cases as require a constantly flowing current of small quantity. Such a cell is
shown in Fig. 64.

The elements of the gravity cell are electrodes of copper and zinc. The solution in
which the copper plate is immersed is primarily a solution of copper sulphate,
commonly known as blue-stone, in water. The zinc plate after the cell is in action is
immersed in a solution of sulphate of zinc which is formed around it.

The glass jar is usually cylindrical, the standard sizes being 5 inches diameter and 7
inches deep; and also 6 inches diameter and 8 inches deep. The copper electrode is of
sheet copper of the form shown, and it is partly covered with crystals of blue-stone or
copper sulphate. Frequently, in later forms of cells, the copper electrode consists
merely of a straight, thick, rectangular bar of copper laid horizontally, directly on top of
the blue-stone crystals. In all cases a rubber-insulated wire is attached by riveting to the
copper electrode, and passes up through the electrolyte to form the positive terminal.

Fig. 64. Gravity Cell
View full size illustration.

The zinc is, as a rule, of crowfoot form, as shown, whence this cell derives the
commonly applied name of crowfoot cell. This is essentially a two-fluid cell, for in its
action zinc sulphate is formed, and this being lighter than copper sulphate rises to the
top of the jar and surrounds the zinc. Gravity, therefore, serves to keep the two fluids
separate.

In the action of the cell, when the external circuit is closed, sulphuric acid is formed
which attacks the zinc to form sulphate of zinc and to liberate hydrogen, which follows
its tendency to attach itself to the copper plate. But in so doing the hydrogen
necessarily passes through the solution of sulphate of copper surrounding the copper
plate. The hydrogen immediately combines with the SO4 radical, forming therewith
sulphuric acid, and liberating metallic copper. This sulphuric acid, being lighter than
the copper sulphate, rises to the surface of the zinc and attacks the zinc, thus forming
more sulphate of zinc. The metallic copper so formed is deposited on the copper plate,
thereby keeping the surface bright and clean. Since hydrogen is thus diverted from the
copper plate, polarization does not ensue.

The zinc sulphate being colorless, while the copper sulphate is of a dark blue color,
the separating line of the two liquids is easily distinguishable. This line is called the
blue line and care should be taken that it does not reach the zinc and cause a deposit of
copper to be placed thereon.

As has been stated, these two liquids do not mix readily, but they will eventually
mingle unless the action of the cell is sufficient to use up the copper sulphate as
speedily as it is dissolved. Thus it will be seen that while the cell is free from
polarization and local action, there is, nevertheless, a deteriorating effect if the cell is

allowed to remain long on open circuit. Therefore, it should be used when a constant
current is required.

Prevention of Creeping:—Much trouble has been experienced in gravity cells due to
the creeping of the salts over the edge of the jar. Frequently the upper edges of the jars
are coated by dipping in hot paraffin wax in the hope of preventing this. Sometimes oil
is poured on top of the fluid in the jar to prevent the creeping of the salts and the
evaporation of the electrolyte. The following account of experiments performed by Mr.
William Reid, of Chicago, throws light on the relative advantages of these and other
methods of preventing creeping.
The experiment was made with gravity cells having 5-inch by 7-inch glass jars. Four cells were made
up and operated in a rather dry, warm place, although perhaps under no more severe local conditions
than would be found in most telephone exchanges. Cell No. 1 was a plain cell as ordinarily used. Cell
No. 2 had the top of the rim of the jar treated with paraffin wax by dipping the rim to about one inch in
depth in melted paraffin wax. Cell No. 3 had melted paraffin wax poured over the surface of the liquid
forming a seal about 3/16 inch in thickness. After cooling, a few small holes were bored through the seal
to let gases escape. Cell No. 4 had a layer of heavy paraffin oil nearly 1/2 inch in thickness (about 6 oz.
being used) on top of the solutions.

These cells were all run on a load of .22 to .29 amperes for 15-1/2 hours per day for thirty days, after
which the following results were noted:

(a) The plain cell, or cell No. 1, had to have 26 ounces of water added to it to replace that which had
evaporated. The creeping of zinc sulphate salts was very bad.

(b) The waxed rim cell, or cell No. 2, evaporated 26 ounces of water and the creeping of zinc sulphate
salts was not prevented by the waxed rim. The wax proved of no value.

(c) The wax sealed cell, or cell No. 3, showed practically no evaporation and only very slight creeping
of zinc sulphate salts. The creeping of salts that took place was only around spots where the edges of the
seal were loose from the jar.

(d) The paraffin oil sealed cell, or cell No. 4, showed no evaporation and no creeping of salts.

It was concluded by Mr. Reid from the above experiments that the wax applied to
the rim of the jar is totally ineffective and has no merits. The wax seal loosens around
the edges and does not totally prevent creeping of the zinc sulphate salts, although
nearly so. The wax-sealed jar must have holes drilled in it to allow the gases to escape.
The method is hardly commercial, as it is difficult to make a neat appearing cell,
besides making it almost impossible to manipulate its contents. A coat of paraffin oil
approximately 1/2 inch in thickness (about 6 ounces) gives perfect protection against
evaporation and creeping of the zinc sulphate salts. The cell, having the paraffin-oil
seal, had a very neat, clean appearance as compared with cells No. 1 and No. 2. It was
found that the zinc could be drawn out through the oil, cleaned, and replaced with no
appreciable effect on voltage or current.

Setting Up:—In setting up the battery the copper electrode is first unfolded to form
a cross and placed in the bottom of the jar. Enough copper sulphate, or blue-stone
crystals, is then dropped into the jar to almost cover the copper. The zinc crowfoot is

then hung in place, occupying a position about 4 inches above the top of the copper.
Clear water is then poured in sufficient to fill the jar within about an inch of the top.

If it is not required to use the cell at once, it may be placed on short circuit for a
time and allowed to form its own zinc sulphate. The cell may, however, be made
immediately available for use by drawing about one-half pint of a solution of zinc
sulphate from a cell already in use and pouring it into the jar, or, when this is not
convenient, by putting into the liquid four or five ounces of pulverized sulphate of zinc,
or by adding about ten drops of sulphuric acid. When the cell is in proper working
condition, one-half inch in thickness of heavy paraffin oil of good quality may be
added.

If the blue line gets too low, and if there is in the bottom of the cell a sufficient
quantity of sulphate of copper, it may be raised by drawing off a portion of the zinc
sulphate with a battery syringe and replacing this with water. If the blue line gets too
high, it may be lowered by short-circuiting the cell for a time, or by the addition of
more sulphate of zinc solution from another battery. If the copper sulphate becomes
exhausted, it should be replenished by dropping in more crystals.

Care should be taken in cold weather to maintain the temperature of the battery
above 65° or 70° Fahrenheit. If below this temperature, the internal resistance of a cell
increases very rapidly, so much so that even at 50° Fahrenheit the action becomes very
much impaired. This follows from the facts that the resistance of a liquid decreases as
its temperature rises, and that chemical action is much slower at lower temperatures.

The gravity cell has a practically constant voltage of 1.08 volts. Its internal
resistance is comparatively high, seldom falling below 1 ohm and often rising to 6
ohms. At best, therefore, it is only capable of producing about 1 ampere. The gravity
cell is perhaps the most common type of cell wherein depolarization is affected by
electro-chemical means.

Fuller Cell:—A form of cell that is adapted to very heavy open-circuit work and also
closed-circuit work where heavier currents are required than can be supplied by the
gravity battery is the Fuller. In this the electrodes are of zinc and carbon, respectively,
the zinc usually being in the form of a heavy cone and placed within a porous cup. The
electrolyte of the Fuller cell is known as electropoion fluid, and consists of a mixture of
sodium or potassium bichromate, sulphuric acid, and water.

The various parts of the standard Fuller cell, as once largely employed by the
various Bell operating companies, are shown in Fig. 65. In this the jar was made of flint
glass, cylindrical in form, six inches in diameter and eight inches deep. It is important
that a good grade of glass be used for the jar in this cell, because, on account of the
nature of the electrolyte, breakage is disastrous in the effects it may produce on

adjacent property. The carbon plate is rectangular in form, about four inches wide, eight
and three-quarters inches long, and one-quarter inch thick. The metal terminal at the
top of the carbon block is of bronze, both it and the lock nuts and bolts being nickel-
plated to minimize corrosion. The upper end of the carbon block is soaked in paraffin
so hot as to drive all of the moisture out of the paraffin and out of the pores of the block
itself.

The zinc, as is noted from the cut, is in the form of a truncated cone. It is about two
and one-eighth inches in diameter at the base and two and one-half inches high. Cast
into the zinc is a soft copper wire about No. 12 B. & S. gauge. This wire extends above
the top of the jar so as to form a convenient terminal for the cell.

The porous cup is cylindrical in form, about three inches in diameter and seven
inches deep. The wooden cover is of kiln-dried white wood thoroughly coated with two
coats of asphalt paint. It is provided with a slot for the carbon and a hole for the copper
wire extending to the zinc.

The electrolyte for this cell is made as follows:

Sodium bichromate 6 oz.
Sulphuric acid 17 oz.
Soft water 56 oz.

This solution is mixed by dissolving the bichromate of sodium in the water and then
adding slowly the sulphuric acid. Potassium bichromate may be substituted for the
sodium bichromate.

In setting up this cell, the amalgamated zinc is placed within the porous cup, in the
bottom of which are about two teaspoonfuls of mercury, the latter serving to keep the
zinc well amalgamated. The porous cup is then placed in the glass jar and a sufficient
quantity of the electrolyte is placed in the outer jar to come within about one and one-
half inches of the top of the porous cup. About two teaspoonfuls of salt are then placed
in the porous cup and sufficient soft water added to bring the level of the liquid within
the porous cup even with the level of the electrolyte in the jar surrounding the cup. The
carbon is then placed through the slot in the cover, and the wire from the zinc is passed
through the hole in the cover provided for it, and the cover is allowed to fall in place.
The cell is now ready for immediate use.

The action of this cell is as follows: The sulphuric acid attacks the zinc and forms
zinc sulphate, liberating hydrogen. The hydrogen attempts to pass to the carbon plate as
usual, but in so doing it meets with the oxygen of the chromic acid and forms water
therewith. The remainder of the chromic acid combines with the sulphuric acid to form
chromium sulphate.

Fig. 65. Fuller Cell
View full size illustration.

The mercury placed in the bottom of the porous cup with the zinc keeps the zinc in a
state of perpetual amalgamation. This it does by capillary action, as the mercury
spreads over the entire surface of the zinc. The initial amalgamation, while not
absolutely essential, helps in a measure this capillary action.

In another well-known type of the Fuller battery the carbon is a hollow cylinder,
surrounding the porous cup. In this type the zinc usually took the form of a long bar
having a cross-shaped section, the length of this bar being sufficient to extend the entire
depth of the porous cup. This type of cell has the advantage of a somewhat lower
internal resistance than the standard form just described.

Should the electrolyte become supersaturated by virtue of the battery being
neglected or too heavily overworked, a set of secondary reactions will occur in the cell,
resulting in the formation of the yellow crystals upon the carbon. This seriously affects
the e.m.f. of the cell and also its internal resistance. Should this occur, some of the
solution should be withdrawn and dilute sulphuric acid inserted in its place and the
crystals which have formed on the carbon should be carefully washed off. Should the
solution lose its orange tint and turn blue, it indicates that more bichromate of potash or
bichromate of sodium is needed. This cell gives an electromotive force of 2.1 volts and
a very large current when it is in good condition, since its internal resistance is low.

The Fuller cell was once largely used for supplying current to telephone transmitters
at subscribers' stations, where very heavy service was demanded, but the advent of the
so-called common-battery systems, in some cases, and of the high-resistance
transmitter, in other cases, has caused a great lessening in its use. This is fortunate as
the cell is a "dirty" one to handle and is expensive to maintain.

The Fuller cell still warrants attention, however, as an available source of current,
which may be found useful in certain cases of emergency work, and in supplying
special but temporary needs for heavier current than the LeClanché or gravity cell can
furnish.

Lalande Cell:—A type of cell, specially adapted to constant-current work, and
sometimes used as a central source of current in very small common-battery exchanges
is the so-called copper oxide, or Lalande cell, of which the Edison and the Gordon are
types. In all of these the negatively charged element is of zinc, the positively charged
element a mass of copper oxide, and the electrolyte a solution of caustic potash in
water. In the Edison cell the copper oxide is in the form of a compressed slab which
with its connecting copper support forms the electrode. In the Gordon and other cells of
this type the copper oxide is contained loosely in a perforated cylinder of sheet copper.
The copper oxide serves not only as an electrode, but also as a depolarizing agent, the
liberated hydrogen in the electrolyte uniting with the oxygen of the copper oxide to
form water, and leaving free metallic copper.

On open circuit the elements are not attacked, therefore there is no waste of material
while the cell is not in use. This important feature, and the fact that the internal
resistance is low, make this cell well adapted for all forms of heavy open-circuit work.
The fact that there is no polarizing action within the cell makes it further adaptable to
heavy closed-circuit service.

These cells are intended to be so proportioned that all of their parts become
exhausted at once so that when the cell fails, complete renewals are necessary.
Therefore, there is never a question as to which of the elements should be renewed.

After the elements and solution are in place about one-fourth of an inch of heavy
paraffin oil is poured upon the surface of the solution in order to prevent evaporation.
This cell requires little attention and will maintain a constant e.m.f. of about two-thirds
of a volt until completely exhausted. It is non-freezable at all ordinary temperatures. Its
low voltage is its principal disadvantage.

Standard Cell. Chloride of Silver Cell:—The chloride of silver cell is largely used as
a standard for testing purposes. Its compactness and portability and its freedom from
local action make it particularly adaptable to use in portable testing outfits where
constant electromotive force and very small currents are required.

Fig. 66. Chloride of Silver Cell
View full size illustration.

A cross-section of one form of the cell is shown in Fig. 66. Its elements are a rod of
chemically-pure zinc and a rod of chloride of silver immersed in a water solution of sal
ammoniac. As ordinarily constructed, the glass jar or tube is usually about 2-1/2 inches
long by 1 inch in diameter. After the solution is poured in and the elements are in place
the glass tube is hermetically sealed with a plug of paraffin wax.

The e.m.f. of a cell of this type is 1.03 volts and the external resistance varies with
the age of the cell, being about 4 ohms at first. Care should be taken not to short-circuit
these cells, or use them in any but high-resistance circuits, as they have but little energy
and become quickly exhausted if compelled to work in low-resistance circuits.

Conventional Symbol. The conventional symbol for a cell, either of the primary or
the secondary type, consists of a long thin line and a short heavy line side by side and
parallel. A battery is represented by a number of pairs of such lines, as in Fig. 67. The
two lines of each pair are supposed to represent the two electrodes of a cell. Where any
significance is to be placed on the polarity of the cell or battery the long thin line is
supposed to represent the positively charged plate and the short thick line the
negatively charged plate. The number of pairs may indicate the number of cells in the
battery. Frequently, however, a few pairs of such lines are employed merely for the
purpose of indicating a battery without regard to its polarity or its number of cells.

Fig. 67. Battery Symbols
View full size illustration.

In Fig. 67 the representation at A is that of a battery of a number of cells connected
in parallel; that at B of a battery with the cells connected in series; and that at C of a
battery with one of its poles grounded.

ToC

CHAPTER VIII
MAGNETO SIGNALING APPARATUS

Method of Signaling. The ordinary apparatus, by which speech is received
telephonically, is not capable of making sufficiently loud sounds to attract the attention
of people at a distance from the instrument. For this reason it is necessary to employ
auxiliary apparatus for the purpose of signaling between stations. In central offices
where an attendant is always on hand, the sense of sight is usually appealed to by the
use of signals which give a visual indication, but in the case of telephone instruments
for use by the public, the sense of hearing is appealed to by employing an audible
rather than a visual signal.

Battery Bell. The ordinary vibrating or battery bell, such as is employed for door
bells, is sometimes, though not often, employed in telephony. It derives its current from
primary batteries or from any direct-current source. The reason why they are not
employed to a greater extent in telephony is that telephone signals usually have to be
sent over lines of considerable length and the voltage that would be required to furnish
current to operate such bells over such lengths of line is higher than would ordinarily
be found in the batteries commonly employed in telephone work. Besides this the
make-and-break contacts on which the, ordinary battery bell depends for its operation
are an objectionable feature from the standpoint of maintenance.

Magneto Bell. Fortunately, however, there has been developed a simpler type of
electric bell, which operates on smaller currents, and which requires no make-and-
break contacts whatever. This simpler form of bell is commonly known as the
polarized, or magneto, bell or ringer. It requires for its operation, in its ordinary form,
an alternating current, though in its modified forms it may be used with pulsating
currents, that is, with periodically recurring impulses of current always in the same
direction.

Magneto Generator. In the early days of telephony there was nearly always
associated with each polarized bell a magneto generator for furnishing the proper kind
of current to ring such bells. Each telephone was therefore equipped, in addition to the
transmitter and receiver, with a signal-receiving device in the form of a polarized bell,
and with a current generator by which the user was enabled to develop his own currents
of suitable kind and voltage for ringing the bells of other stations.

Considering the signaling apparatus of the telephones alone, therefore, each
telephone was equipped with a power plant for generating currents used by that station

in signaling other stations, the prime mover being the muscles of the user applied to the
turning of a crank on the side of the instrument; and also with a current-consuming
device in the form of a polarized electromagnetic bell adapted to receive the currents
generated at other stations and to convert a portion of their energy into audible signals.

The magneto generator is about the simplest type of dynamo-electric machine, and
it depends upon the same principles of operation as the much larger generators,
employed in electric-lighting and street-railway power plants, for instance. Instead of
developing the necessary magnetic field by means of electromagnets, as in the case of
the ordinary dynamo, the field of the magneto generator is developed by permanent
magnets, usually of the horseshoe form. Hence the name magneto.

Fig. 68. Principles of Magneto Generator
View full size illustration.

In order to concentrate the magnetic field within the space in which the armature
revolves, pole pieces of iron are so arranged in connection with the poles of the
permanent magnet as to afford a substantially cylindrical space in which the armature
conductors may revolve and through which practically all the magnetic lines of force
set up by the permanent magnets will pass. In Fig. 68 there is shown, diagrammatically,
a horseshoe magnet with such a pair of pole pieces, between which a loop of wire is
adapted to rotate. The magnet 1 is of hardened steel and permanently magnetized. The
pole pieces are shown at 2 and 3, each being of soft iron adapted to make good
magnetic contact on its flat side with the inner flat surface of the bar magnet, and being
bored out so as to form a cylindrical recess between them as indicated. The direction of
the magnetic lines of force set up by the bar magnet through the interpolar space is
indicated by the long horizontal arrows, this flow being from the north pole (N) to the
south pole (S) of the magnet. At 4 there is shown a loop of wire supposed to revolve in
the magnetic field of force on the axis 5-5.

Theory. In order to understand how currents will be generated in this loop of wire
4, it is only necessary to remember that if a conductor is so moved as to cut across
magnetic lines of force, an electromotive force will be set up in the conductor which
will tend to make the current flow through it. The magnitude of the electromotive force
will depend on the rate at which the conductor cuts through the lines of force, or, in
other words, on the number of lines of force that are cut through by the conductor in a
given unit of time. Again, the direction of the electromotive force depends on the
direction of the cutting, so that if the conductor be moved in one direction across the
lines of force, the electromotive force and the current will be in one direction; while if
it moves in the opposite direction across the lines of force, the electromotive force and
the current will be in the reverse direction.

It is, evident that as the loop of wire 4 revolves in the field of force about the axis 5-
5, the portions of the conductor parallel to the axis will cut through the lines of force,
first in one direction and then in the other, thus producing electromotive forces therein,
first in one direction and then in the other.

Referring now to Fig. 68, and supposing that the loop 4 is revolving in the direction
of the curved arrow shown between the upper edges of the pole pieces, it will be
evident that just as the loop stands in the vertical position, its horizontal members will
be moving in a horizontal direction, parallel with the lines of force and, therefore, not
cutting them at all. The electromotive force and the current will, therefore, be zero at
this time.

As the loop advances toward the position shown in dotted lines, the upper portion of
the loop that is parallel with the axis will begin to cut downwardly through the lines of
force, and likewise the lower portion of the loop that is parallel with the axis will begin
to cut upwardly through the lines of force. This will cause electromotive forces in
opposite directions to be generated in these portions of the loop, and these will tend to
aid each other in causing a current to circulate in the loop in the direction shown by the
arrows associated with the dotted representation of the loop. It is evident that as the
motion of the loop progresses, the rate of cutting the lines of force will increase and
will be a maximum when the loop reaches a horizontal position, or at that time the two
portions of the loop that are parallel with the axis will be traveling at right angles to the
lines of force. At this point, therefore, the electromotive force and the current will be a
maximum.

From this point until the loop again assumes a vertical position, the cutting of the
lines of force will still be in the same direction, but at a constantly decreasing rate,
until, finally, when the loop is vertical the movement of the parts of the loop that are
parallel with the axis will be in the direction of the lines of force and, therefore, no
cutting will take place. At this point, therefore, the electromotive force and the current
in the loop again will be zero. We have seen, therefore, that in this half revolution of

the loop from the time when it was in a vertical position to a time when it was again in
a vertical position but upside down, the electromotive force varied from zero to a
maximum and back to zero, and the current did the same.

It is easy to see that, as the loop moves through the next half revolution, an exactly
similar rise and fall of electromotive force and current will take place; but this will be
in the opposite direction, since that portion of the loop which was going down through
the lines of force is now going up, and the portion which was previously going up is
now going down.

The law concerning the generation of electromotive force and current in a conductor
that is cutting through lines of magnetic force, may be stated in another way, when the
conductor is bent into the form of a loop, as in the case under consideration: Thus, if
the number of lines of force which pass through a conducting loop be varied,
electromotive forces will be generated in the loop. This will be true whether the number
of lines passing through the loop be varied by moving the loop within the field of force
or by varying the field of force itself. In any case, if the number of lines of force be
increased, the current will flow in one way, and if it be diminished the current will flow
in the other way. The amount of the current will depend, other things being equal, on
the rate at which the lines of force through the loop are being varied, regardless of the
method by which the variation is made to take place. One revolution of the loop,
therefore, results in a complete cycle of alternating current consisting of one positive
followed by one negative impulse.

The diagram of Fig. 68 is merely intended to illustrate the principle involved. In the
practical construction of magneto generators more than one bar magnet is used, and, in
addition, the conductors in the armature are so arranged as to include a great many
loops of wire. Furthermore, the conductors in the armature are wound around an iron
core so that the path through the armature loops or turns, may present such low
reluctance to the passage of lines of force as to greatly increase the number of such
lines and also to cause practically all of them to go through the loops in the armature
conductor.

Armature. The iron upon which the armature conductors are wound is called the
core. The core of an ordinary armature is shown in Fig. 69. This is usually made of soft
gray cast iron, turned so as to form bearing surfaces at 1 and 2, upon which the entire
armature may rotate, and also turned so that the surfaces 3 will be truly cylindrical with
respect to the axis through the center of the shaft. The armature conductors are put on
by winding the space between the two parallel faces 4 as full of insulated wire as space
will admit. One end of the armature winding is soldered to the pin 5 and, therefore,
makes contact with the frame of the generator, while the other end of the winding is
soldered to the pin 6, which engages the stud 7, carried in an insulating bushing in a
longitudinal hole in the end of the armature shaft. It is thus seen that the frame of the

machine will form one terminal of the armature winding, while the insulated stud 7 will
form the other terminal.

Fig. 69. Generator Armature
View full size illustration.

Another form of armature largely employed in recent magneto generators is
illustrated in Fig. 70. In this the shaft on which the armature revolves does not form an
integral part of the armature core but consists of two cylindrical studs 2 and 3
projecting from the centers of disks 4 and 5, which are screwed to the ends of the core
1. This H type of armature core, as it is called, while containing somewhat more parts
than the simpler type shown in Fig. 69, possesses distinct advantages in the matter of
winding. By virtue of its simpler form of winding space, it is easier to insulate and
easier to wind, and furthermore, since the shaft does not run through the winding space,
it is capable of holding a considerably greater number of turns of wire. The ends of the
armature winding are connected, one directly to the frame and the other to an insulated
pin, as is shown in the illustration.

Fig. 70. Generator Armature
View full size illustration.

Fig. 71. Generator Field and Armature
View full size illustration.

The method commonly employed of associating the pole pieces with each other and
with the permanent magnets is shown in Fig. 71. It is very important that the space in
which the armature revolves shall be truly cylindrical, and that the bearings for the
armature shall be so aligned as to make the axis of rotation of the armature coincide
with the axis of the cylindrical surface of the pole pieces. A rigid structure is, therefore,
required and this is frequently secured, as shown in Fig. 71, by joining the two pole
pieces 1 and 2 together by means of heavy brass rods 3 and 4, the rods being
shouldered and their reduced ends passed through holes in flanges extending from the
pole pieces, and riveted. The bearing plates in which the armature is journaled are then
secured to the ends of these pole pieces, as will be shown in subsequent illustrations.
This assures proper rigidity between the pole pieces and also between the pole pieces
and the armature bearings.

The reason why this degree of rigidity is required is that it is necessary to work with
very small air gaps between the armature core and its pole pieces and unless these
generators are mechanically well made they are likely to alter their adjustment and thus
allow the armature faces to scrape or rub against the pole pieces. In Fig. 71 one of the
permanent horseshoe magnets is shown, its ends resting in grooves on the outer faces
of the pole pieces and usually clamped thereto by means of heavy iron machine screws.

With this structure in mind, the theory of the magneto generator developed in
connection with Fig. 68 may be carried a little further. When the armature lies in the
position shown at the left of Fig. 71, so that the center position of the core is horizontal,
a good path is afforded for the lines of force passing from one pole to the other.
Practically all of these lines will pass through the iron of the core rather than through
the air, and, therefore, practically all of them will pass through the convolutions of the
armature winding.

When the armature has advanced, say 45 degrees, in its rotation in the direction of
the curved arrow, the lower right-hand portion of the armature flange will still lie
opposite the lower face of the right-hand pole piece and the upper left-hand portion of
the armature flange will still lie opposite the upper face of the left-hand pole piece. As a
result there will still be a good path for the lines of force through the iron of the core
and comparatively little change in the number of lines passing through the armature
winding. As the corners of the armature flange pass away from the corners of the pole
pieces, however, there is a sudden change in condition which may be best understood
by reference to the right-hand portion of Fig. 71. The lines of force now no longer find
path through the center portion of the armature core—that lying at right angles to their
direction of flow. Two other paths are at this time provided through the now horizontal
armature flanges which serve almost to connect the two pole pieces. The lines of force
are thus shunted out of the path through the armature coils and there is a sudden
decrease from a large number of lines through the turns of the winding to almost none.
As the armature continues in its rotation the two paths through the flanges are broken,
and the path through the center of the armature core and, therefore, through the coils
themselves, is reëstablished.

As a result of this consideration it will be seen that in actual practice the change in
the number of lines passing through the armature winding is not of the gradual nature
that would be indicated by a consideration of Fig. 68 alone, but rather, is abrupt, as the
corners of the armature flanges leave the corners of the pole pieces. This abrupt change
produces a sudden rise in electromotive force just at these points in the rotation, and,
therefore, the electromotive force and the current curves of these magneto generators is
not usually of the smooth sine-wave type but rather of a form resembling the sine wave
with distinct humps added to each half cycle.

Fig. 72. Generator with Magnets Removed
View full size illustration.

As is to be expected from any two-pole alternating generator, there is one cycle of
current for each revolution of the armature. Under ordinary conditions a person is able

to turn the generator handle at the rate of about two hundred revolutions a minute, and
as the ratio of gearing is about five to one, this results in about one thousand
revolutions per minute of the generator, and, therefore, in a current of about one
thousand cycles per minute, this varying widely according to the person who is doing
the turning.

The end plates which support the bearings for the armature are usually extended
upwardly, as shown in Fig. 72, so as to afford bearings for the crank shaft. The crank
shaft carries a large spur gear which meshes with a pinion in the end of the armature
shaft, so that the user may cause the armature to revolve rapidly. The construction
shown in Fig. 72 is typical of that of a modern magneto generator, it being understood
that the permanent magnets are removed for clearness of illustration.

Fig. 73 is a view of a completely assembled generator such as is used for service
requiring a comparatively heavy output. Other types of generators having two, three, or
four permanent magnets instead of five, as shown in this figure, are also standard.

Fig. 73. Five-Bar Generator
View full size illustration.

Referring again to Fig. 69, it will be remembered that one end of the armature
winding shown diagrammatically in that figure, is terminated in the pin 5, while the
other terminates in the pin 7. When the armature is assembled in the frame of the
generator it is evident that the frame itself is in metallic connection with one end of the
armature winding, since the pin 5 is in metallic contact with the armature casting and
this is in contact with the frame of the generator through the bearings. The frame of the
machine is, therefore, one terminal of the generator. When the generator is assembled a
spring of one form or another always rests against the terminal pin 7 of the armature so
as to form a terminal for the armature winding of such a nature as to permit the
armature to rotate freely. Such spring, therefore, forms the other terminal of the
generator.

Automatic Shunt. Under nearly all conditions of practice it is desirable to have the
generator automatically perform some switching function when it is operated. As an

example, when the generator is connected so that its armature is in series in a telephone
line, it is quite obvious that the presence of the resistance and the impedance of the
armature winding would be objectionable if left in the circuit through which the voice
currents had to pass. For this reason, what is termed an automatic shunt is employed on
generators designed for series work; this shunt is so arranged that it will automatically
shunt or short-circuit the armature winding when it is at rest and also break this shunt
when the generator is operated, so as to allow the current to pass to line.

Fig. 74. Generator Shunt Switch
View full size illustration.

A simple and much-used arrangement for this purpose is shown in Fig. 74, where 1
is the armature; 2 is a wire leading from the frame of the generator and forming one
terminal of the generator circuit; and 3 is a wire forming the other terminal of the
generator circuit, this wire being attached to the spring 4, which rests against the center
pin of the armature so as to make contact with the opposite end of the armature
winding to that which is connected with the frame. The circuit through the armature
may be traced from the terminal wire 2 through the frame; thence through the bearings
to the armature 1 and through the pin to the right-hand side of the armature winding.
Continuing the circuit through the winding itself, it passes to the center pin projecting
from the left-hand end of the armature shaft; thence to the spring 4 which rests against
this pin; and thence to the terminal wire 3.

Normally, this path is shunted by what is practically a short circuit, which may be
traced from the terminal 2 through the frame of the generator to the crank shaft 5;
thence to the upper end of the spring 4 and out by the terminal wire 3. This is the
condition which ordinarily exists and which results in the removal of the resistance and
the impedance on the armature winding from any circuit in which the generator is
placed, as long as the generator is not operated.

An arrangement is provided, however, whereby the crank shaft 5 will be withdrawn
automatically from engaging with the upper end of the spring 4, thus breaking the shunt
around the armature circuit, whenever the generator crank is turned. In order to

accomplish this the crank shaft 5 is capable of partial rotation and of slight longitudinal
movement within the hub of the large gear wheel. A spring 7 usually presses the crank
shaft toward the left and into engagement with the spring 4. A pin 8 carried by the
crank shaft, rests in a V-shaped notch in the end of the hub 6 and as a result, when the
crank is turned the pin rides on the surface of this notch before the large gear wheel
starts to turn, and thus moves the crank shaft 5 to the right and breaks the contact
between it and the spring 4. Thus, as long as the generator is being operated, its
armature is connected in the circuit of the line, but as soon as it becomes idle the
armature is automatically short-circuited. Such devices as this are termed automatic
shunts.

In still other cases it is desirable to have the generator circuit normally open so that
it will not affect in any way the electrical characteristics of the line while the line is
being used for talking. In this case the arrangement is made so that the generator will
automatically be placed in proper circuit relation with the line when it is operated.

Fig. 75. Generator Cut-in Switch
View full size illustration.

A common arrangement for doing this is shown in Fig. 75, wherein the spring 1
normally rests against the contact pin of the armature and forms one terminal of the
armature circuit. The spring 2 is adapted to form the other terminal of the armature
circuit but it is normally insulated from everything. The circuit of the generator is,
therefore, open between the spring 2 and the shaft 3, but as soon as the generator is
operated the crank shaft is bodily moved to the left by means of the V-shaped notch in
the driving collar 4 and is thus made to engage the spring 2. The circuit of the generator
is then completed from the spring 1 through the armature pin to the armature winding;
thence to the frame of the machine and through shaft 3 to the spring 2. Such devices as
this are largely used in connection with so-called "bridging" telephones in which the
generators and bells are adapted to be connected in multiple across the line.

A better arrangement for accomplishing the automatic switching on the part of the
generator is to make no use of the crank shaft as a part of the conducting path as is the
case in both Figs. 74 and 75, but to make the crank shaft, by its longitudinal movement,

impart the necessary motion to a switch spring which, in turn, is made to engage or
disengage a corresponding contact spring. An arrangement of this kind that is in
common use is shown in Fig. 76. This needs no further explanation than to say that the
crank shaft is provided on its end with an insulating stud 1, against which a switching
spring 2 bears. This spring normally rests against another switch spring 3, but when the
generator crank shaft moves to the right upon the turning of the crank, the spring 2
disengages spring 3 and engages spring 4, thus completing the circuit of the generator
armature. It is seen that this operation accomplishes the breaking of one circuit and the
making of another, a function that will be referred to later on in this work.

Fig. 76. Generator Cut-in Switch
View full size illustration.

Pulsating Current. Sometimes it is desirable to have a generator capable of
developing a pulsating current instead of an alternating current; that is, a current which
will consist of impulses all in one direction rather than of impulses alternating in
direction. It is obvious that this may be accomplished if the circuit of the generator be
broken during each half revolution so that its circuit is completed only when current is
being generated in one direction.

Such an arrangement is indicated diagrammatically in Fig. 77. Instead of having one
terminal of the armature winding brought out through the frame of the generator as is
ordinarily done, both terminals are brought out to a commuting device carried on the
end of the armature shaft. Thus, one end of the loop representing the armature winding
is shown connected directly to the armature pin 1, against which bears a spring 2, in the
usual manner. The other end of the armature winding is carried directly to a disk 3,
mounted on but insulated from the shaft and revolving therewith. One-half of the
circumferential surface of this disk is of insulating material 4 and a spring 5 rests
against this disk and bears alternately upon the conducting portion 3 or the insulating
portion 4, according to the position of the armature in its revolution. It is obvious that
when the generator armature is in the position shown the circuit through it is from the
spring 2 to the pin 1; thence to one terminal of the armature loop; thence through the
loop and back to the disk 3 and out by the spring 5. If, however, the armature were

turned slightly, the spring 5 would rest on the insulating portion 4 and the circuit would
be broken.

Fig. 77. Pulsating-Current Commutator
View full size illustration.

Fig. 78. Generator Symbols
View full size illustration.

It is obvious that if the brush 5 is so disposed as to make contact with the disk 3
only during that portion of the revolution while positive current is being generated, the
generator will produce positive pulsations of current, all the negative ones being cut
out. If, on the other hand, the spring 5 may be made to bear on the opposite side of the
disk, then it is evident that the positive impulses would all be cut out and the generator
would develop only negative impulses. Such a generator is termed a "direct-current"
generator or a "pulsating-current" generator.

The symbols for magneto or hand generators usually embody a simplified side view,
showing the crank and the gears on one side and the shunting or other switching device
on the other. Thus in Fig. 78 are shown three such symbols, differing from each other
only in the details of the switching device. The one at the left shows the simple shunt,
adapted to short-circuit the generator at all times save when it is in operation. The one
in the center shows the cut-in, of which another form is described in connection with
Fig. 75; while the symbol at the right of Fig. 78 is of the make-and-break device,
discussed in connection with Fig. 76. In such diagrammatic representations of
generators it is usual to somewhat exaggerate the size of the switching springs, in order
to make clear their action in respect to the circuit connections in which the generator is
used.

Polarized Ringer. The polarized bell or ringer is, as has been stated, the device
which is adapted to respond to the currents sent out by the magneto generator. In order
that the alternately opposite currents may cause the armature to move alternately in
opposite directions, these bells are polarized, i.e., given a definite magnetic set, so to

speak; so the effect of the currents in the coils is not to create magnetism in normally
neutral iron, but rather to alter the magnetism in iron already magnetized.

Western Electric Ringer. A typical form of polarized bell is shown in Fig. 79, this
being the standard bell or ringer of the Western Electric Company. The two
electromagnets are mounted side by side, as shown, by attaching their cores to a yoke
piece 1 of soft iron. This yoke piece also carries the standards 2 upon which the gongs
are mounted. The method of mounting is such that the standards may be adjusted
slightly so as to bring the gongs closer to or farther from, the tapper.

The soft iron yoke piece 1 also carries two brass posts 3 which, in turn, carry
another yoke 4 of brass. In this yoke 4 is pivoted, by means of trunnion screws, the
armature 5, this extending on each side of the pivot so that its ends lie opposite the free
poles of the electromagnets. From the center of the armature projects the tapper rod
carrying the ball or striker which plays between the two gongs.

In order that the armature and cores may be normally polarized, a permanent magnet
6 is secured to the center of the yoke piece 1. This bends around back of the
electromagnets and comes into close proximity to the armature 5. By this means one
end of each of the electromagnet cores is given one polarity—say north—while the
armature is given the other polarity—say south. The two coils of the electromagnet are
connected together in series in such a way that current in a given direction will act to
produce a north pole in one of the free poles and a south pole in the other. If it be
assumed that the permanent magnet maintains the armature normally of south polarity
and that the current through the coils is of such direction as to make the left-hand core
north and the right-hand core south, then it is evident that the left-hand end of the
armature will be attracted and the right-hand end repelled. This will throw the tapper
rod to the right and sound the right-hand bell. A reversal in current will obviously
produce the opposite effect and cause the tapper to strike the left-hand bell.

An important feature in polarized bells is the adjustment between the armature and
the pole pieces. This is secured in the Western Electric bell by means of the nuts 7, by
which the yoke 4 is secured to the standards 3. By moving these nuts up or down on the
standards the armature may be brought closer to or farther from the poles, and the
device affords ready means for clamping the parts into any position to which they may
have been adjusted.

Fig. 79. Polarized Bell
View full size illustration.

Kellogg Ringer. Another typical ringer is that of the Kellogg Switchboard and
Supply Company, shown in Fig. 80. This differs from that of the Western Electric
Company mainly in the details by which the armature adjustment is obtained. The
armature supporting yoke 1 is attached directly to the cores of the magnets, no
supporting side rods being employed. Instead of providing means whereby the
armature may be adjusted toward or from the poles, the reverse practice is employed,
that is, of making the poles themselves extensible. This is done by means of the iron
screws 2 which form extensions of the cores and which may be made to approach or
recede from the armature by turning them in such direction as to screw them in or out
of the core ends.

Fig. 80. Polarized Bell
View full size illustration.

Fig. 81. Biased Bell
View full size illustration.

Biased Bell. The pulsating-current generator has already been discussed and its
principle of operation pointed out in connection with Fig. 77. The companion piece to
this generator is the so-called biased ringer. This is really nothing but a common
alternating-current polarized ringer with a light spring so arranged as to hold the
armature normally in one of its extreme positions so that the tapper will rest against one
of the gongs. Such a ringer is shown in Fig. 81 and needs no further explanation. It is
obvious that if a current flows in the coils of such a ringer in a direction tending to
move the tapper toward the left, then no sound will result because the tapper is already
moved as far as it can be in that direction. If, however, currents in the opposite
direction are caused to flow through the windings, then the electromagnetic attraction
on the armature will overcome the pull of the spring and the tapper will move over and
strike the right-hand gong. A cessation of the current will allow the spring to exert itself
and throw the tapper back into engagement with the left-hand gong. A series of such
pulsations in the proper direction will, therefore, cause the tapper to play between the
two gongs and ring the bell as usual. A series of currents in a wrong direction will,
however, produce no effect.

Conventional Symbols. In Fig. 82 are shown six conventional symbols of polarized
bells. The three at the top, consisting merely of two circles representing the magnets in
plan view, are perhaps to be preferred as they are well standardized, easy to draw, and
rather suggestive. The three at the bottom, showing the ringer as a whole in side
elevation, are somewhat more specific, but are objectionable in that they take more
space and are not so easily drawn.

Fig. 82. Ringer Symbols
View full size illustration.

Symbols A or B may be used for designating any ordinary polarized ringer. Symbols
C and D are interchangeably used to indicate a biased ringer. If the bell is designed to
operate only on positive impulses, then the plus sign is placed opposite the symbol,
while a minus sign so placed indicates that the bell is to be operated only by negative
impulses.

Some specific types of ringers are designed to operate only on a given frequency of
current. That is, they are so designed as to be responsive to currents having a frequency
of sixty cycles per second, for instance, and to be unresponsive to currents of any other
frequency. Either symbols E or F may be used to designate such ringers, and if it is
desired to indicate the particular frequency of the ringer this is done by adding the
proper numeral followed by a short reversed curve sign indicating frequency. Thus 50~
would indicate a frequency of fifty cycles per second.

ToC

CHAPTER IX
THE HOOK SWITCH

Purpose. In complete telephone instruments, comprising both talking and signaling
apparatus, it is obviously desirable that the two sets of apparatus, for talking and
signaling respectively, shall not be connected with the line at the same time. A certain
switching device is, therefore, necessary in order that the signaling apparatus alone
may be left operatively connected with the line while the instrument is not being used
in the transmission of speech, and in order that the signaling apparatus may be cut out
when the talking apparatus is brought into play.

In instruments employing batteries for the supply of transmitter current, another
switching function is the closing of the battery circuit through the transmitter and the
induction coil when the instrument is in use for talking, since to leave the battery
circuit closed all the time would be an obvious waste of battery energy.

In the early forms of telephones these switching operations were performed by a
manually operated switch, the position of which the user was obliged to change before
and after each use of the telephone. The objection to this was not so much in the
manual labor imposed on the user as in the tax on his memory. It was found to be
practically a necessity to make this switching function automatic, principally because
of the liability of the user to forget to move the switch to the proper position after using
the telephone, resulting not only in the rapid waste of the battery elements but also in
the inoperative condition of the signal-receiving bell. The solution of this problem, a
vexing one at first, was found in the so-called automatic hook switch or switch hook,
by which the circuits of the instrument were made automatically to assume their proper
conditions by the mere act, on the part of the user, of removing the receiver from, or
placing it upon, a conveniently arranged hook or fork projecting from the side of the
telephone casing.

Automatic Operation. It may be taken as a fundamental principle in the design of
any piece of telephone apparatus that is to be generally used by the public, that the
necessary acts which a person must perform in order to use the device must, as far as
possible, follow as a natural result from some other act which it is perfectly obvious to
the user that he must perform. So in the case of the switch hook, the user of a telephone
knows that he must take the receiver from its normal support and hold it to his ear; and
likewise, when he is through with it, that he must dispose of it by hanging it upon a
support obviously provided for that purpose.

In its usual form a forked hook is provided for supporting the receiver in a
convenient place. This hook is at the free end of a pivoted lever, which is normally
pressed upward by a spring when the receiver is not supported on it. When, however,
the receiver is supported on it, the lever is depressed by its weight. The motion of the
lever is mechanically imparted to the members of the switch proper, the contacts of
which are usually enclosed so as to be out of reach of the user. This switch is so
arranged that when the hook is depressed the circuits are held in such condition that the
talking apparatus will be cut out, the battery circuit opened, and the signaling apparatus
connected with the line. On the other hand, when the hook is in its raised position, the
signaling apparatus is cut out, the talking apparatus switched into proper working
relation with the line, and the battery circuit closed through the transmitter.

In the so-called common-battery telephones, where no magneto generator or local
battery is included in the equipment at the subscriber's station, the mere raising of the
hook serves another important function. It acts, not only to complete the circuit through
the substation talking apparatus, but, by virtue of the closure of the line circuit, permits
a current to flow over the line from the central-office battery which energizes a signal
associated with the line at the central office. This use of the hook switch in the case of
the common-battery telephone is a good illustration of the principle just laid down as to
making all the functions which the subscriber has to perform depend, as far as possible,
on acts which his common sense alone tells him he must do. Thus, in the common-
battery telephone the subscriber has only to place the receiver at his ear and ask for
what he wants. This operation automatically displays a signal at the central office and
he does nothing further until the operator inquires for the number that he wants. He has
then nothing to do but wait until the called-for party responds, and after the
conversation his own personal convenience demands that he shall dispose of the
receiver in some way, so he hangs it up on the most convenient object, the hook switch,
and thereby not only places the apparatus at his telephone in proper condition to
receive another call, but also conveys to the central office the signal for disconnection.

Likewise in the case of telephones operating in connection with automatic
exchanges, the hook switch performs a number of functions automatically, of which the
subscriber has no conception; and while, in automatic telephones, there are more acts
required of the user than in the manual, yet a study of these acts will show that they all
follow in a way naturally suggested to the user, so that he need have but the barest
fundamental knowledge in order to properly make use of the instrument. In all cases, in
properly designed apparatus, the arrangement is such that the failure of the subscriber
to do a certain required act will do no damage to the apparatus or to the system, and,
therefore, will inconvenience only himself.

Design. The hook switch is in reality a two-position switch, and while at present it
is a simple affair, yet its development to its high state of perfection has been slow, and
its imperfections in the past have been the cause of much annoyance.

Several important points must be borne in mind in the design of the hook switch.
The spring provided to lift the hook must be sufficiently strong to accomplish this
purpose and yet must not be strong enough to prevent the weight of the receiver from
moving the switch to its other position. The movement of this spring must be somewhat
limited in order that it will not break when used a great many times, and also it must be
of such material and shape that it will not lose its elasticity with use. The shape and
material of the restoring spring are, of course, determined to a considerable extent by
the length of the lever arm which acts on the spring, and on the space which is
available for the spring.

The various contacts by which the circuit changes are brought about upon the
movement of the hook-switch lever usually take the form of springs of German silver
or phosphor-bronze, hard rolled so as to have the necessary resiliency, and these are
usually tipped with platinum at the points of contact so as to assure the necessary
character of surface at the points where the electric circuits are made or broken. A
slight sliding movement between each pair of contacts as they are brought together is
considered desirable, in that it tends to rub off any dirt that may have accumulated, yet
this sliding movement should not be great, as the surfaces will then cut each other and,
therefore, reduce the life of the switch.

Contact Material. On account of the high cost of platinum, much experimental
work has been done to find a substitute metal suitable for the contact points in hook
switches and similar uses in the manufacture of telephone apparatus. Platinum is
unquestionably the best known material, on account of its non-corrosive and heat-
resisting qualities. Hard silver is the next best and is found in some first-class
apparatus. The various cheap alloys intended as substitutes for platinum or silver in
contact points may be dismissed as worthless, so far as the writers' somewhat extensive
investigations have shown.

In the more recent forms of hook switches, the switch lever itself does not form a
part of the electrical circuit, but serves merely as the means by which the springs that
are concerned in the switching functions are moved into their alternate cooperative
relations. One advantage in thus insulating the switch lever from the current-carrying
portions of the apparatus and circuits is that, since it necessarily projects from the box
or cabinet, it is thus liable to come in contact with the person of the user. By insulating
it, all liability of the user receiving shocks by contact with it is eliminated.

Wall Telephone Hooks. Kellogg. A typical form of hook switch, as employed in the
ordinary wall telephone sets, is shown in Fig. 83, this being the standard hook of the
Kellogg Switchboard and Supply Company. In this the lever 1 is pivoted at the point 3
in a bracket 5 that forms the base of all the working parts and the means of securing the
entire hook switch to the box or framework of the telephone. This switch lever is
normally pressed upward by a spring 2, mounted on the bracket 5, and engaging the

under side of the hook lever at the point 4. Attached to the lever arm 1 is an insulated
pin 6. The contact springs by which the various electrical circuits are made and broken
are shown at 7, 8, 9, 10, and 11, these being mounted in one group with insulated
bushings between them; the entire group is secured by machine screws to a lug
projecting horizontally from the bracket 5. The center spring 9 is provided with a
forked extension which embraces the pin 6 on the hook lever. It is obvious that an up-
and-down motion of the hook lever will move the long spring 9 in such manner as to
cause electrical contact either between it and the two upper springs 7 and 8, or between
it and the two lower springs 10 and 11. The hook is shown in its raised position, which
is the position required for talking. When lowered the two springs 7 and 8 are
disengaged from the long spring 9 and from each other, and the three springs 9, 10, and
11 are brought into electrical engagement, thus establishing the necessary signaling
conditions.

Fig. 83. Long Lever Hook Switch
View full size illustration.

The right-hand ends of the contact springs are shown projecting beyond the
insulating supports. This is for the purpose of facilitating making electrical joints
between these springs and the various wires which lead from them. These projecting
ends are commonly referred to as ears, and are usually provided with holes or notches
into which the connecting wire is fastened by soldering.

Western Electric. Fig. 84 shows the type of hook switch quite extensively employed
by the Western Electric Company in wall telephone sets where the space is somewhat
limited and a compact arrangement is desired. It will readily be seen that the principle
on which this hook switch operates is similar to that employed in Fig. 83, although the
mechanical arrangement of the parts differs radically. The hook lever 1 is pivoted at 3
on a bracket 2, which serves to support all the other parts of the switch. The contact
springs are shown at 4, 5, and 6, and this latter spring 6 is so designed as to make it
serve as an actuating spring for the hook. This is accomplished by having the curved
end of this spring press against the lug 7 of the hook and thus tend to raise the hook
when it is relieved of the weight of the receiver. The two shorter springs 8 and 9 have
no electrical function but merely serve as supports against which the springs 4 and 5
may rest, when the receiver is on the hook, these springs 4 and 5 being given a light
normal tension toward the stop springs 8 and 9. It is obvious that in the particular
arrangement of the springs in this switch no contacts are closed when the receiver is on
the hook.

Fig. 84. Short Lever Hook Switch
View full size illustration.

Concerning this latter feature, it will be noted that the particular form of Kellogg
hook switch, shown in Fig. 83, makes two contacts and breaks two when it is raised.
Similarly the Western Electric Company's makes two contacts but does not break any
when raised. From such considerations it is customary to speak of a hook such as that
shown in Fig. 83 as having two make and two break contacts, and such a hook as that
shown in Fig. 84 as having two make contacts.

It will be seen from either of these switches that the modification of the spring
arrangement, so as to make them include a varying number of make-and-break
contacts, is a simple matter, and switches of almost any type are readily modified in
this respect.

Fig. 85. Removable Lever Hook Switch
View full size illustration.

Dean. In Fig. 85 is shown a decidedly unique hook switch for wall telephone sets
which forms the standard equipment of the Dean Electric Company. The hook lever 1

is pivoted at 2, an auxiliary lever 3 also being pivoted at the same point. The auxiliary
lever 3 carries at its rear end a slotted lug 4, which engages the long contact spring 5,
and serves to move it up and down so as to engage and disengage the spring 6, these
two springs being mounted on a base lug extending from the base plate 7, upon which
the entire hook-switch mechanism is mounted. The curved spring 8, also mounted on
this same base, engages the auxiliary lever 3 at the point 9 and normally serves to press
this up so as to maintain the contact springs 5 in engagement with contact spring 6. The
switch springs are moved entirely by the auxiliary lever 3, but in order that this lever 3
may be moved as required by the hook lever 1, this lever is provided with a notched lug
10 on its lower side, which notch is engaged by a forwardly projecting lug 11 that is
integral with the auxiliary lever 3. The switch lever may be bodily removed from the
remaining parts of the hook switch by depressing the lug 11 with the finger, so that it
disengages the notch in lug 10, and then drawing the hook lever out of engagement
with the pivot stud 2, as shown in the lower portion of the figure. It will be noted that
the pivotal end of the hook lever is made with a slot instead of a hole as is the
customary practice.

The advantage of being able to remove the hook switch bodily from the other
portions arises mainly in connection with the shipment or transportation of instruments.
The projecting hooks cause the instruments to take up more room and thus make larger
packing boxes necessary than would otherwise be used. Moreover, in handling the
telephones in store houses or transporting them to the places where they are to be used,
the projecting hook switch is particularly liable to become damaged. It is for
convenience under such conditions that the Dean hook switch is made so that the
switch lever may be removed bodily and placed, for instance, inside the telephone box
for transportation.

Desk-Stand Hooks. The problem of hook-switch design for portable desk
telephones, while presenting the same general characteristics, differs in the details of
construction on account of the necessarily restricted space available for the switch
contacts in the desk telephone.

Western Electric. In Fig. 86 is shown an excellent example of hook-switch design as
applied to the requirements of the ordinary portable desk set. This figure is a cross-
sectional view of the base and standard of a familiar type of desk telephone. The base
itself is of stamped metal construction, as indicated, and the standard which supports
the transmitter and the switch hook for the receiver is composed of a black enameled or
nickel-plated brass tube 1, attached to the base by a screw-threaded joint, as shown.
The switch lever 2 is pivoted at 3 in a brass plug 4, closing the upper end of the tube
forming the standard. This brass plug supports also the transmitter, which is not shown
in this figure. Attached to the plug 4 by the screw 5 is a heavy strip 6, which reaches
down through the tube to the base plate of the standard and is held therein by a screw 7.
The plug 4, carrying with it the switch-hook lever 2 and the brass strip 6, may be lifted

bodily out of the standard 1 by taking out the screw 7 which holds the strip 6 in place,
as is clearly indicated. On the strip 6 there is mounted the group of switch springs by
which the circuit changes of the instrument are brought about when the hook is raised
or lowered. The spring 8 is longer than the others, and projects upwardly far enough to
engage the lug on the switch-hook lever 2. This spring, which is so bent as to close the
contacts at the right when not prevented by the switch lever, also serves as an actuating
spring to raise the lever 2 when the receiver is removed from it. This spring, when the
receiver is removed from the hook, engages the two springs at the right, as shown, or
when the receiver is placed on the hook, breaks contact with the two right-hand springs
and makes contact respectively with the left-hand spring and also with the contact 9
which forms the transmitter terminal.

Fig. 86. Desk-Stand Hook Switch
View full size illustration.

It is seen from an inspection of this switch hook that it has two make and two break
contacts. The various contact springs are connected with the several binding posts
shown, these forming the connectors for the flexible cord conductors leading into the
base and up through the standard of the desk stand. By means of the conductors in this
cord the circuits are led to the other parts of the instrument, such as the induction coil,
call bell, and generator, if there is one, which, in the case of the Western Electric
Company's desk set, are all mounted separately from the portable desk stand proper.

This hook switch is accessible in an easy manner and yet not subject to the
tampering of idle or mischievous persons. By taking out the screw 7 the entire hook
switch may be lifted out of the tube forming the standard, the cords leading to the
various binding posts being slid along through the tube. By this means the connections

to the hook switch, as well as the contact of the switch itself, are readily inspected or
repaired by those whose duty it is to perform such operations.

Kellogg. In Fig. 87 is shown a sectional view of the desk-stand hook switch of the
Kellogg Switchboard and Supply Company. In this it will be seen that instead of
placing the switch-hook springs within the standard or tube, as in the case of the
Western Electric Company, they are mounted in the base where they are readily
accessible by merely taking off the base plate from the bottom of the stand. The hook
lever operates on the long spring of the group of switch springs by means of a toggle
joint in an obvious manner. This switch spring itself serves by its own strength to raise
the hook lever when released from the weight of the receiver.

Fig. 87. Desk-Stand Hook Switch
View full size illustration.

In this switch, the hook lever, and in fact the entire exposed metal portions of the
instrument, are insulated from all of the contact springs and, therefore, there is little
liability of shocks on the part of the person using the instrument.

Conventional Symbols. The hook switch plays a very important part in the
operation of telephone circuits; for this reason readily understood conventional
symbols, by which they may be conveniently represented in drawings of circuits, are
desirable. In Fig. 88 are shown several symbols such as would apply to almost any
circuit, regardless of the actual mechanical details of the particular hook switch which
happened to be employed. Thus diagram A in Fig. 88 shows a hook switch having a
single make contact and this diagram might be used to refer to the hook switch of the

Dean Electric Company shown in Fig. 85, in which only a single contact is made when
the receiver is removed, and none is made when it is on the hook. Similarly, diagram B
might be used to represent the hook switch of the Kellogg Company, shown in Fig. 83,
the arrangement being for two make and two break contacts. Likewise diagram C
might be used to represent the hook switch of the Western Electric Company, shown in
Fig. 84, which, as before stated, has two make contacts only. Diagram D shows another
modification in which contacts made by the hook switch, when the receiver is removed,
control two separate circuits. Assuming that the solid black portion represents
insulation, it is obvious that the contacts are divided into two groups, one insulated
from the other.

Fig. 88. Hook Switch Symbols
View full size illustration.

ToC

CHAPTER X
ELECTROMAGNETS AND INDUCTIVE COILS

Electromagnet. The physical thing which we call an electromagnet, consisting of a
coil or helix of wire, the turns of which are insulated from each other, and within which
is usually included an iron core, is by far the most useful of all the so-called translating
devices employed in telephony. In performing the ordinary functions of an
electromagnet it translates the energy of an electrical current into the energy of
mechanical motion. An almost equally important function is the converse of this, that
is, the translation of the energy of mechanical motion into that of an electrical current.
In addition to these primary functions which underlie the art of telephony, the
electromagnetic coil or helix serves a wide field of usefulness in cases where no
mechanical motion is involved. As impedance coils, they serve to exert important
influences on the flow of currents in circuits, and as induction coils, they serve to
translate the energy of a current flowing in one circuit into the energy of a current
flowing in another circuit, the translation usually, but not always, being accompanied
by a change in voltage.

When a current flows through the convolutions of an ordinary helix, the helix will
exhibit the properties of a magnet even though the substance forming the core of the
helix is of non-magnetic material, such as air, or wood, or brass. If, however, a mass of
iron, such as a rod or a bundle of soft iron wires, for instance, is substituted as a core,
the magnetic properties will be enormously increased. The reason for this is, that a
given magnetizing force will set up in iron a vastly greater number of lines of magnetic
force than in air or in any other non-magnetic material.

Magnetizing Force. The magnetizing force of a given helix is that force which
tends to drive magnetic lines of force through the magnetic circuit interlinked with the
helix. It is called magnetomotive force and is analogous to electromotive force, that is,
the force which tends to drive an electric current through a circuit.

The magnetizing force of a given helix depends on the product of the current
strength and the number of turns of wire in the helix. Thus, when the current strength is
measured in amperes, this magnetizing force is expressed as ampere-turns, being the
product of the number of amperes flowing by the number of turns. The magnetizing
force exerted by a given current, therefore, is independent of anything except the
number of turns, and the material within the core or the shape of the core has no effect
upon it.

Magnetic Flux. The total magnetization resulting from a magnetizing force is called
the magnetic flux, and is analogous to current. The intensity of a magnetic flux is
expressed by the number of magnetic lines of force in a square centimeter or square
inch.

While the magnetomotive force or magnetizing force of a given helix is independent
of the material of the core, the flux which it sets up is largely dependent on the material
and shape of the core—not only upon this but on the material that lies in the return path
for the flux outside of the core. We may say, therefore, that the amount of flux set up by
a given current in a given coil or helix is dependent on the material in the magnetic
path or magnetic circuit, and on the shape and length of that circuit. If the magnetic
circuit be of air or brass or wood or any other non-magnetic material, the amount of
flux set up by a given magnetizing force will be relatively small, while it will be very
much greater if the magnetic circuit be composed in part or wholly of iron or steel,
which are highly magnetic substances.

Permeability. The quality of material, which permits of a given magnetizing force
setting up a greater or less number of lines of force within it, is called its permeability.
More accurately, the permeability is the ratio existing between the amount of
magnetization and the magnetizing force which produces such magnetization.

The permeability of a substance is usually represented by the Greek letter µ
(pronounced mu). The intensity of the magnetizing force is commonly symbolized by
H, and since the permeability of air is always taken as unity, we may express the
intensity of magnetizing force by the number of lines of force per square centimeter
which it sets up in air.

Now, if the space on which the given magnetizing force H were acting were filled
with iron instead of air, then, owing to the greater permeability of iron, there would be
set up a very much greater number of lines of force per square centimeter, and this
number of lines of force per square centimeter in the iron is the measure of the
magnetization produced and is commonly expressed by the letter B.

From this we have

µ=B÷H

Thus, when we say that the permeability of a given specimen of wrought iron under
given conditions is 2,000, we mean that 2,000 times as many lines of force would be
induced in a unit cross-section of this sample as would be induced by the same
magnetizing force in a corresponding unit cross-section of air. Evidently for air B = H,
hence µ becomes unity.

The permeability of air is always a constant. This means that whether the magnetic
density of the lines of force through the air be great or small the number of lines will
always be proportional to the magnetizing force. Unfortunately for easy calculations in
electromagnetic work, however, this is not true of the permeability of iron. For small
magnetic densities the permeability is very great, but for large densities, that is, under
conditions where the number of lines of force existing in the iron is great, the
permeability becomes smaller, and an increase in the magnetizing force does not
produce a corresponding increase in the total flux through the iron.

Magnetization Curves. This quality of iron is best shown by the curves of Fig. 89,
which illustrate the degree of magnetization set up in various kinds of iron by different
magnetizing forces. In these curves the ordinates represent the total magnetization B,
while the abscissas represent the magnetizing force H. It is seen from an inspection of
these curves that as the magnetizing force H increases, the intensity of flux also
increases, but at a gradually lessening rate, indicating a reduction in permeability at the
higher densities. These curves are also instructive as showing the great differences that
exist between the permeability of the different kinds of iron; and also as showing how,
when the magnetizing force becomes very great, the iron approaches what is called
saturation, that is, a point at which the further increase in magnetizing force will result
in no further magnetization of the core.

From the data of the curves of Fig. 89, which are commonly called magnetization
curves, it is easy to determine other data from which so-called permeability curves may
be plotted. In permeability curves the total magnetization of the given pieces of iron are
plotted as abscissas, while the corresponding permeabilities are plotted as ordinates.

Fig. 89. Magnetization Curve
View full size illustration.

Direction of Lines of Force. The lines of force set up within the core of a helix
always have a certain direction. This direction always depends upon the direction of the
flow of current around the core. An easy way to remember the direction is to consider
the helix as grasped in the right hand with the fingers partially encircling it and the
thumb pointing along its axis. Then, if the current through the convolutions of the helix
be in the direction in which the fingers of the hand are pointed around the helix, the
magnetic lines of force will proceed through the core of the helix along the direction in
which the thumb is pointed.

In the case of a simple bar electromagnet, such as is shown in Fig. 90, the lines of
force emerging from one end of the bar must pass back through the air to the other end
of the bar, as indicated by dotted lines and arrows. The path followed by the magnetic
lines of force is called the magnetic circuit, and, therefore, the magnetic circuit of the
magnet shown in Fig. 90 is composed partly of iron and partly of air. From what has
been said concerning the relative permeability of air and of iron, it will be obvious that
the presence of such a long air path in the magnetic circuit will greatly reduce the
number of lines of force that a given magnetizing force can set up. The presence of an
air gap in a magnetic circuit has much the same effect on the total flow of lines of force
as the presence of a piece of bad conductor in a circuit composed otherwise of good
conductor, in the case of the flow of electric current.

Reluctance. As the property which opposes the flow of electric current in an
electrical circuit is called resistance, so the property which opposes the flow of
magnetic lines of force in a magnetic circuit is called reluctance. In the case of the
electric circuit, the resistance is the reciprocal of the conductivity; in the case of the
magnetic circuit, the reluctance is the reciprocal of the permeability. As in the case of
an electrical circuit, the amount of flow of current is equal to the electromotive force
divided by the resistance; so in a magnetic circuit, the magnetic flux is equal to the
magnetizing force or magnetomotive force divided by the reluctance.

Fig. 90. Bar Electromagnet
View full size illustration.

Types of Low-Reluctance Circuits. As the pull of an electromagnet upon its
armature depends on the total number of lines of force passing from the core to the
armature—that is, on the total flux—and as the total flux depends for a given
magnetizing force on the reluctance of the magnetic circuit, it is obvious that the design

of the electromagnetic circuit is of great importance in influencing the action of the
magnet. Obviously, anything that will reduce the amount of air or other non-magnetic
material that is in the magnetic circuit will tend to reduce the reluctance, and, therefore,
to increase the total magnetization resulting from a given magnetizing force.

Horseshoe Form. One of the easiest and most common ways of reducing reluctance
in a circuit is to bend the ordinary bar electromagnet into horseshoe form. In order to
make clear the direction of current flow, attention is called to Fig. 91. This is intended
to represent a simple bar of iron with a winding of one direction throughout its length.
The gap in the middle of the bar, which divides the winding into two parts, is intended
merely to mark the fact that the winding need not cover the whole length of the bar and
still will be able to magnetize the bar when the current passes through it. In Fig. 92 a
similar bar is shown with similar winding upon it, but bent into U-form, exactly as if it
had been grasped in the hand and bent without further change. The magnetic polarity of
the two ends of the bar remain the same as before for the same direction of current, and
it is obvious that the portion of the magnetic circuit which extends through air has been
very greatly shortened by the bending. As a result, the magnetic reluctance of the
circuit has been greatly decreased and the strength of the magnet correspondingly
increased.

Fig. 91. Bar Electromagnet
View full size illustration.

Fig. 92. Horseshoe Electromagnet
View full size illustration.

Fig. 93. Horseshoe Electromagnet
View full size illustration.

If the armature of the electromagnet shown in Fig. 92 is long enough to extend
entirely across the air gap from the south to the north pole, then the air gap in the
magnetic circuit is still further shortened, and is now represented only by the small gap
between the ends of the armature and the ends of the core. Such a magnet, with an
armature closely approaching the poles, is called a closed-circuit magnet, since the only
gap in the iron of the magnetic circuit is that across which the magnet pulls in attracting
its armature.

In Fig. 93 is shown the electrical and magnetic counterpart of Fig. 92. The fact that
the magnetic circuit is not a single iron bar but is made up of two cores and one
backpiece rigidly secured together, has no bearing upon the principle, but only shows
that a modification of construction is possible. In the construction of Fig. 93 the
armature 1 is shown as being pulled directly against the two cores 2 and 3, these two
cores being joined by a yoke 4, which, like the armature and the core, is of magnetic
material. The path of the lines of force is indicated by dotted lines. This is a very
important form of electromagnet and is largely used in telephony.

Iron-Clad Form. Another way of forming a closed-circuit magnet that is widely
used in telephony is to enclose the helix or winding in a shell of magnetic material
which joins the core at one end. This construction results in what is known as the
tubular or iron-clad electromagnet, which is shown in section and in end view in Fig.
94. In this the core 1 is a straight bar of iron and it lies centrally within a cylindrical
shell 2, also of iron. The bar is usually held in place within the shell by a screw, as
shown. The lines of force set up in the core by the current flowing through the coil,
pass to the center of the bottom of the iron shell and thence return through the metal of
the shell, through the air gap between the edges of the shell and the armature, and then
concentrate at the center of the armature and pass back to the end of the core. This is a
highly efficient form of closed-circuit magnet, since the magnetic circuit is of low
reluctance.

Fig. 94. Iron-Clad Electromagnet
View full size illustration.

Such forms of magnets are frequently used where it is necessary to mount a large
number of them closely together and where it is desired that the current flowing in one
magnet shall produce no inductive effect in the coils of the adjacent magnets. The
reason why mutual induction between adjacent magnets is obviated in the case of the
iron-clad or tubular magnet is that practically all stray field is eliminated, since the
return path for the magnetic lines is so completely provided for by the presence of the
iron shell.

Special Horseshoe Form. In Fig. 95 is shown a type of relay commonly employed in
telephone circuits. The purpose of illustrating it in this chapter is not to discuss relays,
but rather to show an adaptation of an electromagnet wherein low reluctance of the
magnetic circuit is secured by providing a return leg for the magnetic lines developed
in the core, thus forming in effect a horseshoe magnet with a winding on one of its
limbs only. To the end of the core 1 there is secured an L-shaped piece of soft iron 2.
This extends upwardly and then forwardly throughout the entire length of the magnet
core. An L-shaped armature 3 rests on the front edge of the piece 2 so that a slight
rocking motion will be permitted on the "knife-edge" bearing thus afforded. It is seen
from the dotted lines that the magnetic circuit is almost a closed one. The only gap is
that between the lower end of the armature 3 and the front end of the core. When the
coil is energized, this gap is closed by the attraction of the armature. As a result, the
rearwardly projecting end of the armature 3 is raised and this raises the spring 4 and
causes it to break the normally existing contact with the spring 5 and to establish
another contact with the spring 6. Thus the energy developed within the coil of the
magnet is made to move certain parts which in turn operate the switching devices to

produce changes in electrical circuits. These relays and other adaptations of the
electromagnet will be discussed more fully later on.

Fig. 95. Electromagnet of Relay
View full size illustration.

There are almost numberless forms of electromagnets, but we have illustrated here
examples of the principal types employed in telephony, and the modifications of these
types will be readily understood in view of the general principles laid down.

Direction of Armature Motion. It may be said in general that the armature of an
electromagnet always moves or tends to move, when the coil is energized, in such a
way as to reduce the reluctance of the magnetic circuit through the coil. Thus, in all of
the forms of electromagnets discussed, the armature, when attracted, moves in such a
direction as to shorten the air gap and to introduce the iron of the armature as much as
possible into the path of the magnetic lines, thus reducing the reluctance. In the case of
a solenoid type of electromagnet, or the coil and plunger type, which is a better name
than solenoid, the coil, when energized, acts in effect to suck the iron core or plunger
within itself so as to include more and more of the iron within the most densely
occupied portion of the magnetic circuit.

Fig. 96. Parallel Differential Electromagnet
View full size illustration.

Differential Electromagnet. Frequently in telephony, the electromagnets are
provided with more than one winding. One purpose of the double-wound
electromagnet is to produce the so-called differential action between the two windings,
i.e., making one of the windings develop magnetization in the opposite direction from
that of the other, so that the two will neutralize each other, or at least exert different and
opposite influences. The principle of the differential electromagnet may be illustrated
in connection with Fig. 96. Here two wires 1 and 2 are shown wrapped in the same
direction about an iron core, the ends of the wire being joined together at 3. Obviously,

if one of these windings only is employed and a current sent through it, as by
connecting the terminals of a battery with the points 4 and 3, for instance, the core will
be magnetized as in an ordinary magnet. Likewise, the core will be energized if a
current be sent from 5 to 3. Assuming that the two windings are of equal resistance and
number of turns, the effects so produced, when either the coil 1 or the coil 2 is
energized, will be equal. If the battery be connected between the terminals 4 and 5 with
the positive pole, say, at 5, then the current will proceed through the winding 2 and tend
to generate magnetism in the core in the direction of the arrow. After traversing the
winding 2, however, it will then begin to traverse the other winding 1 and will pass
around the core in the opposite direction throughout the length of that winding. This
will tend to set up magnetism in the core in the opposite direction to that indicated by
the arrow. Since the two currents are equal and also the number of turns in each
winding, it is obvious that the two magnetizing influences will be exactly equal and
opposite and no magnetic effect will be produced. Such a winding, as is shown in Fig.
96, where the two wires are laid on side by side, is called a parallel differential
winding.

Another way of winding magnets differentially is to put one winding on one end of
the core and the other winding on the other end of the core and connect these so as to
cause the currents through them to flow around the core in opposite directions. Such a
construction is shown in Fig. 97 and is called a tandem differential winding. The
tandem arrangement, while often good enough for practical purposes, cannot result in
the complete neutralization of magnetic effect. This is true because of the leakage of
some of the lines of force from intermediate points in the length of the core through the
air, resulting in some of the lines passing through more of the turns of one coil than of
the other. Complete neutralization can only be attained by first twisting the two wires
together with a uniform lay and then winding them simultaneously on the core.

Fig. 97. Tandem Differential Electromagnet
View full size illustration.

Mechanical Details. We will now consider the actual mechanical construction of
the electromagnet. This is a very important feature of telephone work, because, not
only must the proper electrical and magnetic effects be produced, but also the whole
structure of the magnet must be such that it will not easily get out of order and not be
affected by moisture, heat, careless handling, or other adverse conditions.

The most usual form of magnet construction employed in telephony is shown in Fig.
98. On the core, which is of soft Norway iron, usually cylindrical in form, are forced

two washers of either fiber or hard rubber. Fiber is ordinarily to be preferred because it
is tougher and less liable to breakage. Around the core, between the two heads, are then
wrapped several layers of paper or specially prepared cloth in order that the wire
forming the winding may be thoroughly insulated from the core. One end of the wire is
then passed through a hole in one of the spool heads or washers, near the core, and the
wire is then wound on in layers. Sometimes a thickness of paper is placed around each
layer of wire in order to further guard against the breaking down of the insulation
between layers. When the last layer is wound on, the end of the wire is passed out
through a hole in the head, thus leaving both ends projecting.

Fig. 98 Construction of Electromagnet
View full size illustration.

Magnet Wire. The wire used in winding magnets is, of course, an important part of
the electromagnet. It is always necessary that the adjacent turns of the wire be insulated
from each other so that the current shall be forced to pass around the core through all
the length of wire in each turn rather than allowing it to take the shorter and easier path
from one turn to the next, as would be the case if the turns were not insulated. For this
purpose the wire is usually covered with a coating of some insulating material. There
are, however, methods of winding magnet coils with bare wire and taking care of the
insulation between the turns in another way, as will be pointed out.

Insulated wire for the purpose of winding magnet coils is termed magnet wire.
Copper is the material almost universally employed for the conductor. Its high
conductivity, great ductility, and low cost are the factors which make it superior to all
other metals. However, in special cases, where exceedingly high conductivity is
required with a limited winding space, silver wire is sometimes employed, and on the
other hand, where very high resistance is desired within a limited winding space, either
iron or German silver or some other high-resistance alloy is used.

Wire Gauges. Wire for electrical purposes is drawn to a number of different standard
gauges. Each of the so-called wire gauges consists of a series of graded sizes of wire,
ranging from approximately one-half an inch in diameter down to about the fineness of
a lady's hair. In certain branches of telephone work, such as line construction, the
existence of the several wire gauges or standards is very likely to lead to confusion.
Fortunately, however, so far as magnet wire is concerned, the so-called Brown and

Sharpe, or American, wire gauge is almost universally employed in this country. The
abbreviations for this gauge are B.&S. or A.W.G.

TABLE III

Copper Wire Table

Giving weights, lengths, and resistances of wire @ 68° F., of Matthiessen's Standard
Conductivity.

AREA RESISTANCE LENGTH WEIGHT
A.W.G. DIAMETER
Circular Ohms per Ohms Per Feet per Feet per Pounds per Pounds per
B.&S. Mils
Mils Pound Foot Pound Ohm Foot Ohm
0000 460. 211,600. 0.00007639 0.0000489 1.561 20,440. 0.6405 13,090.
000 409.6 167,800. 0.0001215 0.0000617 1.969 16,210. 0.5080 8,232.
00 364.8 133,100. 0.0001931 0.0000778 2.482 12,850. 0.4028 5,177.
0 324.9 105,500. 0.0003071 0.0000981 3.130 10,190. 0.3195 3,256.
1 289.3 83,690. 0.0004883 0.0001237 3.947 8,083. 0.2533 2,048.
2 257.6 66,370. 0.0007765 0.0001560 4.977 6,410. 0.2009 1,288.
3 229.4 52,630. 0.001235 0.0001967 6.276 5,084. 0.1593 810.0
4 204.3 41,740. 0.001963 0.0002480 7.914 4,031. 0.1264 509.4
5 181.9 33,100. 0.003122 0.0003128 9.980 3,197. 0.1002 320.4
6 162.0 26,250. 0.004963 0.0003944 12.58 2,535. 0.07946 201.5
7 144.3 20,820. 0.007892 0.0004973 15.87 2,011. 0.06302 126.7
8 128.5 16,510. 0.01255 0.0006271 20.01 1,595. 0.04998 79.69
9 114.4 13,090. 0.01995 0.0007908 25.23 1,265. 0.03963 50.12
10 101.9 10,380. 0.03173 0.0009273 31.82 1,003. 0.03143 31.52
11 90.74 8,234. 0.05045 0.001257 40.12 795.3 0.02493 19.82
12 80.81 6,530. 0.08022 0.001586 50.59 630.7 0.01977 12.47
13 71.96 5,178. 0.1276 0.001999 63.79 500.1 0.01568 7.840
14 64.08 4,107. 0.2028 0.002521 80.44 396.6 0.01243 4.931
15 57.07 3,257. 0.3225 0.003179 101.4 314.5 0.009858 3.101
16 50.82 2,583. 0.5128 0.004009 127.9 249.4 0.007818 1.950
17 45.26 2,048. 0.8153 0.005055 161.3 197.8 0.006200 1.226
18 40.30 1,624. 1.296 0.006374 203.4 156.9 0.004917 0.7713
19 35.89 1,288. 2.061 0.008038 256.5 124.4 0.003899 0.4851
20 31.96 1,022. 3.278 0.01014 323.4 98.66 0.003092 0.3051
21 28.46 810.1 5.212 0.01278 407.8 78.24 0.002452 0.1919
22 25.35 642.4 8.287 0.01612 514.2 62.05 0.001945 0.1207
23 22.57 509.5 13.18 0.02032 648.4 49.21 0.001542 0.07589

24 20.10 404.0 20.95 0.02563 817.6 39.02 0.001223 0.04773
25 17.90 320.4 33.32 0.03231 1,031. 30.95 0.0009699 0.03002
26 15.94 254.1 52.97 0.04075 1,300. 24.54 0.0007692 0.1187
27 14.2 201.5 84.23 0.05138 1,639. 19.46 0.0006100 0.01888
28 12.64 159.8 133.9 0.06479 2,067. 15.43 0.0004837 0.007466
29 11.26 126.7 213.0 0.08170 2,607. 12.24 0.0003836 0.004696
30 10.03 100.5 338.6 0.1030 3,287. 9.707 0.0003042 0.002953
31 8.928 79.70 538.4 0.1299 4,145. 7.698 0.0002413 0.001857
32 7.950 63.21 856.2 0.1638 5,227. 6.105 0.0001913 0.001168
33 7.080 50.13 1,361. 0.2066 6,591. 4.841 0.0001517 0.0007346
34 6.305 39.75 2,165. 0.2605 8,311. 3.839 0.0001203 0.0004620
35 5.615 31.52 3,441. 0.3284 10,480. 3.045 0.00009543 0.0002905
36 5.0 25.0 5,473. 0.4142 13,210. 2.414 0.00007568 0.0001827
37 4.453 19.83 8,702. 0.5222 16,660. 1.915 0.00006001 0.0001149
38 3.965 15.72 13,870. 0.6585 21,010. 1.519 0.00004759 0.00007210
39 3.531 12.47 22,000. 0.8304 26,500. 1.204 0.00003774 0.00004545
40 3.145 9.888 34,980. 1.047 33,410. 0.9550 0.00002993 0.00002858

In the Brown and Sharpe gauge the sizes, beginning with the largest, are
numbered 0000, 000, 00, 0, 1, 2, and so on up to 40. Sizes larger than about
No. 16 B.&S. gauge are seldom used as magnet wire in telephony, but for
the purpose of making the list complete, Table III is given, including all of
the sizes of the B.&S. gauge.

In Table III there is given for each gauge number the diameter of the
wire in mils (thousandths of an inch); the cross-sectional area in circular
mils (a unit area equal to that of a circle having a diameter of one one-
thousandth of an inch); the resistance of the wire in various units of length
and weight; the length of the wire in terms of resistance and of weight; and
the weight of the wire in terms of its length and resistance.

It is to be understood that in Table III the wire referred to is bare wire
and is of pure copper. It is not commercially practicable to use absolutely
pure copper, and the ordinary magnet wire has a conductivity equal to about
98 per cent of that of pure copper. The figures given in this table are
sufficiently accurate for all ordinary practical purposes.

Silk and Cotton Insulation. The insulating material usually employed for
covering magnet wire is of silk or cotton. Of these, silk is by far the better
material for all ordinary purposes, since it has a much higher insulating
property than cotton, and is very much thinner. Cotton, however, is largely
employed, particularly in the larger sizes of magnet wire. Both of these
materials possess the disadvantage of being hygroscopic, that is, of readily
absorbing moisture. This disadvantage is overcome in many cases by
saturating the coil after it is wound in some melted insulating compound,
such as wax or varnish or asphaltum, which will solidify on cooling. Where
the coils are to be so saturated the best practice is to place them in a vacuum
chamber and exhaust the air, after which the hot insulating compound is
admitted and is thus drawn into the innermost recesses of the winding
space.

Silk-insulated wire, as regularly produced, has either one or two layers
of silk. This is referred to commercially as single silk wire or as double silk
wire. The single silk has a single layer of silk fibers wrapped about it, while
the double silk has a double layer, the two layers being put on in reverse

direction. The same holds true of cotton insulated wire. Frequently, also,
there is a combination of the two, consisting of a single or a double
wrapping of silk next to the wire with an outer wrapping of cotton. Where
this is done the cotton serves principally as a mechanical protection for the
silk, the principal insulating properties residing in the silk.

Enamel. A later development in the insulation of magnet wire has
resulted in the so-called enamel wire. In this, instead of coating the wire
with some fibrous material such as silk or cotton, the wire is heated and run
through a bath of fluid insulating material or liquid enamel, which adheres
to the wire in a very thin coating. The wire is then run through baking
ovens, so that the enamel is baked on. This process is repeated several times
so that a number of these thin layers of the enamel are laid on and baked in
succession.

The characteristics sought in good enamel insulation for magnet wire
may be thus briefly set forth: It is desirable for the insulation to possess the
highest insulating qualities; to have a glossy, flawless surface; to be hard
without being brittle; to adhere tenaciously and stand all reasonable
handling without cracking or flaking; to have a coefficient of elasticity
greater than the wire itself; to withstand high temperatures; to be moisture-
proof and inert to corrosive agencies; and not to "dry out" or become brittle
over a long period of time.

Space Utilization. The utilization of the winding space in an
electromagnet is an important factor in design, since obviously the copper
or other conductor is the only part of the winding that is effective in setting
up magnetizing force. The space occupied by the insulation is, in this sense,
waste space. An ideally perfect winding may be conceived as one in which
the space is all occupied by wire; and this would necessarily involve the
conception of wire of square cross-section and insulation of infinite
thinness. In such a winding there would be no waste of space and a
maximum amount of metal employed as a conductor. Of course, such a
condition is not possible to attain and in practice some insulating material
must be introduced between the layers of wire and between the adjacent
convolutions of wire. The ratio of the space occupied by the conductor to
the total space occupied by the winding, that is, by the conductor and the

insulation, is called the coefficient of space utilization of the coil. For the
ideal coil just conceived the coefficient of space utilization would be 1.
Ordinarily the coefficient of space utilization is greater for coarse wire than
for fine wire, since obviously the ratio of the diameter of the wire to the
thickness of the insulation increases as the size of the wire grows larger.

The chief advantage of enamel insulation for magnet wire is its thinness,
and the high coefficient of space utilization which may be secured by its
use. In good enamel wire the insulation will average about one-quarter the
thickness of the standard single silk insulation, and the dielectric strength is
equal or greater. Where economy of winding space is desirable the
advantages of this may readily be seen. For instance, in a given coil wound
with No. 36 single silk wire about one-half of the winding space is taken up
with the insulation, whereas when the same coil is wound with No. 36
enameled wire only about one-fifth of the winding space is taken up by the
insulation. Thus the coefficient of space utilization is increased from .50 to
.80. The practical result of this is that, in the case of any given winding
space where No. 36 wire is used, about 60 per cent more turns can be put on
with enameled wire than with single silk insulation, and of course this ratio
greatly increases when the comparison is made with double silk insulation
or with cotton insulation. Again, where it is desired to reduce the winding
space and keep the same number of turns, an equal number of turns may be
had with a corresponding reduction of winding space where enameled wire
is used in place of silk or cotton.

In the matter of heat-resisting properties the enameled wire possesses a
great advantage over silk and cotton. Cotton or silk insulation will char at
about 260° Fahrenheit, while good enameled wire will stand 400° to 500°
Fahrenheit without deterioration of the insulation. It is in the matter of
liability to injury in rough or careless handling, or in winding coils having
irregular shapes, that enamel wire is decidedly inferior to silk or cotton-
covered wire. It is likely to be damaged if it is allowed to strike against the
sharp corners of the magnet spool during winding, or run over the edge of a
hard surface while it is being fed on to the spool. Coils having other than
round cores, or having sharp corners on their spool heads, should not
ordinarily be wound with enamel wire.

The dielectric strength of enamel insulation is much greater than that of
either silk or cotton insulation of equal thickness. This is a distinct
advantage and frequently a combination of the two kinds of insulation
results in a superior wire. If wire insulated with enamel is given a single
wrapping of silk or of cotton, the insulating and dielectric properties of the
enamel is secured, while the presence of the silk and cotton affords not only
an additional safeguard against bare spots in the enamel but also a certain
degree of mechanical protection to the enamel.

Winding Methods. In winding a coil, the spool, after being properly
prepared, is placed upon a spindle which may be made to revolve rapidly.
Ordinarily the wire is guided on by hand; sometimes, however, machinery
is used, the wire being run over a tool which moves to and fro along the
length of the spool, just fast enough to lay the wire on at the proper rate.
The movement of this tool is much the same as that of the tool in a screw
cutting lathe.

Unless high voltages are to be encountered, it is ordinarily not necessary
to separate the layers of wire with paper, in the case of silk-or cotton-
insulated magnet wire; although where especially high insulation resistance
is needed this is often done. It is necessary to separate the successive layers
of a magnet that is wound with enamel wire, by sheets of paper or thin oiled
cloth.

Fig. 99. Electromagnet with Bare Wire
View full size illustration.

In Fig. 99 is shown a method, that has been used with some success, of
winding magnets with bare wire. In this the various adjacent turns are
separated from each other by a fine thread of silk or cotton wound on beside
the wire. Each layer of wire and thread as it is placed on the core is

completely insulated from the subsequent layer by a layer of paper. This is
essentially a machine-wound coil, and machines for winding it have been so
perfected that several coils are wound simultaneously, the paper being fed
in automatically at the end of each layer.

Another method of winding the bare wire omits the silk thread and
depends on the permanent positioning of the wire as it is placed on the coil,
due to the slight sinking into the layer of paper on which it is wound. In this
case the feed of the wire at each turn of the spool is slightly greater than the
diameter of the wire, so that a small distance will be left between each pair
of adjacent turns.

Upon the completion of the winding of a coil, regardless of what method
is used, it is customary to place a layer of bookbinders' cloth over the coil
so as to afford a certain mechanical protection for the insulated wire.

Winding Terminals. The matter of bringing out the terminal ends of the
winding is one that has received a great deal of attention in the construction
of electromagnets and coils for various purposes. Where the winding is of
fine wire, it is always well to reinforce its ends by a short piece of larger
wire. Where this is done the larger wire is given several turns around the
body of the coil, so that the finer wire with which it connects may be
relieved of all strain which may be exerted upon it from the protruding ends
of the wire. Great care is necessary in the bringing out of the inner terminal
—i.e., the terminal which connects with the inner layer—that the terminal
wire shall not come in contact with any of the subsequent layers that are
wound on.

Fig. 100. Electromagnet with Terminals
View full size illustration.

Where economy of space is necessary, a convenient method of
terminating the winding of the coil consists in fastening rigid terminals to
the spool head. This, in the case of a fiber spool head, may be done by

driving heavy metal terminals into the fiber. The connections of the two
wires leading from the winding are then made with these heavy rigid
terminals by means of solder. A coil having such terminals is shown in its
finished condition in Fig. 100.

Winding Data. The two things principally affecting the manufacture of
electromagnets for telephone purposes are the number of turns in a winding
and the resistance of the wound wire. The latter governs the amount of
current which may flow through the coil with a given difference of potential
at its end, while the former control the amount of magnetism produced in
the core by the current flowing. While a coil is being wound, it is a simple
matter to count the turns by any simple form of revolution counter. When
the coil has been completed it is a simple matter to measure its resistance.
But it is not so simple to determine in advance how many turns of a given
size wire may be placed on a given spool, and still less simple to know what
the resistance of the wire on that spool will be when the desired turns shall
have been wound.

TABLE IV

Winding Data for Insulated Wires—Silk and Cotton Covering

A.W.G. B & S 20 21 22 23 24 25 26
DIAMETER Mils 31.961 28.462 25.34722.57120.100 17.90015.940
AREA Circular Mils 1021.20810.10 642.70509.45404.01 320.40254.01
DIAMETER OVER
INSULATION
Single Cotton 37.861 34.362 31.24728.47126.000 23.80021.840
Double Cotton 42.161 38.662 35.54732.77130.300 28.10026.140
Single Silk 34.261 30.762 27.64724.87122.401 20.20018.240
Double Silk 36.161 32.662 29.54726.77124.300 22.10020.140
TURNS PER
LINEAR INCH
Single Cotton 25.7 28.3 31.0 34.4 36.9 38.0 42.0
Double Cotton 22.5 24.5 26.7 28.97 31.35 33.92 36.29
Single Silk 27.70 30.97 34.39 38.19 42.37 47.02 52.06
Double Silk 26.22 29.07 32.11 35.53 39.14 42.94 46.81

TURNS PER
SQUARE INCH
Single Cotton 660.5 800.9 961.0 1183.0 1321.6 1444.01764.0
Double Cotton 506.3 600.2 712.9 839.2 982.8 1150.8 1317.0
Single Silk 767.3 959.1 1182.7 1458.51795.2 2210.92710.3
Double Silk 687.5 845.0 1031.01262.41532.0 1843.82191.2
OHMS PER
CUBIC INCH
Single Cotton .646 .981 1.502 2.359 3.528 5.831 6.941
Double Cotton .533 .795 1.188 1.772 2.595 3.802 5.552
Single Silk .801 1.261 1.956 3.049 4.739 7.489 9.031

A.W.G. B & S 27 28 29 30 31 32 33
DIAMETER Mils 14.19512.641 11.257 10.025 8.928 7.950 7.080
AREA Circular Mils 201.50159.79 126.72100.50 79.71 63.20 50.13
DIAMETER OVER
INSULATION
Single Cotton 20.09518.541 17.15715.92514.82813.850 12.980
Double Cotton 24.39522.841 21.45720.22519.12818.150 17.280
Single Silk 16.49514.941 13.55712.325 11.228 10.250 9.380
Double Silk 18.39516.841 15.45714.22513.12812.150 11.280
TURNS PER
LINEAR INCH
Single Cotton 48.0 53.0 56.5 59.66 64.12568.600 73.050
Double Cotton 38.95 41.61 44.27 46.93 49.78 52.34 55.10
Single Silk 57.67 63.36 70.11 77.14 84.64 92.72 101.65
Double Silk 51.59 56.43 61.56 66.79 72.39 78.19 84.17
TURNS PER
SQUARE INCH
Single Cotton 2304.02809.9 3192.33359.2 4112.2 4692.5 5333.5
Double Cotton 1517.21731.0 1959.92202.52478.02739.5 3036.1
Single Silk 3326.04014.5 4915.55950.27164.08597.510332.0
Double Silk 2661.63184.5 3789.84461.05240.0 6114.0 7085.0
OHMS PER
CUBIC INCH
Single Cotton 10.81417.617 25.50034.800 48.5 73.8 104.5

Double Cotton 8.078 11.54 16.47 23.43 32.83 46.19 64.30
Single Silk 13.92 26.86 41.29 62.98 95.70 144.70 217.8

A.W.G. B & S 34 35 36 37 38 39 40
DIAMETER Mils 6.304 5.614 5.000 4.453 3.965 3.531 3.144
AREA Circular Mils 39.74 31.52 25.00 19.83 15.72 12.47 9.89
DIAMETER OVER
INSULATION
Single Cotton 12.204 11.514 1090.0 10.353 9.865 9.431 9.044
Double Cotton 16.50415.814 15.200 14.653 14.165 13.731 13.344
Single Silk 8.504 7.914 7.300 6.753 6.265 5.831 5.344
Double Silk 10.504 9.814 9.200 8.653 8.165 7.731 7.344
TURNS PER
LINEAR INCH
Single Cotton 77.90082.600 87.100 91.870 95.000 100.700106.000
Double Cotton 57.57 60.04 62.51 64.70 66.80 68.80 71.20
Single Silk 112.11 119.7 130.15 140.6 151.05 163.04 177.65
Double Silk 90.44 96.90 103.55 110.20 116.85 122.55 129.20
TURNS PER
SQUARE INCH
Single Cotton 6068.56773.3 7586.5 8440.0 9025.0 10140.5 11236.0
Double Cotton 3314.23605.0 3907.5 4186.1 4462.2 4733.6 5069.8
Single Silk 8179.59389.516940.0 19770.022820.0 26700.031559.0
Double Silk 8179.59389.510772.0 12145.013665.0 15018.016692.0
OHMS PER
CUBIC INCH
Single Cotton 151.4 202.0 298.8 418.0 567.0 811.0 1113.0
Double Cotton 70.58 125.9 166.3 225.6 305.5 409.8 545.5
Single Silk 342.1 489.0 721.1 1062.0 1557.0 2266.0 3400.0

If the length and the depth of the winding space of the coil as well as the
diameter of the core are known, it is not difficult to determine how much
bare copper wire of a given size may be wound on it, but it is more difficult

to know these facts concerning copper wire which has been covered with
cotton or silk. Yet something may be done, and tables have been prepared
for standard wire sizes with definite thicknesses of silk and cotton
insulation. As a result of facts collected from a large number of actually
wound coils, the number of turns per linear inch and per square inch of
B.&S. gauge wires from No. 20 to No. 40 have been tabulated, and these,
supplemented by a tabulation of the number of ohms per cubic inch of
winding space for wires of three different kinds of insulation, are given in
Table IV.

Bearing in mind that the calculations of Table IV are all based upon the
"diameter over insulation," which it states at the outset for each of four
different kinds of covering, it is evident what is meant by "turns per linear
inch." The columns referring to "turns per square inch" mean the number of
turns, the ends of which would be exposed in one square inch if the wound
coil were cut in a plane passing through the axis of the core. Knowing the
distance between the head, and the depth to which the coil is to be wound, it
is easy to select a size of wire which will give the required number of turns
in the provided space. It is to be noted that the depth of winding space is
one-half of the difference between the core diameter and the complete
diameter of the wound coil. The resistance of the entire volume of wound
wire may be determined in advance by knowing the total cubic contents of
the winding space and multiplying this by the ohms per cubic inch of the
selected wire; that is, one must multiply in inches the distance between the
heads of the spool by the difference between the squares of the diameters of
the core and the winding space, and this in turn by .7854. This result, times
the ohms per cubic inch, as given in the table, gives the resistance of the
winding.

There is a considerable variation in the method of applying silk
insulation to the finer wires, and it is in the finer sizes that the errors, if any,
pile up most rapidly. Yet the table throughout is based on data taken from
many samples of actual coil winding by the present process of winding
small coils. It should be said further that the table does not take into account
the placing of any layers of paper between the successive layers of the
wires. This table has been compared with many examples and has been
used in calculating windings in advance, and is found to be as close an

approximation as is afforded by any of the formulas on the subject, and with
the further advantage that it is not so cumbersome to apply.

Winding Calculations. In experimental work, involving the winding of
coils, it is frequently necessary to try one winding to determine its effect in
a given circuit arrangement, and from the knowledge so gained to substitute
another just fitted to the conditions. It is in such a substitution that the table
is of most value. Assume a case in which are required a spool and core of a
given size with a winding of, say No. 25 single silk-covered wire, of a
resistance of 50 ohms. Assume also that the circuit regulations required that
this spool should be rewound so as to have a resistance of, say 1,000 ohms.
What size single silk-covered wire shall be used? Manifestly, the winding
space remains the same, or nearly so. The resistance is to be increased from
50 to 1,000 ohms, or twenty times its first value. Therefore, the wire to be
used must show in the table twenty times as many ohms per cubic inch as
are shown in No. 25, the known first size. This amount would be twenty
times 7.489, which is 149.8, but there is no size giving this exact resistance.
No. 32, however, is very nearly of that resistance and if wound to exactly
the same depth would give about 970 ohms. A few turns more would
provide the additional thirty ohms.

Similarly, in a coil known to possess a certain number of turns, the table
will give the size to be selected for rewinding to a greater or smaller
number of turns. In this case, as in the case of substituting a winding of
different resistance, it is unnecessary to measure and calculate upon the
dimensions of the spool and core. Assume a spool wound with No. 30
double silk-covered wire, which requires to be wound with a size to double
the number of turns. The exact size to do this would have 8922. turns per
square inch and would be between No. 34 and No. 35. A choice of these
two wires may be made, using an increased winding depth with the smaller
wire and a shallower winding depth for the larger wire.

Impedance Coils. In telephony electromagnets frequently serve, as
already stated, to perform other functions than the producing of motion by
attracting or releasing their armatures. They are required to act as
impedance coils to present a barrier to the passage of alternating or other
rapidly fluctuating currents, and at the same time to allow the

comparatively free passage of steady currents. Where it is desired that an
electromagnet coil shall possess high impedance, it is usual to employ a
laminated instead of a solid core. This is done by building up a core of
suitable size by laying together thin sheets of soft iron, or by forming a
bundle of soft iron wires. The use of laminated cores is for the purpose of
preventing eddy currents, which, if allowed to flow, would not only be
wasteful of energy but would also tend to defeat the desired high
impedance. Sometimes in iron-clad impedance coils, the iron shell is slotted
longitudinally to break up the flow of eddy currents in the shell.

Frequently electromagnetic coils have only the function of offering
impedance, where no requirements exist for converting any part of the
electric energy into mechanical work. Where this is the case, such coils are
termed impedance, or retardation, or choke coils, since they are employed
to impede or to retard or to choke back the flow of rapidly varying current.
The distinction, therefore, between an impedance coil and the coil of an
ordinary electromagnet is one of function, since structurally they may be
the same, and the same principles of design and construction apply largely
to each.

Number of Turns. It should be remembered that an impedance coil
obstructs the passage of fluctuating current, not so much by ohmic
resistance as by offering an opposing or counter-electromotive force. Other
things being equal, the counter-electromotive force of self-induction
increases directly as the number of turns on a coil and directly as the
number of lines of force threading the coil, and this latter factor depends
also on the reluctance of the magnetic circuit. Therefore, to secure high
impedance we need many turns or low reluctance, or both. Often, owing to
requirements for direct-current carrying capacity and limitations of space, a
very large number of turns is not permissible, in which case sufficiently
high impedance to such rapid fluctuations as those of voice currents may be
had by employing a magnetic circuit of very low reluctance, usually a
completely closed circuit.

Kind of Iron. An important factor in the design of impedance coils is the
grade of iron used in the magnetic circuit. Obviously, it should be of the
highest permeability and, furthermore, there should be ample cross-section

of core to prevent even an approach to saturation. The iron should, if
possible, be worked at that density of magnetization at which it has the
highest permeability in order to obtain the maximum impedance effects.

Types. Open-Circuit:—Where very feeble currents are being dealt with,
and particularly where there is no flow of direct current, an open magnetic
circuit is much used. An impedance coil having an open magnetic circuit is
shown in section in Fig. 101, Fig. 102 showing its external appearance and
illustrating particularly the method of bringing out the terminals of the
winding.

Fig. 101. Section of Open-Circuit Impedance Coil
View full size illustration.

Fig. 102. Open-Circuit Impedance Coil
View full size illustration.

Fig. 103. Closed-Circuit Impedance Coil
View full size illustration.

Closed-Circuit:—A type of retardation coil which is largely used in
systems of simultaneous telegraphy and telephony, known as composite
systems, is shown in Fig. 103. In the construction of this coil the core is
made of a bundle of fine iron wires first bent into U-shape, and then after
the coils are in place, the free ends of the core are brought together to form
a closed magnetic circuit. The coils have a large number of turns of rather
coarse wire. The conditions surrounding the use of this coil are those which
require very high impedance and rather large current-carrying capacity, and
fortunately the added requirement, that it shall be placed in a very small
space, does not exist.

Toroidal:—Another type of retardation coil, called the toroidal type due
to the fact that its core is a torus formed by winding a continuous length of
fine iron wire, is shown in diagram in Fig. 104. The two windings of this
coil may be connected in series to form in effect a single winding, or it may
be used as a "split-winding" coil, the two windings being in series but
having some other element, such as a battery, connected between them in
the circuit. Evidently such a coil, however connected, is well adapted for
high impedance, on account of the low reluctance of its core.

Fig. 104. Symbol of Toroidal Impedance Coil
View full size illustration.

This coil is usually mounted on a base-board, the coil being enclosed in
a protecting iron case, as shown in Fig. 105. The terminal wires of both
windings of each coil are brought out to terminal punchings on one end of
the base-board to facilitate the making of the necessary circuit connections.

Fig. 105. Toroidal Impedance Coil
View full size illustration.

The usual diagrammatic symbol for an impedance coil is shown in Fig.
106. This is the same as for an ordinary bar magnet, except that the parallel
lines through the core may be taken as indicating that the core is laminated,
thus conveying the idea of high impedance. The symbol of Fig. 104 is a
good one for the toroidal type of impedance coil.

Fig. 106. Symbol of Impedance Coil
View full size illustration.

Induction Coil. An induction coil consists of two or more windings of
wire interlinked by a common magnetic circuit. In an induction coil having
two windings, any change in the strength of the current flowing in one of
the windings, called the primary, will cause corresponding changes in the
magnetic flux threading the magnetic circuit, and, therefore, changes in flux
through the other winding, called the secondary. This, by the laws of
electromagnetic induction, will produce corresponding electromotive forces
in the secondary winding and, therefore, corresponding currents in that
winding if its circuit be closed.

Current and Voltage Ratios. In a well-designed induction coil the energy
in the secondary, i.e., the induced current, is for all practical purposes equal
to that of the primary current, yet the values of the voltage and the
amperage of the induced current may vary widely from the values of the
voltage and the amperage of the primary current. With simple periodic
currents, such as the commercial alternating lighting currents, the ratio
between the voltage in the primary and that in the secondary will be equal

to the ratio of the number of turns in the primary to the number of turns in
the secondary. Since the energy in the two circuits will be practically the
same, it follows that the ratio between the current in the primary and that
in the secondary will be equal to the ratio of the number of turns in the
secondary to the number of turns in the primary. In telephony, where the
currents are not simple periodic currents, and where the variations in
current strength take place at different rates, such a law as that just stated
does not hold for all cases; but it may be stated in general that the induced
currents will be of higher voltage and smaller current strength than those of
the primary in all coils where the secondary winding has a greater number
of turns than the primary, and vice versâ.

Functions. The function of the induction coil in telephony is, therefore,
mainly one of transformation, that is, either of stepping up the voltage of a
current, or in other cases stepping it down. The induction coil, however,
does serve another purpose in cases where no change in voltage and current
strength is desired, that is, it serves as a means for electrically separating
two circuits so far as any conductive relation exists, and yet of allowing the
free transmission by induction from one of these circuits to the other. This is
a function that in telephony is scarcely of less importance than the purely
transforming function.

Design. Induction coils, as employed in telephony, may be divided into
two general types: first, those having an open magnetic circuit; and, second,
those having a closed magnetic circuit. In the design of either type it is
important that the core should be thoroughly laminated, and this is done
usually by forming it of a bundle of soft Swedish or Norway iron wire
about .02 of an inch in diameter. The diameter and the length of the coil,
and the relation between the number of turns in the primary and in the
secondary, and the mechanical construction of the coil, are all matters
which are subject to very wide variation in practice. While the proper
relationship of these factors is of great importance, yet they may not be
readily determined except by actual experiment with various coils, owing to
the extreme complexity of the action which takes place in them and to the
difficulty of obtaining fundamental data as to the existing facts. It may be
stated, therefore, that the design of induction coils is nearly always carried

out by "cut-and-try" methods, bringing to bear, of course, such scientific
and practical knowledge as the experimenter may possess.

Fig. 107. Induction Coil
View full size illustration.

Fig. 108. Section of Induction Coil
View full size illustration.

Use and Advantage. The use and advantages of the induction coil in so-
called local-battery telephone sets have already been explained in previous
chapters. Such induction coils are nearly always of the open magnetic
circuit type, consisting of a long, straight core comprised of a bundle of
small annealed iron wires, on which is wound a primary of comparatively
coarse wire and having a small number of turns, and over which is wound a
secondary of comparatively fine wire and having a very much larger
number of turns. A view of such a coil mounted on a base is shown in Fig.
107, and a sectional view of a similar coil is shown in Fig. 108. The method
of bringing out the winding terminals is clearly indicated in this figure, the
terminal wires 2 and 4 being those of the primary winding and 1 and 3 those
of the secondary winding. It is customary to bring out these wires and
attach them by solder to suitable terminal clips. In the case of the coil
shown in Fig. 108 these clips are mounted on the wooden heads of the coil,
while in the design shown in Fig. 107 they are mounted on the base, as is
clearly indicated.

Repeating Coil. The so-called repeating coil used in telephony is really
nothing but an induction coil. It is used in a variety of ways and usually has
for its purpose the inductive association of two circuits that are
conductively separated. Usually the repeating coil has a one to one ratio of

turns, that is, there are the same number of turns in the primary as in the
secondary. However, this is not always the case, since sometimes they are
made to have an unequal number of turns, in which case they are called
step-up or step-down repeating coils, according to whether the primary has
a smaller or a greater number of turns than the secondary. Repeating coils
are almost universally of the closed magnetic circuit type.

Ringing and Talking Considerations. Since repeating coils often serve to
connect two telephones, it follows that it is sometimes necessary to ring
through them as well as talk through them. By this is meant that it is
necessary that the coil shall be so designed as to be capable of transforming
the heavy ringing currents as well as the very much smaller telephone or
voice currents. Ringing currents ordinarily have a frequency ranging from
about 16 to 75 cycles per second, while voice currents have frequencies
ranging from a few hundred up to perhaps ten thousand per second.
Ordinarily, therefore, the best form of repeating coil for transforming voice
currents is not the best for transforming the heavy ringing currents and vice
versâ. If the comparatively heavy ringing currents alone were to be
considered, the repeating coil might well be of heavy construction with a
large amount of iron in its magnetic circuit. On the other hand, for carrying
voice currents alone it is usually made with a small amount of iron and with
small windings, in order to prevent waste of energy in the core, and to give
a high degree of responsiveness with the least amount of distortion of wave
form, so that the voice currents will retain as far as possible their original
characteristics. When, therefore, a coil is required to carry both ringing and
talking currents, a compromise must be effected.

Types. The form of repeating coil largely used for both ringing and
talking through is shown in Fig. 109. This coil comprises a soft iron core
made up of a bundle of wires about .02 inch in diameter, the ends of which
are left of sufficient length to be bent back around the windings after they
are in place and thus form a completely closed magnetic path for the core.
The windings of this particular coil are four in number, and contain about
2,400 turns each, and have a resistance of about 60 ohms. In this coil, when
connected for local battery work, the windings are connected in pairs in
series, thus forming effectively two windings having about 120 ohms
resistance each. The whole coil is enclosed in a protecting case of iron. The

terminals are brought out to suitable clips on the wooden base, as shown.
An external perspective view of this coil is shown in Fig. 110. By bringing
out each terminal of each winding, eight in all, as shown in this figure, great
latitude of connection is provided for, since the windings may be connected
in circuit in any desirable way, either by connecting them together in pairs
to form virtually a primary and a secondary, or, as is frequently the case, to
split the primary and the secondary, connecting a battery between each pair
of windings.

Fig. 109. Repeating Coil
View full size illustration.

Fig. 110. Repeating Coil
View full size illustration.

Fig. 111 illustrates in section a commercial type of coil designed for
talking through only. This coil is provided with four windings of 1,357 turns
each, and when used for local battery work the coils are connected in pairs
in series, thus giving a resistance of about 190 ohms in each half of the
repeating coil. The core of this coil consists of a bundle of soft iron wires,
and the shell which forms the return path for the magnetic lines is of very
soft sheet iron. This shell is drawn into cup shape and its open end is closed,
after the coil is inserted, by the insertion of a soft iron head, as indicated. As
in the case of the coil shown in Figs. 109 and 110, eight terminals are

brought out on this coil, thus providing the necessary flexibility of
connection.

Fig. 111. Repeating Coil
View full size illustration.

Fig. 112. Diagram of Toroidal Repeating Coil
View full size illustration.

Fig. 113. Toroidal Repeating Coil
View full size illustration.

Still another type of repeating coil is illustrated in diagram in Fig. 112,
and in view in Fig. 113. This coil, like the impedance coil shown in Fig.
104, comprises a core made up of a bundle of soft iron wires wound into the
form of a ring. It is usually provided with two primary windings placed
opposite each other upon the core, and with two secondary windings, one

over each primary. In practice these two primary windings are connected in
one circuit and the two secondaries in another. This is the standard
repeating coil now used by the Bell companies in their common-battery
cord circuits.

Fig. 114. Symbol of Induction Coil
View full size illustration.

Conventional Symbols. The ordinary symbol for the induction coil used
in local battery work is shown in Fig. 114. This consists merely of a pair of
parallel zig-zag lines. The primary winding is usually indicated by a heavy
line having a fewer number of zig-zags, and the secondary by a finer line
having a greater number of zig-zags. In this way the fact that the primary is
of large wire and of comparatively few turns is indicated. This
diagrammatic symbol may be modified to suit almost any conditions, and
where a tertiary as well as a secondary winding is provided it may be shown
by merely adding another zig-zag line.

Fig. 115. Repeating-Coil Symbols
View full size illustration.

The repeating coil is indicated symbolically in the two diagrams of Fig.
115. Where there is no necessity for indicating the internal connections of
the coil, the symbol shown in the left of this figure is usually employed.
Where, however, the coil consists of four windings rather than two and the
method of connecting them is to be indicated, the symbol at the right hand
is employed. In Fig. 116 another way of indicating a four-winding repeating
coil or induction coil is shown. Sometimes such windings may be combined
by connection to form merely a primary and a secondary winding, and in

other cases the four windings all act separately, in which case one may be
considered the primary and the others, respectively, the secondary, tertiary,
and quaternary.

Fig. 116. Symbol of Four-Winding Repeating Coil
View full size illustration.

Where the toroidal type of repeating coil is employed, the diagram of
Fig. 112, already referred to, is a good symbolic representation.

ToC

CHAPTER XI
NON-INDUCTIVE RESISTANCE DEVICES

It is often desired to introduce simple ohmic resistance into telephone
circuits, in order to limit the current flow, or to create specific differences of
potential at given points in the circuit.

Temperature Coefficient. The design or selection of resistance devices
for various purposes frequently involves the consideration of the effect of
temperature on the resistance of the conductor employed. The resistance of
conductors is subject to change by changes in temperature. While nearly all
metals show an increase, carbon shows a decrease in its resistance when
heated.

The temperature coefficient of a conductor is a factor by which the
resistance of the conductor at a given temperature must be multiplied in
order to determine the change in resistance of that conductor brought about
by a rise in temperature of one degree.

TABLE V

Temperature Coefficients

Temperature Coefficients
Pure Metals
Centigrade Fahrenheit
Silver (annealed) 0.00400 0.00222
Copper (annealed) 0.00428 0.00242
Gold (99.9%) 0.00377 0.00210
Aluminum (99%) 0.00423 0.00235
Zinc 0.00406 0.00226
Platinum (annealed) 0.00247 0.00137
Iron 0.00625 0.00347
Nickel 0.0062 0.00345

Tin 0.00440 0.00245
Lead 0.00411 0.00228
Antimony 0.00389 0.00216
Mercury 0.00072 0.00044
Bismuth 0.00354 0.00197

Positive and Negative Coefficients. Those conductors, in which a rise in
temperature produces an increase in resistance, are said to have positive
temperature coefficients, while those in which a rise in temperature
produces a lowering of resistance are said to have negative temperature
coefficients.

The temperature coefficients of pure metals are always positive and for
some of the more familiar metals, have values, according to Foster, as in
Table V.

Iron, it will be noticed, has the highest temperature coefficient of all.
Carbon, on the other hand, has a large negative coefficient, as proved by the
fact that the filament of an ordinary incandescent lamp has nearly twice the
resistance when cold as when heated to full candle-power.

Certain alloys have been produced which have very low temperature
coefficients, and these are of value in producing resistance units which have
practically the same resistance for all ordinary temperatures. Some of these
alloys also have very high resistance as compared with copper and are of
value in enabling one to obtain a high resistance in small space.

One of the most valuable resistance wires is of an alloy known as
German silver. The so-called eighteen per cent alloy has approximately
18.3 times the resistance of copper and a temperature coefficient of .00016
per degree Fahrenheit. The thirty per cent alloy has approximately 28 times
the resistance of copper and a temperature coefficient of .00024 per degree
Fahrenheit.

For facilitating the design of resistance coils of German silver wire,
Tables VI and VII are given, containing information as to length, resistance,
and weight of the eighteen per cent and the thirty per cent alloys,
respectively, for all sizes of wire smaller than No. 20 B. & S. gauge.

Special resistance alloys may be obtained having temperature
coefficients as low as .000003 per degree Fahrenheit. Other alloys of nickel
and steel are adapted for use where the wire must carry heavy currents and
be raised to comparatively high temperatures thereby; for such use non-
corrosive properties are specially to be desired. Such wire may be obtained
having a resistance of about fifty times that of copper.

TABLE VI

18 Per Cent German Silver Wire

No. B. & S. Diameter Weight Length Resistance
Gauge Inches Pounds per Foot Feet per Pound Ohms per Foot
21 .02846 .002389 418.6 .2333
22 .02535 .001894 527.9 .2941
23 .02257 .001502 665.8 .3710
24 .02010 .001191 839.5 .4678
25 .01790 .0009449 1058. .5899
26 .01594 .0007493 1335. .7438
27 .01419 .0005943 1683. .9386
28 .01264 .0004711 2123. 1.183
29 .01126 .0003735 2677. 1.491
30 .01003 .0002962 3376. 1.879
31 .008928.0002350 4255. 2.371
32 .007950.0001864 5366. 2.990
33 .007080.0001478 6766. 3.771
34 .006304.0001172 8532. 4.756
35 .005614.00009295 10758. 5.997
36 .005000.00007369 13569. 7.560
37 .004453.00005845 17108. 9.532
38 .003965.00004636 21569. 12.02

39 .003531.00003675 27209. 15.16
40 .003145.00002917 34282. 19.11

TABLE VII

30 Per Cent German Silver Wire

No. B. & S. Diameter Weight Length Resistance
Gauge Inches Pounds per Foot Feet per Pound Ohms per Foot
21 .02846 .002405 415.8 .3581
22 .02535 .001907 524.4 .4513
23 .02257 .001512 661.3 .5693
24 .02010 .001199 833.9 .7178
25 .01790 .0009513 1051. .9051
26 .01594 .0007544 1326. 1.141
27 .01419 .0005983 1671. 1.440
28 .01264 .0004743 2108. 1.815
29 .01126 .0003761 2659. 2.287
30 .01003 .0002982 3353. 2.883
31 .008928.0002366 4227. 3.638
32 .007950.0001876 5330. 4.588
33 .007080.0001488 6721. 5.786
34 .006304.0001180 8475. 7.297
35 .005614.00009358 10686. 9.201
36 .005000.00007419 13478. 11.60
37 .004453.00005885 16994. 14.63
38 .003965.00004668 21424. 18.45
39 .003531.00003700 27026. 23.26
40 .003145.00002937 34053. 29.32

Inductive Neutrality. Where the resistance unit is required to be strictly
non-inductive, and is to be in the form of a coil, special designs must be

employed to give the desired inductive neutrality.

Provisions Against Heating. In cases where a considerable amount of
heat is to be generated in the resistance, due to the necessity of carrying
large currents, special precautions must be taken as to the heat-resisting
properties of the structure, and also as to the provision of sufficient
radiating surface or its equivalent to provide for the dissipation of the heat
generated.

Fig. 117. Mica Card Resistance
View full size illustration.

Types. Mica Card Unit. One of the most common resistance coils used
in practice is shown in Fig. 117. This comprises a coil of fine, bare German
silver wire wound on a card of mica, the windings being so spaced that the
loops are not in contact with each other. The winding is protected by two
cards of mica and the whole is bound in place by metal strips, to which the
ends of the winding are attached. Binding posts are provided on the
extended portions of the terminals to assist in mounting the resistance on a
supporting frame, and the posts terminate in soldering terminals by which
the resistance is connected into the circuit.

Differentially-Wound Unit. Another type of resistance coil is that in
which the winding is placed upon an insulating core of heat-resisting
material and wound so as to overcome inductive effects. In order to
accomplish this, the wire to be bound on the core is doubled back on itself
at its middle portion to form two strands, and these are wound
simultaneously on the core, thus forming two spirals of equal number of
turns. The current in traversing the entire coil must flow through one spiral
in one direction with relation to the core, and in the opposite direction in the
other spiral, thereby nullifying the inductive effects of one spiral by those of
the other. This is called a non-inductive winding and is in reality an example
of differential winding.

Lamp Filament. An excellent type of non-inductive resistance is the
ordinary carbon-filament incandescent lamp. This is used largely in the
circuits of batteries, generators, and other sources of supply to prevent
overload in case of short circuits on the line. These are cheap, durable, have
large current-carrying capacities, and are not likely to set things afire when
overheated. An additional advantage incident to their use for this purpose is
that an overload on a circuit in which they are placed is visibly indicated by
the glowing of the lamp.

Obviously, the carbon-filament incandescent lamp, when used as a
resistance, has, on account of the negative temperature coefficient of
carbon, the property of presenting the highest resistance to the circuit when
carrying no current, and of presenting a lower and lower resistance as the
current and consequent heating increases. For some conditions of practice
this is not to be desired, and the opposite characteristic of presenting low
resistance to small currents and comparatively high resistance to large
currents would best meet the conditions of practice.

Fig. 118. Iron-Wire Ballast
View full size illustration.

Iron-Wire Ballast. Claude D. Enochs took advantage of the very high
positive temperature coefficient of iron to produce a resistance device
having these characteristics. His arrangement possesses the compactness of
the carbon-filament lamp and is shown in Fig. 118. The resistance element
proper is an iron wire, wound on a central stem of glass, and this is included
in an exhausted bulb so as to avoid oxidation. Such a resistance is
comparatively low when cold, but when traversed by currents sufficient to
heat it considerably will offer a very large increase of resistance to oppose
the further increase of current. In a sense, it is a self-adjusting resistance,
tending towards the equalization of the flow of current in the circuit in
which it is placed.

ToC

CHAPTER XII
CONDENSERS

Charge. A conducting body insulated from all other bodies will receive
and hold a certain amount of electricity (a charge), if subjected to an
electrical potential. Thus, referring to Fig. 119, if a metal plate, insulated
from other bodies, be connected with, say, the positive pole of a battery, the
negative pole of which is grounded, a current will flow into the plate until
the plate is raised to the same potential as that of the battery pole to which it
is connected. The amount of electricity that will flow into the plate will
depend, other things being equal, on the potential of the source from which
it is charged; in fact, it is proportional to the potential of the source from
which it is charged. This amount of electricity is a measure of the capacity
of the plate, just as the amount of water that a bath-tub will hold is a
measure of the capacity of the bath-tub.

Capacity. Instead of measuring the amount of electricity by the quart or
pound, as in the case of material things, the unit of electrical quantity is the
coulomb. The unit of capacity of an insulated conductor is the farad, and a
given insulated conductor is said to have unit capacity, that is, the capacity
of one farad, when it will receive a charge of one coulomb of electricity at a
potential of one volt.

Referring to Fig. 119, the potential of the negative terminal of the battery
may be said to be zero, since it is connected to the earth. If the battery
shown be supposed to have exactly one volt potential, then the plate would
be said to have the capacity of one farad if one coulomb of electricity
flowed from the battery to the plate before the plate was raised to the same
potential as that of the positive pole, that is, to a potential of one volt above
the potential of the earth; it being assumed that the plate was also at zero
potential before the connection was made. Another conception of this
quantity may be had by remembering that a coulomb is such a quantity of
current as will result from one ampere flowing one second.

The capacity of a conductor depends, among other things, on its area. If
the plate of Fig. 119 should be made twice as large in area, other things
remaining the same, it would have twice the capacity. But there are other
factors governing the capacity of a conductor. Consider the diagram of Fig.
120, which is supposed to represent two such plates as are shown in Fig.
119, placed opposite each other and connected respectively with the
positive and the negative poles of the battery. When the connection between
the plates and the battery is made, the two plates become charged to a
difference of potential equal to the electromotive force of the battery. In
order to obtain these charges, assume that the plates were each at zero
potential before the connection was made; then current flows from the
battery into the plates until they each assume the potential of the
corresponding battery terminal. If the two plates be brought closer together,
it will be found that more current will now flow into each of them, although
the difference of potential between the two plates must obviously remain
the same, since each of them is still connected to the battery.

Fig. 119. Condenser Plate
View full size illustration.

Theory. Due to the proximity of the plates, the positive electricity on
plate A is drawn by the negative charge on plate B towards plate B, and
likewise the negative electricity on plate B is drawn to the side towards
plate A by the positive charge on that plate. These two charges so drawn
towards each other will, so to speak, bind each other, and they are referred
to as bound charges. The charge on the right-hand side of plate A and on the
left-hand side of plate B will, however, be free charges, since there is

nothing to attract them, and these are, therefore, neutralized by a further
flow of electricity from the battery to the plate.

Fig. 120. Theory of Condenser
View full size illustration.

Obviously, the closer together the plates are the stronger will be the
attractive influence of the two charges on each other. From this it follows
that in the case of plate A, when the two plates are being moved closer
together, more positive electricity will flow into plate A to neutralize the
increasing free negative charges on the right-hand side of the plate. As the
plates are moved closer together still, a new distribution of charges will take
place, resulting in more positive electricity flowing into plate A and more
negative electricity flowing into plate B. The closer proximity of the plates,
therefore, increases the capacity of the plates for holding charges, due to the
increased inductive action across the dielectric separating the plates.

Condenser Defined. A condenser is a device consisting of two adjacent
plates of conducting material, separated by an insulating material, called a
dielectric. The purpose is to increase by the proximity of the plates, each to
the other, the amount of electricity which each plate will receive and hold
when subjected to a given potential.

Dielectric. We have already seen that the capacity of a condenser
depends upon the area of its plates, and also upon their distance apart. There
is still another factor on which the capacity of a condenser depends, i.e., on
the character of the insulating medium separating its plates. The inductive
action which takes place between a charged conductor and other conductors

nearby it, as between plate A and plate B of Fig. 120, is called electrostatic
induction, and it plays an important part in telephony. It is found that the
ability of a given charged conductor to induce charges on other neighboring
conductors varies largely with the insulating medium or dielectric that
separates them. This quality of a dielectric, by which it enables inductive
action to take place between two separated conductors, is called inductive
capacity. Usually this quality of dielectrics is measured in terms of the same
quality in dry air, this being taken as unity. When so expressed, it is termed
specific inductive capacity. To be more accurate the specific inductive
capacity of a dielectric is the ratio between the capacity of a condenser
having that substance as a dielectric, to the capacity of the same condenser
using dry air at zero degrees Centigrade and at a pressure of 14.7 pounds
per square inch as the dielectric. To illustrate, if two condensers having
plates of equal size and equal distance apart are constructed, one using air
as the dielectric and the other using hard crown glass as the dielectric, the
one using glass will have a capacity of 6.96 times that of the one using air.
From this we say that crown glass has a specific inductive capacity of 6.96.

Various authorities differ rather widely as to the specific inductive
capacity of many common substances. The values given in Table VIII have
been chosen from the Smithsonian Physical Tables.

TABLE VIII

Specific Inductive Capacities

Dielectric Referred to Air as 1
Vacuum .99941
Hydrogen .99967
Carbonic Acid 1.00036
Dry Paper 1.25 to 1.75
Paraffin 1.95 to 2.32
Ebonite 1.9 to 3.48
Sulphur 2.24 to 3.90
Shellac 2.95 to 3.73
Gutta-percha 3.3 to 4.9
Plate Glass 3.31 to 7.5

Porcelain 4.38
Mica 4.6 to 8.0
Gutta-percha 3.3 to 4.9
Glass—Light Flint 6.61
Glass—Hard Crown 6.96
Selenium 10.2

This data is interesting as showing the wide divergence in specific
inductive capacities of various materials, and also showing the wide
divergence in different observations of the same material. Undoubtedly, this
latter is due mainly to the fact that various materials differ largely in
themselves, as in the case of paraffin, for instance, which exhibits widely
different specific inductive capacities according to the difference in rapidity
with which it is cooled in changing from a liquid to a solid state.

We see then that the capacity of a condenser varies as the area of its
plates, as the specific inductive capacity of the dielectric employed, and
also inversely as the distance between the plates.

Obviously, therefore, in making a condenser of large capacity, it is
important to have as large an area of the plate as possible; to have them as
close together as possible; to have the dielectric a good insulating medium
so that there will be practically no leakage between the plates; and to have
the dielectric of as high a specific inductive capacity as economy and
suitability of material in other respects will permit.

Dielectric Materials. Mica. Of all dielectrics mica is the most suitable
for condensers, since it has very high insulation resistance and also high
specific inductive capacity, and furthermore may be obtained in very thin
sheets. High-grade condensers, such as are used for measurements and
standardization purposes, usually have mica for the dielectric.

Fig. 121. Rolled Condenser
View full size illustration.

Dry Paper. The demands of telephonic practice are, however, such as to
require condensers of very cheap construction with large capacity in a small
space. For this purpose thin bond paper, saturated with paraffin, has been
found to be the best dielectric. The conductors in condensers are almost
always of tinfoil, this being an ideal material on account of its cheapness
and its thinness. Before telephony made such urgent demands for a cheap
compact condenser, the customary way of making them was to lay up
alternate sheets of dielectric material, either of oiled paper or mica and
tinfoil, the sheets of tinfoil being cut somewhat smaller than the sheets of
dielectric material in order that the proper insulation might be secured at the
edges. After a sufficient number of such plates were built up the alternate
sheets of tinfoil were connected together to form one composite plate of the
condenser, while the other sheets were similarly connected together to form
the other plate. Obviously, in this way a very large area of plates could be
secured with a minimum degree of separation.

Fig. 122. Rolled Condenser
View full size illustration.

There has been developed for use in telephony, however, and its use has
since extended into other arts requiring condensers, what is called the rolled
condenser. This is formed by rolling together in a flat roll four sheets of

thin bond paper, 1, 2, 3, and 4, and two somewhat narrower strips of tinfoil,
5 and 6, Fig. 121. The strips of tinfoil and paper are fed on to the roll in
continuous lengths and in such manner that two sheets of paper will lie
between the two strips of tinfoil in all cases. Thin sheet metal terminals 7
and 8 are rolled into the condenser as it is being wound, and as these project
beyond the edges of the paper they form convenient terminals for the
condenser after it is finished. After it is rolled, the roll is boiled in hot
paraffin so as to thoroughly impregnate it and expel all moisture. It is then
squeezed in a press and allowed to cool while under pressure. In this way
the surplus paraffin is expelled and the plates are brought very close
together. It then appears as in Fig. 122. The condenser is now sealed in a
metallic case, usually rectangular in form, and presents the appearance
shown in Fig. 123.

Fig. 123. Rolled Condenser
View full size illustration.

A later method of condenser making which has not yet been thoroughly
proven in practice, but which bids fair to produce good results, varies from
the method just described in that a paper is used which in itself is coated
with a very thin conducting material. This conducting material is of metallic
nature and in reality forms a part of the paper. To form a condenser of this
the sheets are merely rolled together and then boiled in paraffin and
compressed as before.

Sizes. The condensers ordinarily used in telephone practice range in
capacity from about 1/4 microfarad to 2 microfarads. When larger

capacities than 2 microfarads are desired, they may be obtained by
connecting several of the smaller size condensers in multiple. Table IX
gives the capacity, shape, and dimensions of a variety of condensers
selected from those regularly on the market.

TABLE IX

Condenser Data

Dimensions in Inches
Capacity Shape
Height Width Thickness
2 m. f. Rectangular 9–1/6 4–3/4 11/16
1 m. f. " 9–1/6 4–3/4 11/16
1 m. f. " 4–3/4 2–3/32 13/16
1/2 m. f. " 2–3/4 1–1/4 3/4
1 m. f. " 4–13/16 2–1/32 25/32
1/2 m. f. " 4–3/4 2–3/32 13/16
3/10 m. f. " 4–3/4 2–3/32 13/16
1 m. f. " 2–3/4 3 1

Conventional Symbols. The conventional symbols usually employed to
represent condensers in telephone diagrams are shown in Fig. 124. These all
convey the idea of the adjacent conducting plates separated by insulating
material.

Fig. 124. Condenser Symbols
View full size illustration.

Functions. Obviously, when placed in a circuit a condenser offers a
complete barrier to the flow of direct current, since no conducting path
exists between its terminals, the dielectric offering a very high insulation
resistance. If, however, the condenser is connected across the terminals of a
source of alternating current, this current flows first in one direction and
then in the other, the electromotive force in the circuit increasing from zero
to a maximum in one direction, and then decreasing back to zero and to a

maximum in the other direction, and so on. With a condenser connected so
as to be subjected to such alternating electromotive forces, as the
electromotive force begins to rise the electromotive force at the condenser
terminals will also rise and a current will, therefore, flow into the
condenser. When the electromotive force reaches its maximum, the
condenser will have received its full charge for that potential, and the
current flow into it will cease. When the electromotive force begins to fall,
the condenser can no longer retain its charge and a current will, therefore,
flow out of it. Apparently, therefore, there is a flow of current through the
condenser the same as if it were a conductor.

Means for Assorting Currents. In conclusion, it is obvious that the
telephone engineer has within his reach in the various coils—whether non-
inductive or inductive, or whether having one or several windings—and in
the condenser, a variety of tools by which he may achieve a great many
useful ends in his circuit work. Obviously, the condenser affords a means
for transmitting voice currents or fluctuating currents, and for excluding
steady currents. Likewise the impedance coil affords a means for readily
transmitting steady currents but practically excluding voice currents or
fluctuating currents. By the use of these very simple devices it is possible to
sift out the voice currents from a circuit containing both steady and
fluctuating currents, or it is possible in the same manner to sift out the
steady currents and to leave the voice currents alone to traverse the circuit.

Great use is made in the design of telephone circuits of the fact that the
electromagnets, which accomplish the useful mechanical results in causing
the movement of parts, possess the quality of impedance. Thus, the magnets
which operate various signaling relays at the central office are often used
also as impedance coils in portions of the circuit through which it is desired
to have only steady currents pass. If, on the other hand, it is necessary to
place a relay magnet, having considerable impedance, directly in a talking
circuit, the bad effects of this on the voice currents may be eliminated by
shunting this coil with a condenser, or with a comparatively high non-
inductive resistance. The voice currents will flow around the high
impedance of the relay coil through the condenser or resistance, while the
steady currents, which are the ones which must be depended upon to

operate the relay, are still forced in whole or in part to pass through the
relay coil where they belong.

In a similar way the induction coil affords a means for keeping two
circuits completely isolated so far as the direct flow of current between
them is concerned, and yet of readily transmitting, by electromagnetic
induction, currents from one of these circuits to the other. Here is a means
of isolation so far as direct current is concerned, with complete
communication for alternating current.

ToC

CHAPTER XIII
CURRENT SUPPLY TO TRANSMITTERS

The methods by which current is supplied to the transmitter of a
telephone for energizing it, may be classified under two divisions: first,
those where the battery or other source of current is located at the station
with the transmitter which it supplies; and second, those where the battery
or other source of current is located at a distant point from the transmitter,
the battery in such cases serving as a common source of current for the
supply of transmitters at a number of stations.

The advantages of putting the transmitter and the battery which supplies
it with current in a local circuit with the primary of an induction coil, and
placing the secondary of the induction coil in the line, have already been
pointed out but may be briefly summarized as follows: When the
transmitter is placed directly in the line circuit and the line is of
considerable length, the current which passes through the transmitter is
necessarily rather small unless a battery of high potential is used; and,
furthermore, the total change in resistance which the transmitter is capable
of producing is but a small proportion of the total resistance of the line, and,
therefore, the current changes produced by the transmitter are relatively
small. On the other hand, when the transmitter is placed in a local circuit
with the battery, this circuit may be of small resistance and the current
relatively large, even though supplied by a low-voltage battery; so that the
transmitter is capable of producing relatively large changes in a relatively
large current.

To draw a comparison between these two general classes of transmitter
current supply, a number of cases will be considered in connection with the
following figures, in each of which two stations connected by a telephone
line are shown. Brief reference to the local battery method of supplying
current will be made in order to make this chapter contain, as far as

possible, all of the commonly used methods of current supply to
transmitters.

Local Battery. In Fig. 125 two stations are shown connected by a
grounded line wire. The transmitter of each station is included in a low-
resistance primary circuit including a battery and the primary winding of an
induction coil, the relation between the primary circuits and the line circuits
being established by the inductive action between the primary and the
secondary windings of induction coils, the secondary in each case being in
the line circuits with the receivers.

Fig. 125. Local-Battery Stations with Grounded Circuit
View full size illustration.

Fig. 126 shows exactly the same arrangement but with a metallic circuit
rather than a grounded circuit. The student should become accustomed to
the replacing of one of the line wires of a metallic circuit by the earth, and
to the method, employed in Figs. 125 and 126, of indicating a grounded
circuit as distinguished from a metallic circuit.

Fig. 126. Local-Battery Stations with Metallic Circuit
View full size illustration.

In Fig. 127 is shown a slight modification of the circuit shown in Fig.
126, which consists of connecting one end of the primary winding to one
end of the secondary winding of the induction coil, thus linking together the

primary circuit and the line circuit, a portion of each of these circuits being
common to a short piece of the local wiring. There is no difference
whatever in the action of the circuits shown in Figs. 126 and 127, the latter
being shown merely for the purpose of bringing out this fact. It is very
common, particularly in local-battery circuits, to connect one end of the
primary and the secondary windings, as by doing so it is often possible to
save a contact point in the hook switch and also to simplify the wiring.

Fig. 127. Local-Battery Stations with Metallic Circuit
View full size illustration.

The advantages to be gained by employing a local battery at each
subscriber's station associated with the transmitter in the primary circuit of
an induction coil are attended by certain disadvantages from a commercial
standpoint. The primary battery is not an economical way to generate
electric energy. In all its commercial forms it involves the consumption of
zinc and zinc is an expensive fuel. The actual amount of current in watts
required by a telephone is small, however, and this disadvantage due to the
inexpensive method of generating current would not in itself be of great
importance. A more serious objection to the use of local batteries at
subscribers' stations appears when the subject is considered from the
standpoint of maintenance. Batteries, whether of the so-called "dry" or
"wet" type, gradually deteriorate, even when not used, and in cases where
the telephone is used many times a day the deterioration is comparatively
rapid. This makes necessary the occasional renewals of the batteries with
the attendant expense for new batteries or new material, and of labor and
transportation in visiting the station. The labor item becomes more serious
when the stations are scattered in a sparsely settled community, in which
case the visiting of the stations, even for the performance of a task that
would require but a few minutes' time, may consume some hours on the
part of the employes in getting there and back.

Common Battery. Advantages. It would be more economical if all of
the current for the subscribers' transmitters could be supplied from a single
comparatively efficient generating source instead of from a multitude of
inefficient small sources scattered throughout the community served by the
exchange. The advantage of such centralization lies not only in more
economic generating means, but also in having the common source of
current located at one place, where it may be cared for with a minimum
amount of expense. Such considerations have resulted in the so-called
"common-battery system," wherein the current for all the subscribers'
transmitters is furnished from a source located at the central office.

Where such a method of supplying current is practiced, the result has
also been, in nearly all cases, the doing away with the subscriber's magneto
generators, relying on the central-office source of current to furnish the
energy for enabling the subscriber to signal the operator. Such systems,
therefore, concentrate all of the sources of energy at the central office and
for that reason they are frequently referred to as central-energy systems.

NOTE. In this chapter the central-energy or common-battery
system will be considered only in so far as the supply of current for
energizing the subscribers' transmitters is concerned, the discussion of
the action of signaling being reserved for subsequent chapters.

Series Battery. If but a single pair of lines had to be considered, the
arrangement shown in Fig. 128 might be employed. In this the battery is
located at the central office and placed in series with the two grounded lines
leading from the central office to the two subscribers' stations. The voltage
of this battery is made sufficient to furnish the required current over the
resistance of the entire line circuit with its included instruments. Obviously,
changes in resistance in the transmitter at Station A will affect the flow of
current in the entire line and the fluctuations resulting from the vibration of
the transmitter diaphragm will, therefore, reproduce these sounds in the
receiver at Station B, as well as in that at Station A.

Fig. 128. Battery in Series with Two Lines
View full size illustration.

An exactly similar arrangement applied to a metallic circuit is shown in
Fig. 129. In thus placing the battery in series in the circuit between the two
stations, as shown in Figs. 128 and 129, it is obvious that the transmitter at
each station is compelled to vary the resistance of the entire circuit
comprising the two lines in series, in order to affect the receiver at distant
stations. This is in effect making the transmitter circuit twice as long as is
necessary, as will be shown in the subsequent systems considered.
Furthermore, the placing of the battery in series in the circuit of the two
combined lines does not lend itself readily to the supply of current from a
common source to more than a single pair of lines.

Fig. 129. Battery in Series with Two Lines
View full size illustration.

Series Substation Circuit. The arrangement at the substations—
consisting in placing the transmitter and the receiver in series in the line
circuit, as shown in Figs. 128 and 129—is the simplest possible one, and
has been used to a considerable extent, but it has been subject to the serious
objection, where receivers having permanent magnets were used, of making
it necessary to so connect the receiver in the line circuit that the steady
current from the battery would not set up a magnetization in the cores of the
receiver in such a direction as to neutralize or oppose the magnetization of
the permanent magnets. As long as the current flowed through the receiver
coils in such a direction as to supplement the magnetization of the
permanent magnets, no harm was usually done, but when the current flowed

through the receiver coils in such a way as to neutralize or oppose the
magnetizing force of the permanent magnets, the action of the receiver was
greatly interfered with. As a result, it was necessary to always connect the
receivers in the line circuit in a certain way, and this operation was called
poling.

In order to obviate the necessity for poling and also to bring about other
desirable features, it has been, until recently, almost universal practice to so
arrange the receiver that it would be in the circuit of the voice currents
passing over the line, but would not be traversed by direct currents, this
condition being brought about by various arrangements of condensers,
impedance coils, or induction coils, as will be shown later. During the year
1909, however, the adoption by several concerns of the so-called "direct-
current" receiver has made it necessary for the direct current to flow
through the receiver coils in order to give the proper magnetization to the
receiver cores, and this has brought about a return to the very simple form
of substation circuit, which includes the receiver and the transmitter directly
in the circuit of the line. This illustrates well an occurrence that is
frequently observed by those who have opportunity to watch closely the
development of an art. At one time the conditions will be such as to call for
complicated arrangements, and for years the aim of inventors will be to
perfect these arrangements; then, after they are perfected, adopted, and
standardized, a new idea, or a slight alteration in the practice in some other
respect, will demand a return to the first principles and wipe out the
necessity for the things that have been so arduously striven for.

Fig. 130. Bridging Battery with Repeating Coil
View full size illustration.

Bridging Battery with Repeating Coil. As pointed out, the placing of the
battery in series in the line circuit in the central office is not desirable, and,
so far as we are aware, has never been extensively used. The universal

practice, therefore, is to place it in a bridge path across the line circuit, and
a number of arrangements employing this basic idea are in wide use. In Fig.
130 is shown the standard arrangement of the Western Electric Company,
employed by practically all the Bell operating companies. In this the battery
at the central office is connected in the middle of the two sides of a
repeating coil so that the current from the battery is fed out to the two
connected lines in multiple.

Referring to the middle portion of this figure showing the central-office
apparatus, 1 and 2 may be considered as the two halves of one side of a
repeating coil divided so that the battery may be cut into their circuit.
Likewise, 3 and 4 may be considered as the two halves of the other side of
the repeating coil similarly divided for the same purpose. The windings of
this repeating coil are ordinarily alike; that is, 1 and 2 combined have the
same resistance, number of turns, and impedance as 3 and 4 combined. The
two sides of this coil are alternately used as primary and secondary, 1 and 2
forming the primary when Station A is talking, and 3 and 4, the secondary;
and vice versâ when Station B is talking.

As will be seen, the current flowing from the positive pole of the battery
will divide and flow through the windings 2 and 4; thence over the upper
limb of each line, through the transmitter at each station, and back over the
lower limbs of the line, through the windings 1 and 3, where the two paths
reunite and pass to the negative pole of the battery. It is evident that when
neither transmitter is being used the current flowing through both lines will
be a steady current and that, therefore, neither line will have an inductive
effect on the other. When, however, the transmitter at Station A is used the
variations in the resistance caused by it will cause undulations in the
current. These undulations, passing through the windings 1 and 2 of the
repeating coil, will cause, by electromagnetic induction, alternating currents
to flow in the windings 3 and 4, and these alternating currents will be
superimposed on the steady currents flowing in that line and will affect the
receiver at Station B, as will be pointed out. The reverse conditions exist
when Station B is talking.

Bell Substation Arrangement. The substation circuits at the stations in
Fig. 130 are illustrative of one of the commonly employed methods of

preventing the steady current from the battery from flowing through the
receiver coil. This particular arrangement is that employed by the common-
battery instruments of the various Bell companies. Considering the action at
Station B, it is evident that the steady current will pass through the
transmitter and through the secondary winding of the induction coil, and
that as long as this current is steady no current will flow through the
telephone receiver. The receiver, transmitter, and primary winding of the
induction coil are, however, included in a local circuit with the condenser.
The presence of the condenser precludes the possibility of direct current
flowing in this path. Considering Station A as a receiving station, it is
evident that the voice currents coming to the station over the line will pass
through the secondary winding and will induce alternating currents in the
primary winding which will circulate through the local circuit containing
the receiver and the condenser, and thus actuate the receiver. The
considerations are not so simple when the station is being treated as a
transmitting station. Under this condition the steady current passes through
the transmitter in an obvious manner. It is clear that if the local circuit
containing the receiver did not exist, the circuit would be operative as a
transmitting circuit because the transmitter would produce fluctuations in
the steady current flowing in the line and thus be able to affect the distant
station. The transmitter, therefore, has a direct action on the currents
flowing in the line by the variation in resistance which it produces in the
line circuit. There is, however, a subsidiary action in this circuit. Obviously,
there is a drop of potential across the transmitter terminals due to the flow
of steady current. This means that the upper terminal of the condenser will
be charged to the same potential as the upper terminal of the transmitter,
while the lower terminal of the condenser will be of the same potential as
the lower terminal of the transmitter. When, now, the transmitter varies its
resistance, a variation in the potential across its terminals will occur; and as
a result, a variation in potential across the terminals of the condenser will
occur, and this means that alternating currents will flow through the primary
winding of the induction coil. The transmitter, therefore, by its action,
causes alternating currents to flow through the primary of this induction coil
and it causes, by direct action on the circuit of the line, fluctuations in the
steady current flowing in the line. The alternating currents flowing in the
primary of the coil induce currents in the secondary of the coil which
supplement and augment the fluctuations produced by the direct action of

the transmitter. This circuit may be looked at, therefore, in the light of
combining the direct action which the transmitter produces in the current in
the line with the action which the transmitter produces in the local circuit
containing the primary of the induction coil, this action being repeated in
the line circuit through the secondary of the induction coil.

The receiver in this circuit is placed in the local circuit, and is thus not
traversed by the steady currents flowing in the line. There is thus no
necessity for poling it. This circuit is very efficient, but is subject to the
objection of producing a heavy side tone in the receiver of the transmitting
station. By "side tone" is meant the noises which are produced in the
receiver at a station by virtue of the action of the transmitter at that station.
Side tone is objectionable for several reasons: first, it is sometimes
annoying to the subscriber; second, and of more importance, the subscriber
who is talking, hearing a very loud noise in his own receiver, unconsciously
assumes that he is talking too loud and, therefore, lowers his voice,
sometimes to such an extent that it will not properly reach the distant
station.

Fig. 131. Bridging Battery with Impedance Coils
View full size illustration.

Bridging Battery with Impedance Coils. The method of feeding current
to the line from the common battery, shown in Fig. 130, is called the "split
repeating-coil" method. As distinguished from this is the impedance-coil
method which is shown in Fig. 131. In this the battery is bridged across the
circuit of the combined lines in series with two impedance coils, 1 and 2,
one on each side of the battery. The steady currents from the battery find
ready path through these impedance coils which are of comparatively low
ohmic resistance, and the current divides and passes in multiple over the
circuits of the two lines. Voice currents, however, originating at either one
of the stations, will not pass through the shunt across the line at the central

office on account of the high impedance offered by these coils, and as a
result they are compelled to pass on to the distant station and affect the
receiver there, as desired.

This impedance-coil method seems to present the advantage of greater
simplicity over the repeating-coil method shown in Fig. 130, and so far as
talking efficiency is concerned, there is little to choose between the two.
The repeating-coil method, however, has the advantage over this
impedance-coil method, because by it the two lines are practically divided
except by the inductive connection between the two windings, and as a
result an unbalanced condition of one of the connected lines is not as likely
to produce an unbalanced condition in the other as where the two lines are
connected straight through, as with the impedance-coil method. The
substation arrangement of Fig. 131 is the same as that of Fig. 130.

Fig. 132. Double-Battery Kellogg System
View full size illustration.

Double Battery with Impedance Coils. A modification of the impedance-
coil method is used in all of the central-office work of the Kellogg
Switchboard and Supply Company. This employs a combination of
impedance coils and condensers, and in effect isolates the lines
conductively from each other as completely as the repeating-coil method. It
is characteristic of all the Kellogg common-battery systems that they
employ two batteries instead of one, one of these being connected in all
cases with the calling line of a pair of connected lines and the other in all
cases with the called line. As shown in Fig. 132, the left-hand battery is
connected with the line leading to Station A through the impedance coils 1
and 2. Likewise, the right-hand battery is connected to the line of Station B
through the impedance coils 3 and 4. These four impedance coils are wound
on separate cores and do not have any inductive relation whatsoever with
each other. Condensers 5 and 6 are employed to completely isolate the lines

conductively. Current from the left-hand battery, therefore, passes only to
Station A, and current from the right-hand battery to Station B. Whenever
the transmitter at Station A is actuated the undulations of current which it
produces in the line cause a varying difference of potential across the
outside terminals of the two impedance coils 1 and 2. This means that the
two left-hand terminals of condensers 5 and 6 are subjected to a varying
difference of potential and these, of course, by electrostatic induction, cause
the right-hand terminals of these condensers to be subject to a
correspondingly varying difference of potential. From this it follows that
alternating currents will be impressed upon the right-hand line and these
will affect the receiver at Station B.

A rough way of expressing the action of this circuit is to consider it in
the same light as that of the impedance-coil circuit shown in Fig. 131, and
to consider that the voice currents originating in one line are prevented from
passing through the bridge paths at the central office on account of the
impedance, and are, therefore, forced to continue on the line, being allowed
to pass readily by the condensers in series between the two lines.

Kellogg Substation Arrangement. An interesting form of substation
circuit which is employed by the Kellogg Company in all of its common-
battery telephones is shown in Fig. 132. In passing, it may be well to state
that almost any of the substation circuits shown in this chapter are capable
of working with any of the central-office circuits. The different ones are
shown for the purpose of giving a knowledge of the various substation
circuits that are employed, and, as far as possible, to associate them with the
particular central-office arrangements with which they are commonly used.

In this Kellogg substation arrangement the line circuit passes first
through the transmitter and then divides, one branch passing through an
impedance coil 7 and the other through the receiver and the condenser 8, in
series. The steady current from the central-office battery finds ready path
through the transmitter and the impedance coil, but is prevented from
passing through the receiver by the barrier set up by the condenser 8. Voice
currents, however, coming over the line to the station, find ready path
through the receiver and the condenser but are barred from passing through
the impedance coil by virtue of its high impedance.

In considering the action of the station as a transmitting station, the
variations set up by the transmitter pass through the condenser and the
receiver at the same station, while the steady current which supplies the
transmitter passes through the impedance coil. Impedance coils used for this
purpose are made of low ohmic resistance but of a comparatively great
number of turns, and, therefore, present a good path for steady currents and
a difficult path for voice currents. This divided circuit arrangement
employed by the Kellogg Company is one of the very simple ways of
eliminating direct currents from the receiver path, at the same time allowing
the free passage of voice currents.

Fig. 133. Dean System
View full size illustration.

Dean Substation Arrangement. In marked contrast to the scheme for
keeping steady current out of the receiver circuit employed by the Kellogg
Company, is that shown in Fig. 133, which has been largely used by the
Dean Electric Company, of Elyria, Ohio. The central-office arrangement in
this case is that using the split repeating coil, which needs no further
description. The substation arrangement, however, is unique and is a
beautiful example of what can be done in the way of preventing a flow of
current through a path without in any way insulating that path or placing
any barrier in the way of the current. It is an example of the prevention of
the direct flow of current through the receiver by so arranging the circuits
that there will always be an equal potential on each side of it, and, therefore,
no tendency for current to flow through it.

In this substation arrangement four coils of wire—1, 2, 3, and 4—are so
arranged as to be connected in the circuit of the line, two in series and two
in multiple. The current flowing from the battery at the central office, after
passing through the transmitter, divides between the two paths containing,
respectively, the coils 1 and 3 and the coils 2 and 4. The receiver is

connected between the junction of the coils 2 and 4 and that of 1 and 3. The
resistances of the coils are so chosen that the drop of potential through the
coil 2 will be equal to that through the coil 1, and likewise that through the
coil 4 will be equal to that through the coil 3. As a result, the receiver will
be connected between two points of equal potential, and no direct current
will flow through it. How, then, do voice currents find their way through the
receiver, as they evidently must, if the circuit is to fulfill any useful
function? The coils 2 and 3 are made to have high impedance, while 1 and 4
are so wound as to be non-inductive and, therefore, offer no impedance save
that of their ohmic resistance. What is true, therefore, of direct currents does
not hold for voice currents, and as a result, the voice currents, instead of
taking the divided path which the direct currents pursued, are debarred from
the coils 2 and 3 by their high impedance and thus pass through the non-
inductive coil 1, the receiver, and the non-inductive coil 4.

This circuit employs a Wheatstone-bridge arrangement, adjusted to a
state of balance with respect to direct currents, such currents being excluded
from the receiver, not because the receiver circuit is in any sense opaque to
such direct currents, but because there is no difference of potential between
the terminals of the receiver circuit, and, therefore, no tendency for current
to flow through the receiver. In order that fluctuating currents may not, for
the same reason, be caused to pass by, rather than through, the receiver
circuit, the diametrically-opposed arms of the Wheatstone bridge are made
to possess, in large degree, self-induction, thereby giving these two arms a
high impedance to fluctuating currents. The conditions which exist for
direct currents do not, therefore, exist for fluctuating currents, and it is this
distinction which allows alternating currents to pass through the receiver
and at the same time excludes direct currents therefrom.

In practice, the coils 1, 2, 3, and 4 of the Dean substation circuit are
wound on the same core, but coils 1 and 4—the non-inductive ones—are
wound by doubling the wire back on itself so as to neutralize their self-
induction.

Stromberg-Carlson. Another modification of the central-office
arrangement and also of the subscribers' station circuits, is shown in Fig.
134, this being a simplified representation of the circuits commonly

employed by the Stromberg-Carlson Telephone Manufacturing Company.
The battery feed at the central office differs only from that shown in Fig.
132, in that a single battery rather than two batteries is used, the current
being supplied to one of the lines through the impedance coils 1 and 2, and
to the other line through the impedance coils 3 and 4; condensers 5 and 6
serve conductively to isolate the two lines. At the subscriber's station the
line circuit passes through the secondary of an induction coil and the
transmitter. The receiver is kept entirely in a local circuit so that there is no
tendency for direct current to flow through it, but it is receptive to voice
currents through the electromagnetic induction between the primary and the
secondary of the induction coil.

Fig. 134. Stromberg-Carlson System
View full size illustration.

Fig. 135. North Electric Company System
View full size illustration.

North. Another arrangement of central-office battery feed is employed
by the North Electric Company, and is shown in Fig. 135. In this two
batteries are used which supply current respectively to the two connected
lines, condensers being employed to conductively isolate the lines. This
differs from the Kellogg arrangement shown in Fig. 132 in that the two
coils 1 and 2 are wound on the same core, while the coils 3 and 4 are wound
together upon another core. In this case, in order that the inductive action of
one of the coils may not neutralize that of the other coil on the same core,
the two coils are wound in such relative direction that their magnetizing
influence will always be cumulative rather than differential.

The central-office arrangements discussed in Figs. 130 to 135, inclusive,
are those which are in principal use in commercial practice in common-
battery exchanges.

Current Supply over Limbs of Line in Parallel. As indicating further
interesting possibilities in the method of supplying current from a common
source to a number of substations, several other systems will be briefly
referred to as being of interest, although these have not gone into wide
commercial use. The system shown in Fig. 136 is one proposed by Dean in

the early days of common-battery working, and this arrangement was put
into actual service and gave satisfactory results, but was afterwards
supplanted by the Bell equipment operating under the system shown in Fig.
130, which became standardized by that company. In this the current from
the common battery at the central office is not fed over the two line wires in
series, but in multiple, using a ground return from the subscriber's station to
the central office. Across the metallic circuit formed by two connected lines
there is bridged, at the central office, an impedance coil 1, and between the
center point of this impedance coil and the ground is connected the
common battery. At the subscriber's station is placed an impedance coil 2,
also bridged across the two limbs of the line, and between the center point
of this impedance coil and the ground is connected the transmitter, which is
shunted by the primary winding of an induction coil. Connected between
the two limbs of the line at the substation there is also the receiver and the
secondary of an induction coil in series.

Fig. 136. Current Supply over Parallel Limbs of Line
View full size illustration.

The action of this circuit at first seems a little complex, but if taken step
by step may readily be understood. The transmitter supply circuit may be
traced from the central-office battery through the two halves of the
impedance coil 1 in multiple; thence over the two limbs of the line in
multiple to Station A, for instance; thence in multiple through the two
halves of impedance coil 2, to the center point of that coil; thence through
the two paths offered respectively by the primary of the induction coil and
by the transmitter; then to ground and back to the other pole of the central-
office battery. By this circuit the transmitter at the substation is supplied
with current.

Variations in the resistance of the transmitter when in action, cause
complementary variations in the supply current flowing through the primary

of the induction coil. These variations induce similar alternating currents in
the secondary of this coil, which is in series in the line circuit. The currents,
so induced in this secondary, flow in series through one side of the line to
the distant station; thence through the secondary and the receiver at that
station to the other side of the line and back through that side of the line to
the receiver. These currents are not permitted to pass through the bridged
paths across the metallic circuit that are offered by the impedance coils 1
and 2, because they are voice currents and are, therefore, debarred from
these paths by virtue of the impedance.

Fig. 137. Current Supply over Parallel Limbs of Line
View full size illustration.

An objection to this form of current supply and to other similar forms,
wherein the transmitter current is fed over the two sides of the line in
multiple with a ground return, is that the ground-return circuit formed by
the two sides of the line in multiple is subject to inductive disturbances
from other lines in the same way as an ordinary grounded line is subject to
inductive disturbance. The current-supply circuit is thus subject to external
disturbances and such disturbances find their way into the metallic circuit
and, therefore, through the instruments by means of the electromagnetic
induction between the primary and the secondary coils at the substations.

Another interesting method of current supply from a central-office
battery is shown in Fig. 137. This, like the circuit just considered, feeds the
energy to the subscriber's station over the two sides of the line in multiple
with a ground return. In this case, however, a local circuit is provided at the
substation, in which is placed a storage battery 1 and the primary 2 of an
induction coil, together with the transmitter. The idea in this is that the
current supply from the central office will pass through the storage battery
and charge it. Upon the use of the transmitter, this storage battery acts to
supply current to the local circuit containing the transmitter and the primary

coil 2 in exactly the same manner as in a local battery system. The
fluctuating current so produced by the action of the transmitter in this local
circuit acts on the secondary winding 3 of the induction coil, and produces
therein alternating currents which pass to the central office and are in turn
repeated to the distant station.

Supply Many Lines from Common Source. We come now to the
consideration of the arrangement by which a single battery may be made to
supply current at the central office to a large number of pairs of connected
lines simultaneously. Up to this point in this discussion it has been shown
only how each battery served a single pair of connected lines and no others.

Repeating Coil:—In Fig. 138 is shown how a single battery supplies
current simultaneously to four different pairs of lines, the lines of each pair
being connected for conversation. It is seen that the pairs of lines shown in
this figure are arranged in each case in accordance with the system shown
in Fig. 130. Let us inquire why it is that, although all of these four pairs of
lines are connected with a common source of energy and are, therefore, all
conductively joined, the stations will be able to communicate in pairs
without interference between the pairs. In other words, why is it that voice
currents originating at Station A will pass only to the receiver at Station B
and not to the receivers at Station C or Station H, for instance? The reason
is that separate supply conductors lead from the points such as 1 and 2 at
the junctions of the repeating-coil windings on each pair of circuits to the
battery terminals, and the resistance and impedance of the battery itself and
of the common leads to it are so small that although the feeble voice
currents originating in the pair of lines connecting Station A and Station B
pass through the battery, they are not able to alter the potential of the battery
in any appreciable degree. As a result, therefore, the supply wires leading
from the common-battery terminals to the points 7 and 8, for instance,
cannot be subjected to any variations in potential by virtue of currents
flowing through the battery from the points 1 and 2 of the lines joining
Station A and Station B.

Fig. 138. Common Source for Many Lines
View full size illustration.

Fig. 139. Common Source for Many Lines
View full size illustration.

Retardation Coil—Single Battery:—In Fig. 139 is shown in similar
manner the current supply from a single battery to four different pairs of
lines, the battery being associated with the lines by the combined
impedance coil and condenser method, which was specifically dealt with in
connection with Fig. 133. The reasons why there will be no interference
between the conversations carried on in the various pairs of connected lines

in this case are the same as those just considered in connection with the
system shown in Fig. 138. The impedance coils in this case serve to keep
the telephone currents confined to their respective pairs of lines in which
they originate, and this same consideration applies to the system of Fig.
138, for each of the separate repeating-coil windings of Fig. 138 is in itself
an impedance coil with respect to such currents as might leak away from
one pair of lines on to another.

Retardation Coil—Double Battery:—The arrangement of feeding a
number of pairs of lines according to the Kellogg two-battery system is
indicated in Fig. 140, which needs no further explanation in view of the
description of the preceding figures. It is interesting to note in this case that
the left-hand battery serves only the left-hand lines and the right-hand
battery only the right-hand lines. As this is worked out in practice, the left-
hand battery is always connected to those lines which originate a call and
the right-hand battery always to those lines that are called for. The energy
supplied to a calling line is always, therefore, from a different source than
that which supplies a called line.

Fig. 140. Two Sources for Many Lines
View full size illustration.

Fig. 141. Current Supply from Distant Point
View full size illustration.

Current Supply from Distant Point. Sometimes it is convenient to supply
current to a group of lines centering at a certain point from a source of
current located at a distant point. This is often the case in the so-called
private branch exchange, where a given business house or other institution
is provided with its own switchboard for interconnecting the lines leading to
the various telephones of that concern or institution among themselves, and
also for connecting them with lines leading to the city exchange. It is not
always easy or convenient to maintain at such private switchboards a
separate battery for supplying the current needed by the local exchange.

In such cases the arrangement shown in Fig. 141 is sometimes
employed. This shows two pairs of lines connected by the impedance-coil
system with common terminals 1 and 2, between which ordinarily the
common battery would be connected. Instead of putting a battery between
these terminals, however, at the local exchange, a condenser of large
capacity is connected between them and from these terminals circuit wires 3
and 4 are led to a battery of suitable voltage at a distant central office. The
condenser in this case is used to afford a short-circuit path for the voice
currents that leak from one side of one pair of lines to the other, through the
impedance coils bridged across the line. In this way the effect of the
necessarily high resistance in the common leads 3 and 4, leading to the

storage battery, is overcome and the tendency to cross-talk between the
various pairs of connected lines is eliminated. Frequently, instead of
employing this arrangement, a storage battery of small capacity will be
connected between the terminals 1 and 2, instead of the condenser, and
these will be charged over the wires 3 and 4 from a source of current at a
distant point.

A consideration of the various methods of supplying current from a
common source to a number of lines will show that it is essential that the
resistance of the battery itself be very low. It is also necessary that the
resistance and the impedance of the common leads from the battery to the
point of distribution to the various pairs of lines be very low, in order that
the voice currents which flow through them, by virtue of the conversations
going on in the different pairs of lines, shall not produce any appreciable
alteration in the difference of potential between the battery terminals.

ToC

CHAPTER XIV
THE TELEPHONE SET

We have considered what may be called the elemental parts of a
complete telephone; that is, the receiver, transmitter, hook switch, battery,
generator, call bell, condenser, and the various kinds of coils which go to
make up the apparatus by which one is enabled to transmit and receive
speech and signals. We will now consider the grouping of these various
elements into a complete working organization known as a telephone.

Before considering the various types it is well to state that the term
telephone is often rather loosely used. We sometimes hear the receiver
proper called a telephone or a hand telephone. Since this was the original
speaking telephone, there is some reason for so calling the receiver. The
modern custom more often applies the term telephone to the complete
organization of talking and signaling apparatus, together with the associated
wiring and cabinet or standard on which it is mounted. The name telephone
set is perhaps to be preferred to the word telephone, since it tends to avoid
misunderstanding as to exactly what is meant. Frequently, also, the
telephone or telephone set is referred to as a subscriber's station equipment,
indicating the equipment that is to be found at a subscriber's station. This, as
applying to a telephone alone, is not proper, since the subscriber's station
equipment includes more than a telephone. It includes the local wiring
within the premises of the subscriber and also the lightning arrester and
other protective devices, if such exist.

To avoid confusion, therefore, the collection of talking and signaling
apparatus with its wiring and containing cabinet or standard will be referred
to in this work as a telephone or telephone set. The receiver will, as a rule,
be designated as such, rather than as a telephone. The term subscriber's
station equipment will refer to the complete equipment at a subscriber's
station, and will include the telephone set, the interior wiring, and the

protective devices, together with any other apparatus that may be associated
with the telephone line and be located within the subscriber's premises.

Classification of Sets. Telephones may be classified under two general
headings, magneto telephones and common-battery telephones, according
to the character of the systems in which they are adapted to work.

Magneto Telephone. The term magneto telephone, as it was originally
employed in telephony, referred to the type of instrument now known as a
receiver, particularly when this was used also as a transmitter. As the use of
this instrument as a transmitter has practically ceased, the term magneto
telephone has lost its significance as applying to the receiver, and, since
many telephones are equipped with magneto generators for calling
purposes, the term magneto telephone has, by common consent, come to be
used to designate any telephone including, as a part of its equipment, a
magneto generator. Magneto telephones usually, also, include local batteries
for furnishing the transmitter with current, and this has led to these
telephones being frequently called local battery telephones. However, a
local battery telephone is not necessarily a magneto telephone and vice
versâ, since sometimes magneto telephones have no local batteries and
sometimes local battery telephones have no magnetos. Nearly all of the
telephones which are equipped with magneto generators are, however, also
equipped with local batteries for talking purposes, and, therefore, the terms
magneto telephone and local battery telephone usually refer to the same
thing.

Common-Battery Telephone. Common-battery telephones, on the other
hand, are those which have no local battery and no magneto generator, all
the current for both talking and signaling being furnished from a common
source of current at the central office.

Wall and Desk Telephones. Again we may classify telephones or
telephone sets in accordance with the manner in which their various parts
are associated with each other for use, regardless of what parts are
contained in the set. We may refer to all sets adapted to be mounted on a
wall or partition as wall telephones, and to all in which the receiver,
transmitter, and hook are provided with a standard of their own to enable
them to rest on any flat surface, such as a desk or table, as desk telephones.

These latter are also referred to as portable telephones and as portable desk
telephones.

In general, magneto or local battery telephones differ from common-
battery telephones in their component parts, the difference residing
principally in the fact that the magneto telephone always has a magneto
generator and usually a local battery, while the common-battery telephone
has no local source of current whatever. On the other hand, the differences
between wall telephones and desk telephones are principally structural, and
obviously either of these types of telephones may be for common-battery or
magneto work. The same component parts go to make up a desk telephone
as a wall telephone, provided the two instruments are adapted for the same
class of service, but the difference between the two lies in the structural
features by which these same parts are associated with each other and
protected from exposure.

Fig. 142. Magneto Wall Set
View full size illustration.

Fig. 143. Magneto Wall Set
View full size illustration.

Magneto-Telephone Sets. Wall. In Fig. 142 is shown a familiar type of
wall set. The containing box includes within it all of the working parts of
the apparatus except that which is necessarily left outside in order to be
within the reach of the user. Fig. 143 shows the same set with the door
open. This gives a good idea of the ordinary arrangement of the apparatus
within. It is seen that the polarized bell or ringer has its working parts
mounted on the inside of the door or cover of the box, the tapper projecting
through so as to play between the gongs on the outside. Likewise the
transmitter arm, which supports the transmitter and allows its adjustment up
and down to accommodate itself to the height of the user, is mounted on the
front of the door, and the conductors leading to it may be seen fastened to
the rear of the door in Fig. 143.

In some wall sets the wires leading to the bell and transmitter are
connected to the wiring of the rest of the set through the hinges of the door,
thus allowing the door to be opened and closed repeatedly without breaking
off the wires. In order to always insure positive electrical contact between
the stationary and movable parts of the hinge a small wire is wound around
the hinge pin, one end being soldered to the stationary part and the other
end to the movable part of the hinge. In other forms of wall set the wires to
the bell and the transmitter lead directly from the stationary portion of the
cabinet to the back of the door, the wires being left long enough to have

sufficient flexibility to allow the door to be opened and closed without
injuring the wires.

At the upper portion of the box there is mounted the hook switch, this
being, in this case, of the short lever type. The lever of the hook projects
through the side of the box so as to make the hook available as a support for
the receiver. Immediately at the right of the hook switch is mounted the
induction coil, and immediately below this the generator, its crank handle
projecting through the right-hand side of the box so as to be available for
use there. The generator is usually mounted on a transverse shelf across the
middle of the cabinet, this shelf serving to form a compartment below it in
which the dry battery of two or three cells is placed.

The wall telephone-set cabinets have assumed a multitude of forms.
When wet cells rather than dry cells were ordinarily employed, as was the
case up to about the year 1895, the magneto generator, polarized bell, and
hook switch were usually mounted in a rectangular box placed at the top of
a long backboard. Immediately below this on the backboard was mounted
the transmitter arm, and sometimes the base of this included the induction
coil. Below this was the battery box, this being a large affair usually
adapted to accommodate two and sometimes three ordinary LeClanché cells
side by side.

The dry cell has almost completely replaced the wet cell in this country,
and as a result, the general type of wall set as shown in Figs. 142 and 143,
has gradually replaced the old wet-cell type, which was more cumbrous and
unsightly. It is usual on wall sets to provide some sort of a shelf, as
indicated in Fig. 142, for the convenience of the user in making notes and
memoranda.

Desk. In the magneto desk-telephone sets, the so-called desk stand,
containing the transmitter, the receiver, and the hook switch, with the
standard upon which they are mounted, is shown in Fig. 144. This desk
stand evidently does not comprise the complete equipment for a magneto
desk-telephone set, since the generator, polarized bell, and battery are
lacking. The generator and bell are usually mounted together in a box,
either on the under side of the desk of the user or on the wall within easy
reach of his chair. Connections are made between the apparatus in the desk

stand proper and the battery, generator, and bell by means of flexible
conducting cords, these carrying a plurality of conductors, as required by
the particular circuit of the telephone in question. Such a complete magneto
desk-telephone set is shown in Fig. 145, this being one of the types
manufactured by the Stromberg-Carlson Manufacturing Company.

Fig. 144. Desk Stand
View full size illustration.

A great variety of arrangements of the various parts of magneto desk-
telephone apparatus is employed in practice. Sometimes, as shown in Fig.
145, the magneto bell box is equipped with binding posts for terminating all
of the conductors in the cord, the line wires also running to some of these
binding posts.

In the magneto-telephone set illustrated the box is made large enough to
accommodate only the generator and call bell, and the batteries are mounted
elsewhere, as in a drawer of the desk, while in other cases there is no other
equipment but that shown in the cut, the batteries being mounted within the
magneto bell box itself. In still other cases, the polarized bell is contained in
one box, the generator in another, the batteries in the drawer of the desk, the
induction coil being mounted either in the base of the desk stand, in the bell
box, or in the generator box. In such cases all of the circuits of the various
scattered parts are wired to a terminal strip, located at some convenient

point, this strip containing terminals for all the wires leading from the
various parts and for the line wires themselves. By combining the various
wires on the terminals of this terminal strip, the complete circuits of the
telephone are built up. In still other cases the induction coil is mounted on
the terminal strip and separate wires or sets of wires are run to the polarized
bell and generator, to the desk stand itself, and to the batteries. These
various arrangements are subject largely to the desire or personal ideas of
the manufacturer or user. All of them work on the same principle so far as
the operation of the talking and signaling circuits is concerned.

Fig. 145. Magneto Desk Set
View full size illustration.

Circuits of Magneto-Telephone Sets. Magneto telephones, whether of
the wall or desk type, may be divided into two general classes, series and
bridging, according to whether the magnet of the bell is included in series
or bridge relation with the telephone line when the hook is down.

Series. In the so-called series telephone line, where several telephones
are placed in series in a single line circuit, the employment of the series
type of telephone results in all of the telephone bells being in series in the
line circuit. This means that the voice currents originating in the telephones
that are in use at a given time must pass in series through the magnets of the
bells of the stations that are not in use. In order that these magnets, through

which the voice currents must pass, may interfere to as small a degree as
possible with the voice currents, it is common to employ low-resistance
magnets in series telephones, these magnets being wound with
comparatively few turns and on rather short cores so that the impedance
will be as small as possible. Likewise, since the generators are required to
ring all of the bells in series, they need not have a large current output, but
must have sufficient voltage to ring through all of the bells in series and
through the resistance of the line. For this reason the generators are usually
of the three-bar type and sometimes have only two bars.

In Fig. 146 are shown, in simplified form, the circuits of an ordinary
series telephone. The receiver in this is shown as being removed from the
hook and thus the talking apparatus is brought into play. The line wires 1
and 2 connect respectively to the binding posts 3 and 4 which form the
terminals of the instrument. When the hook is up, the circuit between the
binding posts 3 and 4 includes the receiver and the secondary winding of
the induction coil, together with one of the upper contacts 5 of the switch
hook and the hook lever itself. This completes the circuit for receiving
speech. The hook switch is provided with another upper contact 6, between
which and the contact 5 is connected the local circuit containing the
transmitter, the battery, and the primary of the induction coil in series. The
primary and the secondary windings are connected together at one end and
connected with the switch contact 5, as shown. It is thus seen that when the
hook is up the circuit through the receiver is automatically closed and also
the local circuit containing the primary, the battery, and the transmitter.
Thus, all the conditions for transmitting and receiving speech are fulfilled.

Fig. 146. Circuit of Series Magneto Set
View full size illustration.

When the hook is down, however, the receiving and transmitting circuits
are broken, but another circuit is completed by the engagement of the hook-
switch lever with the lower hook contact 7. Between this contact and one
side of the line is connected the polarized ringer and the generator. With the
hook down, therefore, the circuit may be traced from the line wire 1 to
binding post 3, thence through the generator shunt to the call bell, and
thence through the lower switching contact 7 to the binding post 4 and line
wire 2. The generator shunt, as already described in Chapter VIII, normally
keeps the generator shunted out of circuit. When, however, the generator is
operated the shunt is broken, which allows the armature of the generator to
come into the circuit in series with the winding of the polarized bell. The
normal shunting of the generator armature from the circuit of the line is
advantageous in several ways. In the first place, the impedance of the
generator winding is normally cut out of the circuit so that in the case of a
line with several stations the talking or voice currents do not have to flow
through the generator armatures at the stations which are not in use. Again,
the normal shunting of the generator tends to save the generator armature
from injury by lightning.

Fig. 147. Circuit of Series Magneto Set
View full size illustration.

The more complete circuits of a series magneto telephone are shown in
Fig. 147. In this the line binding posts are shown as 1 and 2. At the bottom
of the telephone cabinet are four other binding posts marked 3, 4, 5, and 6.
Of these 3 and 4 serve for the receiver terminals and 5 and 6 for the
transmitter and battery terminals. The circuits of this diagram will be found
to be essentially the same as those of Fig. 146, except that they are shown in
greater detail. This particular type of circuit is one commonly employed
where the generator, ringer, hook switch, and induction coil are all mounted
in a so-called magneto bell box at the top of the instrument, and where the
transmitter is mounted on an arm just below this box, and the battery in a
separate compartment below the transmitter. The only wiring that has to be
done between the bell box and the other parts of the instrument in
assembling the complete telephone is to connect the receiver to the binding
posts 3 and 4 and to connect the battery and transmitter circuit to the
binding posts 5 and 6.

Bridging. In other cases, where several telephones are placed on a
single-line circuit, the bells are arranged in multiple across the line. For this
reason their magnets are wound with a very great number of turns and
consequently to a high resistance. In order to further increase the
impedance, the cores are made long and heavy. Since the generators on
these lines must be capable of giving out a sufficient volume of current to
divide up between all of the bells in multiple, it follows that these
generators must have a large current output, and at the same time a
sufficient voltage to ring the bells at the farthest end of the line. Such
instruments are commonly called bridging instruments, on account of the
method of connecting their bells across the circuit of the line.

Fig. 148. Circuit of Bridging Magneto Set
View full size illustration.

The fundamental characteristic of the bridging telephone is that it
contains three possible bridge paths across the line wires. The first of these
bridge paths is through the talking apparatus, the second through the
generator, and the third through the ringer. This is shown in simplified form
in Fig. 148. The talking apparatus is associated with the two upper contacts
of the hook switch in the usual manner and needs no further description.
The generator is the second separate bridge path, normally open, but
adapted to be closed when the generator is operated, this automatic closure
being performed by the movement of the crank shaft. The third bridge
contains the polarized bell, and this, as a rule, is permanently closed.
Sometimes, however, the arrangement is such that the bell path is normally
closed through the switch which is operated by the generator crank shaft,
and this path is automatically broken when the generator is operated, at
which time, also, the generator path is automatically closed. This
arrangement brings about the result that the generator never can ring its
own bell, because its switch always operates to cut out the bell at its own
station just before the generator itself is cut into the circuit.

In Fig. 149 is shown the complete circuit of a bridging telephone. The
circuit given in this figure is for a local-battery wall set similar in type to
that shown in Figs. 142 and 143. A simplified diagrammatic arrangement is
shown in the lower left-hand corner of this figure, and from a consideration
of this it will be seen that the bell circuit across the line is normally
completed through the two right-hand normally closed contacts of the
switch on the generator. When, however, the generator is operated these two
contacts are made to disengage each other while the long spring of the
generator switch engages the left-hand spring and thus brings the generator
itself into the circuit.

Fig. 149. Circuit of Bridging Magneto Set
View full size illustration.

Of the three binding posts, 1, 2, and 3, at the top of Fig. 149, 1 and 2 are
for connecting with the line wires, while 8 is for a ground connection,
acting in conjunction with the lightning arrester mounted at the top of the
telephone and indicated at 4 in Fig. 149. This has no function in talking or
ringing, and will be referred to more fully in Chapter XIX. Suffice it to say
at this point that these arresters usually consist of two conducting bodies,
one connected permanently to each of the line binding posts, and a third
conducting body connected to the ground binding post. These three
conducting bodies are in close proximity but carefully insulated from each
other; the idea being that when the line wires are struck by lightning or
subjected otherwise to a dangerous potential, the charge on the line will
jump across the space between the conducting bodies and pass harmlessly
to ground.

NOTE. The student should practice making simplified diagrams
from actual wiring diagrams. The difference between the two is that
one is laid out for ease in understanding it, while the other is laid out to
show the actual course of the wires as installed.

If the large detailed circuit of Fig. 149 be compared with the small
theoretical circuit in the same figure, the various conducting paths will be
found to be the same. Such a simplified circuit does more to enable one to
grasp the fundamental scheme of a complex circuit than much description,

since it shows at a glance the general arrangement. The more detailed
circuits are, however, necessary to show the actual paths followed by the
wiring.

The circuits of desk stands do not differ from those of wall sets in any
material degree, except as may be necessitated by the fact that the various
parts of the telephone set are not all mounted in the same cabinet or on the
same standard. To provide for the necessary relative movement between the
desk stand and the other portions of the set, flexible conductors are run
from the desk stand itself to the stationary portions of the equipment, such
as the battery and the parts contained in the generator and bell box.

Fig. 150. Circuit of Bridging Magneto Desk Set
View full size illustration.

In Fig. 150 is shown the circuit of the Stromberg-Carlson magneto desk-
telephone set, illustrated in Fig. 145. This diagram needs no explanation in
view of what has already been said. The conductors, leading from the desk-
stand group of apparatus to the bell-box group of apparatus, are grouped
together in a flexible cord, as shown in Fig. 145, and are connected
respectively to the various binding posts or contact points within the desk
stand at one end and at the base of the bell box at the other end. These
flexible conductors are insulated individually and covered by a common
braided covering. They usually are individualized by having a colored
thread woven into their insulating braid, so that it is an easy matter to

identify the two ends of the same conductor at either end of the flexible
cord or cable.

Fig. 151. Common-Battery Wall Set
View full size illustration.

Fig. 152. Common-Battery Wall Set
View full size illustration.

Common-Battery Telephone Sets. Owing to the fact that common-
battery telephones contain no sources of current, they are usually somewhat
simpler than the magneto type. The component parts of a common-battery
telephone, whether of the wall or desk type, are the transmitter, receiver,
hook switch, polarized bell, condenser, and sometimes an induction coil.
The purpose of the condenser is to prevent direct or steady currents from
passing through the windings of the ringer while the ringer is connected
across the circuit of the line during the time when the telephone is not in
use. The requirements of common-battery signaling demand that the ringer
shall be connected with the line so as to be receptive of a call at any time
while the telephone is not in use. The requirements also demand that no
conducting path shall normally exist between the two sides of the line.
These two apparently contradictory requirements are met by placing a
condenser in series with the ringer so that the ringer will be in a path that
will readily transmit the alternating ringing currents sent out from the
central-office generator, while at the same time the condenser will afford a
complete bar to the passage of steady currents. Sometimes the condenser is
also used as a portion of the talking apparatus, as will be pointed out.

Wall. In Figs. 151 and 152 are given two views of a characteristic form
of common-battery wall-telephone set, made by the Stromberg-Carlson
Manufacturing Company. The common-battery wall set has usually taken
this general form. In it the transmitter is mounted on an adjustable arm at
the top of the backboard, while the box containing the bell and all working
parts of the instrument is placed below the transmitter, the top of the box
affording a shelf for writing purposes. In Fig. 151 are shown the hook
switch and the receiver; just below these may be seen the magnets of the
polarized bell, back of which is shown a rectangular box containing the
condenser. Immediately in front of the ringer magnets is the induction coil.

Fig. 153. Stromberg-Carlson Common-Battery Wall Set
View full size illustration.

In Fig. 153 are shown the details of the circuit of this instrument. This
figure also includes a simplified circuit arrangement from which the
principles involved may be more readily understood. It is seen that the
primary of the induction coil and the transmitter are included in series
across the line. The secondary of the induction coil, in series with the
receiver, is connected also across the line in series with a condenser and the
transmitter.

Hotel. Sometimes, in order to economize space, the shelf of common-
battery wall sets is omitted and the entire apparatus mounted in a small
rectangular box, the front of which carries the transmitter mounted on the
short arm or on no arm at all. Such instruments are commonly termed hotel