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[Transcriber's Note: References to page numbers in table of contents
and index removed, as well as the numbers themselves.]

[Illustration: ALEXANDER GRAHAM BELL The Inventor of the Telephone.]


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





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."

[Illustration: A TYPICAL SMALL MAGNETO SWITCHBOARD INSTALLATION]

[Illustration: A TYPICAL CENTRAL OFFICE FOR RURAL EXCHANGE Line
Protectors on Wall at Left.]




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]

Acoustics--Characteristics of Sound--Loudness--Pitch--Vibration of
Diaphragms--Timbre--Human Voice--Human Ear--Speech--Magneto
Telephones--Loose-Contact Principle--Induction Coils--Simple Telephone
Circuit--Capacity--Telephone Currents--Audible and Visible
Signals--Telephone Lines--Conductors--Inductance--Insulation


SUBSTATION EQUIPMENT _By K. B. Miller and S. G. McMeen_

Transmitters--Variable Resistance--Materials--Single and Multiple
Electrodes--Solid-Back Transmitter--Types of
Transmitters--Electrodes--Packing--Acousticon Transmitter--Switchboard
Transmitter--Receivers--Types of Receivers--Operator's
Receiver--Primary Cells--Series and Multiple Connections--Types of
Primary Cells--Magneto Signaling Apparatus--Battery Bell--Magneto
Bell--Magneto Generator--Armature--Automatic Shunt--Polarized
Ringer--Hook Switch--Electromagnets--Impedance, Induction, and
Repeating Coils--Non-Inductive Resistance
Devices--Differentially-Wound Unit--Condensers--Materials--Current
Supply to Transmitters--Local Battery--Common Battery--Diagrams of
Common-Battery Systems--Telephone Sets: Magneto, Series and Bridging,
Common-Battery

PARTY-LINE SYSTEMS _By K. B. Miller and S. G. McMeen_

Non-Selective Party-Line Systems--Series and Bridging--Signal
Code--Selective Party-Line Systems: Polarity, Harmonic, Step-by-Step,
and Broken-Line--Lock-Out Party-Line Systems: Poole, Step-by-Step, and
Broken-Line

PROTECTION _By K. B. Miller and S. G. McMeen_

Electrical Hazards--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_

The Telephone Exchange--Subscribers', Trunk, and Toll
Lines--Districts--Switchboards--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

INDEX

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

[Illustration: OLD BRANCH-TERMINAL MULTIPLE BOARD, PARIS, FRANCE]




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.




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.




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.

[Illustration: Fig. 1. Type of Magneto Telephone]

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.

[Illustration: Fig. 2. Magneto Telephones and Line]

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.

[Illustration: No. 10 SERIES MULTIPLE SWITCHBOARD _Monarch Telephone
Mfg. Co._]

[Illustration: Fig. 3. Magneto Telephones without Permanent Magnets]

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:

[Illustration: Fig. 4. Reis Transmitter]

"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.

[Illustration: Fig. 5. Typical Telegraph Line]

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.

[Illustration: Fig. 6. Telegraph Equipment Converted into Telephone
Equipment]

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.

[Illustration: Fig. 7. Electrostatic Telephone]

_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.

[Illustration: Fig. 8. Hughes' Microphone]

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.

[Illustration: Fig. 9. General Types of Transmitters]

_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.

[Illustration: Fig. 10. Battery in Line Circuit]

_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.

[Illustration: Fig 11. Battery in Local Circuit]

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.

[Illustration: Fig. 12. Conventional Diagram of Talking Circuit]

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.

[Illustration: Fig. 13. Induction-Coil Action]

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.

[Illustration: Fig. 14. Oscillogram of Telephone Currents]

_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.




CHAPTER III

ELECTRICAL SIGNALS


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

[Illustration: Fig. 15. Telegraph Sounder and Key]

[Illustration: Fig. 16. Vibrating Bell]

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.

[Illustration: Fig. 17. Elemental Magneto-Generator]

_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.

[Illustration: Fig. 18. Extension of a Permanent Magnet]

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.

[Illustration: Fig. 19. Extension of a Permanent Magnet]

"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.

[Illustration: Fig. 20. Electromagnet]

[Illustration: Fig. 21. Polarized Ringer]

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.

[Illustration: OPERATOR'S EQUIPMENT
Clement Automanual System.]

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.

[Illustration: Fig. 22. Gravity-Drop]

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.

[Illustration: Fig. 23. Electromagnetic Visible Signal]

[Illustration: Fig. 24. Lamp Signal and Lens]

_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.

[Illustration: Fig. 25. Lamp Signal Controlled by Relay]

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.

[Illustration: Fig. 26. Lamp Signal Directly in Line]

[Illustration: Fig. 27. Lamp Signal and Ballast]

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.




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_.

[Illustration: Fig. 28. Simple Condenser]

[Illustration: Fig. 29. Condenser Symbols]

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.

[Illustration: Fig. 30. Line with Shunt Capacity]

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.

[Illustration: Fig. 31. Test of Line with Varying Shunt Capacity]

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_[omega]

wherein _C_ is the capacity in farads and [omega] is 2[pi]_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_.

[Illustration: Fig. 32. Spiral of Wire]

[Illustration: Fig. 33. Spiral of Wire Around Iron Core]

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_[omega]

wherein _L_ is the inductance in henrys and [omega] is _2_[pi]_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.

[Illustration: Fig. 34. Test of Line with Varying Serial Inductance]

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,

       -------------------------
      /                1
Z =  /  R^{2} + ----------------
   \/           C^{2}[omega]^{2}


and


      --------------------------
Z =  /  R^{2} + L^{2}[omega]^{2}
   \/


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.

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

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.

[Illustration: Fig. 35. Loaded Line]

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

      -----------------------------------------------------------------------
     /   -----------------------------------------------
a=  /½  /(R^{2}+L^{2}[omega]^{2})(S^{2}+C^{2}[omega]^{2} + ½(RS-LC[omega]^{2}
  \/  \/


The quantities are

R = Resistance in ohms
L = Inductance in henrys
C = Mutual (shunt) capacity in farads
[omega] = 2[pi]_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.

[Illustration: Fig. 36. Shreeve Repeater and Circuit]

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.

[Illustration: Fig. 37. Mercury-Arc Telephone Relay]

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_.




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.

[Illustration: Fig. 38. Blake Transmitter]

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.

[Illustration: Fig. 39. Multiple-Electrode Transmitter]

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.

[Illustration: Fig. 40. White Solid-Back Transmitter]

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.

[Illustration: Fig. 41. White Solid-Back Transmitter]

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.

[Illustration: Fig. 42. New Western Electric Transmitter]

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.

[Illustration: Fig. 43. Kellogg Transmitter]

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

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.

[Illustration: Fig. 45. Monarch Transmitter]

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.

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

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.

[Illustration: Fig 46. Acousticon Transmitter]

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.

[Illustration: Fig. 47. Switchboard Transmitter]

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.

[Illustration: Fig. 48. Transmitter Symbols]

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.




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.

[Illustration: Fig. 49. Single-Pole Receiver]

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.

[Illustration: Fig. 50. Western Electric Receiver]

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.

[Illustration: Fig. 51. Kellogg Receiver]

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.

[Illustration: Fig. 52. Automatic Electric Company Direct-Current
Receiver]

[Illustration: Fig. 53. Automatic Electric Company Direct-Current
Receiver]

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.

[Illustration: Fig. 54. Monarch Direct-Current Receiver]

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.

[Illustration: Fig. 55. Dean Steel Shell Receiver]

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.

[Illustration: Fig. 56. Working Parts of Dean Receiver]

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.

[Illustration: Fig. 57. Operator's Receiver]

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.

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

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.

[Illustration: Fig. 58. Operator's Receiver and Cord]

[Illustration: Fig. 59. Receiver Symbols]

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.




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 SO_{4} ions bearing negative charges. The solution as a whole is
neutral in potential, having an equal number of equal and opposite
charges.

[Illustration: Fig. 60. Simple Voltaic Cell]

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
SO_{4} 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.

[Illustration: Fig. 61. LeClanché Cell]

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[typo was 'wth'] 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.

[Illustration: Fig. 62. Carbon Cylinder LeClanché Cell]

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.

[Illustration: Fig. 63. Dry Cell]

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.

[Illustration: Fig. 64. Gravity Cell]

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.

[Illustration: INTERIOR OF WAREHOUSE FOR TELEPHONE CONSTRUCTION
MATERIAL]

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 SO_{4}
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.

[Illustration: Fig 65. Fuller Cell]

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.

[Illustration: Fig. 66. Chloride of Silver Cell]

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.

[Illustration: Fig. 67. Battery Symbols]

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.




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_.

[Illustration: Fig. 68. Principles of Magneto Generator]

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.

[Illustration: Fig. 69. Generator Armature]

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.

[Illustration: Fig. 70. Generator Armature]

[Illustration: Fig. 71. Generator Field and Armature]

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.

[Illustration: Fig. 72. Generator with Magnets Removed]

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.

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

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.

[Illustration: Fig. 73. Five-Bar Generator]

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.

[Illustration: Fig 74. Generator Shunt Switch]

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.

[Illustration: Fig. 75. Generator Cut-in Switch]

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.

[Illustration: Fig. 76. Generator Cut-in Switch]

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.

[Illustration: Fig. 77. Pulsating-Current Commutator]

[Illustration: Fig. 78. Generator Symbols]

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.

[Illustration: Fig. 79. Polarized Bell]

_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.

[Illustration: Fig. 80. Polarized Bell]

[Illustration: Fig. 81. Biased Bell]

_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.

[Illustration: Fig. 82. Ringer Symbols]

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.




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.

[Illustration: Fig. 83. Long Lever Hook Switch]

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.

[Illustration: Fig. 84. Short Lever Hook Switch]

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.

[Illustration: Fig. 85. Removable Lever Hook Switch]

_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.

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

_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.

[Illustration: Fig. 86. Desk-Stand Hook Switch]

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.

[Illustration: Fig. 87. Desk-Stand Hook Switch]

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.

[Illustration: Fig. 88. Hook Switch Symbols]

[Illustration: COMPRESSED AIR WAGON FOR PNEUMATIC DRILLING AND
CHIPPING IN MANHOLES]




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.

[Illustration: Fig. 89. Magnetization Curve]

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.

[Illustration: Fig. 90. Bar Electromagnet]

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.

[Illustration: Fig. 91. Bar Electromagnet]

[Illustration: Fig. 92. Horseshoe Electromagnet]

[Illustration: Fig. 93. Horseshoe Electromagnet]

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.

[Illustration: Fig. 94. Iron-Clad Electromagnet]

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.

[Illustration: Fig. 95. Electromagnet of Relay]

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.

[Illustration: Fig. 96. Parallel Differential Electromagnet]


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.

[Illustration: Fig. 97. Tandem Differential Electromagnet]

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.

[Illustration: Fig. 98 Construction of Electromagnet]

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.

+-------+----------+----------+-----------------------+--------------------+-----------------------+
|       |          |          |       RESISTANCE      |       LENGTH       |       WEIGHT          |
| A.W.G.| DIAMETER |   AREA   +-----------------------+--------------------+-----------------------+
| B.&S. |   MILS   | CIRCULAR | OHMS PER  | OHMS PER  | FEET PER | FEET PER| POUNDS PER |POUNDS PER|
|       |          |   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|
+-------+----------+----------+-----------+-----------+----------+---------+------------+----------+

[Illustration: SOUTH OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO]

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.

[Illustration: Fig. 99. Electromagnet with Bare Wire]

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.

[Illustration Fig. 100. Electromagnet with Terminals]

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
---------------------------------------------------------------------
DIAMETER      |
Mils          |   31.961   28.462   25.347   22.571   20.100   17.900
---------------------------------------------------------------------
AREA          |
Circular Mils | 1021.20   810.10   642.70   509.45   404.01   320.40
---------------------------------------------------------------------
DIAMETER OVER |
INSULATION    |
  SINGLE      |
  COTTON      |   37.861   34.362   31.247   28.471   26.000   23.800
              |
  DOUBLE      |
  COTTON      |   42.161   38.662   35.547   32.771   30.300   28.100
              |
  SINGLE SILK |   34.261   30.762   27.647   24,871   22.401   20.200
              |
  DOUBLE SILK |   36.161   32.662   29.547   26.771   24.300   22.100
---------------------------------------------------------------------
TURNS PER     |
LINEAR INCH   |
  SINGLE      |
  COTTON      |   25.7     28.3     31.0     34.4     36.9     38.0
              |
  DOUBLE      |   22.5     24.5     26.7     28.97    31.35    33.92
  COTTON      |
              |
  SINGLE SILK |   27.70    30.97    34.39    38.19    42.37    47.02
              |
  DOUBLE SILK |   26.22    29.07    32.11    35.53    39.14    42.94
---------------------------------------------------------------------
TURNS PER     |
SQUARE INCH   |
  SINGLE      |
  COTTON      |  660.5    800.9    961.0   1183.0   1321.6   1444.0
              |
  DOUBLE      |
  COTTON      |  506.3    600.2    712.9    839.2     982.8  1150.8
              |
  SINGLE SILK |  767.3    959.1   1182.7   1458.5    1795.2  2210.9
              |
  DOUBLE SILK |  687.5    845.0   1031.0   1262.4    1532.0  1843.8
---------------------------------------------------------------------
OHMS PER      |
CUBIC INCH    |
  SINGLE      |
  COTTON      |     .646     .981    1.502    2.359     3.528   5.831
              |
  DOUBLE      |
  COTTON      |     .533     .795    1.188    1.772     2.595   3.802
              |
  SINGLE SILK |     .801    1.261    1.956    3.049     4.739   7.489
---------------------------------------------------------------------


A.W.G. B & S  |   26       27       28       29       30       31
---------------------------------------------------------------------
DIAMETER      |
Mils          |   15.940   14.195   12.641   11.257   10.025    8.928
---------------------------------------------------------------------
AREA          |
Circular Mils |  254.01   201.50   159.79   126.72   100.50    79.71
---------------------------------------------------------------------
DIAMETER OVER |
INSULATION    |
  SINGLE      |
  COTTON      |   21.840   20.095   18.541   17.157   15.925   14.828
              |
  DOUBLE      |
  COTTON      |   26.140   24.395   22.841   21.457   20.225   19.128
              |
  SINGLE SILK |   18.240   16.495   14.941   13.557   12.325   11.228
              |
  DOUBLE SILK |   20.140   18.395   16.841   15.457   14.225   13.128
---------------------------------------------------------------------
TURNS PER     |
LINEAR INCH   |
  SINGLE      |
  COTTON      |   42.0     48.0     53.0     56.5     59.66    64.125
              |
  DOUBLE      |
  COTTON      |   36.29    38.95    41.61    44.27    46.93    49.78
              |
  SINGLE SILK |   52.06    57.67    63.36    70.11    77.14    84.64
              |
  DOUBLE SILK |   46.81    51.59    56.43    61.56    66.79    72.39
---------------------------------------------------------------------
TURNS PER     |
SQUARE INCH   |
  SINGLE      |
  COTTON      | 1764.0   2304.0   2809.9   3192.3   3359.2   4112.2
              |
  DOUBLE      |
  COTTON      | 1317.0   1517.2   1731.0   1959.9   2202.5   2478.0
              |
  SINGLE SILK | 2710.3   3326.0   4014.5   4915.5   5950.2   7164.0
              |
  DOUBLE SILK | 2191.2   2661.6   3184.5   3789.8   4461.0   5240.0
---------------------------------------------------------------------
OHMS PER      |
CUBIC INCH    |
  SINGLE      |
  COTTON      |    6.941   10.814   17.617   25.500   34.800   48.5
              |
  DOUBLE      |
  COTTON      |    5.552    8.078   11.54    16.47    23.43    32.83
              |
  SINGLE SILK |    9.031   13.92    26.86    41.29    62.98    95.70
---------------------------------------------------------------------


A.W.G. B & S  |    32       33       34       35       36       37
----------------------------------------------------------------------
DIAMETER      |
Mils          |     7.950    7.080    6.304    5.614    5.000    4.453
----------------------------------------------------------------------
AREA          |
Circular Mils |    63.20    50.13    39.74    31.52    25.00    19.83
----------------------------------------------------------------------
DIAMETER OVER |
INSULATION    |
  SINGLE      |
  COTTON      |    13.850   12.980   12.204   11.514   10.900   10.353
              |
  DOUBLE      |
  COTTON      |    18.150   17.280   16.504   15.814   15.200   14.653
              |
  SINGLE SILK |    10.250    9.380    8.504    7.914    7.300    6.753
              |
  DOUBLE SILK |    12.150   11.280   10.504    9.814    9.200    8.653
----------------------------------------------------------------------
TURNS PER     |
LINEAR INCH   |
  SINGLE      |
  COTTON      |    68.600   73.050   77.900   82.600   87.100   91.870
              |
  DOUBLE      |
  COTTON      |    52.34    55.10    57.57    60.04    62.51    64.70
              |
  SINGLE SILK |    92.72   101.65   112.11   119.7    130.15   140.6
              |
  DOUBLE SILK |    78.19    84.17    90.44    96.90   103.55   110.20
----------------------------------------------------------------------
TURNS PER     |
SQUARE INCH   |
  SINGLE      |  4692.5   5333.5   6068.5   6773.3   7586.5   8440.0
  COTTON      |
              |
  DOUBLE      |
  COTTON      |  2739.5   3036.1   3314.2   3605.0   3907.5   4186.1
              |
  SINGLE SILK |  8597.5  10332.0  12570.0  14327.0  16940.0  19770.0
              |
  DOUBLE SILK |  6114.0   7085.0   8179.5   9389.5  10772.0  12145.0
---------------------------------------------------------------------
OHMS PER      |
CUBIC INCH    |
  SINGLE      |
  COTTON      |    73.8    104.5    151.4    202.0    298.8    418.0
              |
  DOUBLE      |
  COTTON      |    46.19    64.30    70.58   125.9    166.3    225.6
              |
  SINGLE SILK |   144.70   217.8    342.1    489.0    721.1   1062.0
---------------------------------------------------------------------


A.W.G. B & S  |    38        39       40
--------------------------------------------
DIAMETER      |
Mils          |     3.965     3.531    3.144
--------------------------------------------
AREA          |
Circular Mils |    15.72     12.47     9.89
--------------------------------------------
DIAMETER OVER |
INSULATION    |
  SINGLE      |
  COTTON      |     9.865     9.431    9.044
              |
  DOUBLE      |
  COTTON      |    14.165    13.731   13.344
              |
  SINGLE SILK |     6.265     5.831    5.344
              |
  DOUBLE SILK |     8.165     7.731    7.344
--------------------------------------------
TURNS PER     |
LINEAR INCH   |
  SINGLE      |
  COTTON      |    95.000   100.700  106.000
              |
  DOUBLE      |
  COTTON      |    66.80     68.80    71.20
              |
  SINGLE SILK |   151.05    163.04   177.65
              |
  DOUBLE SILK |   116.85    122.55   129.20
--------------------------------------------
TURNS PER     |
SQUARE INCH   |
  SINGLE      |
  COTTON      |  9025.0   10140.5  11236.0
              |
  DOUBLE      |  4462.2    4733.6   5069.8
  COTTON      |
              |
  SINGLE SILK | 22820.0   26700.0  31559.0
              |
  DOUBLE SILK | 13655.0   15018.0  16692.0
--------------------------------------------
OHMS PER      |
CUBIC INCH    |
  SINGLE      |
  COTTON      |  567.0    811.0    1113.0
              |
  DOUBLE      |  305.5    409.8     545.5
  COTTON      |
              |
  SINGLE SILK | 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.

[Illustration: Fig. 101. Section of Open-Circuit Impedance Coil]

[Illustration: Fig. 102. Open-Circuit Impedance Coil]

[Illustration: Fig. 103. Closed-Circuit Impedance Coil]

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.

[Illustration: Fig. 104. Symbol of Toroidal Impedance Coil]

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.

[Illustration: Fig. 105. Toroidal Impedance Coil]

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.

[Illustration: Fig. 106. Symbol of Impedance Coil]

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.

[Illustration: Fig. 107. Induction Coil]

[Illustration: Fig. 108. Section of Induction Coil]

_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.

[Illustration: Fig. 109. Repeating Coil]

[Illustration: Fig. 110. Repeating Coil]

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.

[Illustration: Fig. 111. Repeating Coil]

[Illustration: Fig. 112. Diagram of Toroidal Repeating Coil]

[Illustration: Fig. 113. Toroidal Repeating Coil]

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.

[Illustration: THE OPERATING ROOM OF THE EXCHANGE AT WEBB CITY,
MISSOURI]

[Illustration: Fig. 114. Symbol of Induction Coil]

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.

[Illustration: Fig. 115. Repeating-Coil Symbols]

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.

[Illustration: Fig. 116. Symbol of Four-Winding Repeating Coil]

Where the toroidal type of repeating coil is employed, the diagram of
Fig. 112, already referred to, is a good symbolic representation.




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

+---------------------------+-----------------------------+
|       PURE METALS         |  TEMPERATURE  COEFFICIENTS  |
+---------------------------+--------------+--------------+
|                           |  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       |
+---------+----------+-----------------+----------------+---------------+

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.

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.

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       |
+---------+----------+-----------------+----------------+---------------+


_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.

[Illustration: Fig. 117. Mica Card Resistance]

[Illustration: Fig. 118. Iron-Wire Ballast]

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.

_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.




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.

[Illustration: Fig. 119. Condenser Plate]

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.

[Illustration: Fig. 120. Theory of Condenser]

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           |
|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.

[Illustration: Fig. 121. Rolled Condenser]

_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.

[Illustration: Fig. 122. Rolled Condenser]

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.

[Illustration: Fig. 123. Rolled Condenser]

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       |     l     |
+------------+---------------+----------+----------+-----------+


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.

[Illustration: Fig. 124. Condenser Symbols]

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.




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.

[Illustration: A TYPICAL MEDIUM-SIZED MULTIPLE SWITCHBOARD EQUIPMENT]

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.

[Illustration: Fig. 125. Local-Battery Stations with Grounded Circuit]

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.

[Illustration: Fig. 126. Local-Battery Stations with Metallic Circuit]

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.

[Illustration: Fig. 127. Local-Battery Stations with Metallic Circuit]

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.

[Illustration: Fig. 128. Battery in Series with Two Lines]

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.

[Illustration: Fig. 129. Battery in Series with Two Lines]

_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.

[Illustration: Fig. 130. Bridging Battery with Repeating Coil]

_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.

[Illustration: Fig. 131. Bridging Battery with Impedance Coils]

_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.

[Illustration: Fig. 132. Double-Battery Kellogg System]

_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.

[Illustration: Fig. 133. Dean System]

_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.

[Illustration: Fig. 134. Stromberg-Carlson System]

[Illustration: Fig. 135. North Electric Company System]

_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.

[Illustration: Fig. 136. Current Supply over Parallel Limbs of Line]

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.

[Illustration: Fig. 137. Current Supply over Parallel Limbs of Line]

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.

[Illustration: MAIN OFFICE, KEYSTONE TELEPHONE COMPANY, PHILADELPHIA,
PA.]

[Illustration: Fig. 138. Common Source for Many Lines]

[Illustration: Fig. 139. Common Source for Many Lines]

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.

[Illustration: Fig. 140. Two Sources for Many Lines]

[Illustration: Fig. 141. Current Supply from Distant Point]

_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.




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.

[Illustration: Fig. 142. Magneto Wall Set]

[Illustration: Fig. 143. Magneto Wall Set]

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.

[Illustration: Fig. 144. Desk Stand]

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.

[Illustration: Fig. 145. Magneto Desk Set]

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]

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.

[Illustration: Fig. 147. Circuit of Series Magneto Set.]

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.

[Illustration: Fig. 148. Circuit of Bridging Magneto Set]

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.

[Illustration: Fig. 149. Circuit of Bridging Magneto Set]

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.

[Illustration: Fig. 150. Circuit of Bridging Magneto Desk Set]

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.

[Illustration: Fig. 151. Common-Battery Wall Set]

[Illustration: Fig. 152. Common-Battery Wall Set]

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.

[Illustration: MAIN OFFICE, KANSAS CITY HOME TELEPHONE CO., KANSAS
CITY, MO.]

_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.

[Illustration: Fig. 153. Stromberg-Carlson Common-Battery Wall Set]

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 sets, because of the fact that their use was
first confined largely to the rooms in hotels. Later, however, these
instruments have become very popular in general use, particularly in
residences. Sometimes the boxes or cabinets of these sets are made of
wood, but of recent years the tendency has been growing to make them
of pressed steel. The steel box is usually finished in black enamel,
baked on, the color being sometimes varied to match the color of the
surrounding woodwork. In Figs. 154 and 155 are shown two views of a
common-battery hotel set manufactured by the Dean Electric Company.

Such sets are extremely neat in appearance and have the advantage of
taking up little room on the wall and the commercial advantage of
being light and compact for shipping purposes. A possible disadvantage
of this type of instrument is the somewhat crowded condition which
necessarily follows from the placing of all the parts in so confined a
space. This interferes somewhat with the accessibility of the various
parts, but great ingenuity has been manifested in making the parts
readily get-at-able in case of necessity for repairs or alterations.

[Illustration: Fig. 154. Steel Box Hotel]

[Illustration: Fig. 155. Steel Box Hotel Set]

_Desk_. The common-battery desk telephone presents a somewhat simpler
problem than the magneto desk telephone for the reason that the
generator and local battery, the two most bulky parts of a magneto
telephone, do not have to be provided for. Some companies, in
manufacturing desk stands for common-battery purposes, mount the
condenser and the induction coil or impedance coil, or whatever device
is used in connection with the talking circuit, in the base of the
desk stand itself, and mount the polarized ringer and the condenser
used for ringing purposes in a separate bell box adapted to be
mounted on the wall or some portion of the desk. Other companies mount
only the transmitter, receiver, and hook switch on the desk stand
proper and put the condenser or induction coil, or other device
associated with the talking circuit, in the bell box. There is little
to choose between the two general practices. The number of conducting
strands in the flexible cord is somewhat dependent on the arrangement
of the circuit employed.

[Illustration: Fig. 156. Common-Battery Desk Set]

[Illustration: Fig. 157. Bell for Common-Battery Desk Set.]

The Kellogg Switchboard and Supply Company is one which places all the
parts, except the polarized ringer and the associated condenser, in
the desk stand itself. In Fig. 156 is shown a bottom view of the desk
stand with the bottom plate removed. In the upper portion of the
circle of the base is shown a small condenser which is placed in the
talking circuit in series with the receiver. In the right-hand portion
of the circle of the base is shown a small impedance coil, which is
placed in series with the transmitter but in shunt relation with the
condenser and the receiver.

[Illustration: Fig. 158. Bell for Common-Battery Desk Set]

In Figs. 157 and 158 are shown two views of the type of bell box
employed by the Kellogg Company in connection with the common-battery
desk sets, this box being of pressed-steel construction and having a
removable lid, as shown in Fig. 158, by which the working parts of the
ringer are made readily accessible, as are also the terminals for the
cord leading from the desk stand and for the wires of the line
circuit. The condenser that is placed in series with the ringer is
also mounted in this same box. By employing two condensers, one in the
bell box large enough to transmit ringing currents and the other in
the base of the desk stand large enough only to transmit voice
currents, a duplication of condensers is involved, but it has the
corresponding advantages of requiring only two strands to the flexible
cord leading from the bell box to the desk stand proper.

[Illustration: Fig. 159. Microtelephone Set]

A form of desk-telephone set that is used largely abroad, but that has
found very little use in this country, is shown in Fig. 159. In this
the transmitter and the receiver are permanently attached together,
the receiver being of the watch-case variety and so positioned
relatively to the transmitter that when the receiver is held at the
ear, the mouthpiece of the transmitter will be just in front of the
lips of the user. In order to maintain the transmitter in a vertical
position during use, this necessitates the use of a curved mouthpiece
as shown. This transmitter and receiver so combined is commonly
called, in this country, the _microtelephone set_, although there
seems to be no logical reason for this name. The combined transmitter
and receiver, instead of being supported on an ordinary form of hook
switch, are supported on a forked bracket as shown, this bracket
serving to operate the switch springs which are held in one position
when the bracket is subjected to the weight of the microtelephone, and
in the alternate position when relieved therefrom. This particular
microtelephone set is the product of the L.M. Ericsson Telephone
Manufacturing Company, of Buffalo, New York. The circuits of such sets
do not differ materially from those of the ordinary desk telephone
set.

[Illustration: Fig. 160. Kellogg Common-Battery Desk Set]

[Illustration: Fig. 161. Dean Common-Battery Set]

Circuits of Common-Battery Telephone Sets. The complete circuits of
the Kellogg desk-stand arrangement are shown in Fig. 160, the
desk-stand parts being shown at the left and the bell-box parts at
the right. As is seen, but two conductors extend from the former to
the latter. A simplified theoretical sketch is also shown in the upper
right-hand corner of this figure.

The details of the common-battery telephone circuits of the Dean
Electric Company are shown in Fig. 161. This involves the use of the
balanced Wheatstone bridge. The only other thing about this circuit
that needs description, in view of what has previously been said about
it, is that the polarized bell is placed in series with a condenser so
that the two sides of the circuit may be insulated from each other
while the telephone is not in use, and yet permit the passage of
ringing current through the bell.

[Illustration: Fig. 162. Monarch Common-Battery Wall Set]

The use of the so-called direct-current receiver has brought about a
great simplification in the common-battery telephone circuits of
several of the manufacturing companies. By this use the transmitter
and the receiver are placed in series across the line, this path being
normally opened by the hook-switch contacts. The polarized bell and
condenser are placed in another bridge path across the line, this path
not being affected by the hook-switch contacts. All that there is to
such a complete common-battery telephone set, therefore, is a
receiver, transmitter, hook switch, bell, condenser, and cabinet, or
other support.

The extreme simplicity of the circuits of such a set is illustrated in
Fig. 162, which shows how the Monarch Telephone Manufacturing Company
connect up the various parts of their telephone set, using the
direct-current receiver already described in connection with Fig. 54.

[Illustration: VENTILATING PLANT FOR LARGE TELEPHONE OFFICE BUILDING]




CHAPTER XV

NON-SELECTIVE PARTY-LINE SYSTEMS


A party line is a line that is for the joint use of several stations.
It is, therefore, a line that connects a central office with two or
more subscribers' stations, or where no central office is involved, a
line that connects three or more isolated stations with each other.
The distinguishing feature of a party line, therefore, is that it
serves more than two stations, counting the central office, if there
is one, as a station.

Strictly speaking, the term _party_ line should be used in
contradistinction to the term _private_ line. Companies operating
telephone exchanges, however, frequently lease their wires to
individuals for private use, with no central-office switchboard
connections, and such lines are, by common usage, referred to as
"private lines." Such lines may be used to connect two or more
isolated stations. A _private_ line, in the parlance of telephone
exchange working, may, therefore, be a _party_ line, as inconsistent
as this may seem.

A telephone line that is connected with an exchange is an exchange
line, and it is a party line if it has more than one station on it. It
is an individual line or a single party line if it has but a single
station on it. A line which has no central-office connection is called
an "isolated line," and it is a party line if it has more than two
stations on it.

The problem of mere speech transmission on party lines is comparatively
easy, being scarcely more complex than that involved in private or
single party lines. This is not true, however, of the problem of
signaling the various stations. This is because the line is for the
common use of all its patrons or subscribers, as they are termed, and
the necessity therefore exists that the person sending a signal, whether
operator or subscriber, shall be able in some way to inform a person at
the desired station that the call is intended for that station. There
are two general ways of accomplishing this purpose.

(_1_) The first and simplest of these ways is to make no provision for
ringing any one bell on the line to the exclusion of the others, and
thus allow all bells to ring at once whenever any station on the line
is wanted. Where this is done, in order to prevent all stations from
answering, it is necessary, in some way, to convey to the desired
station the information that the call is intended for that station,
and to all of the other stations the information that the call is not
intended for them. This is done on such lines by what is called "code
ringing," the code consisting of various combinations of long and
short rings.

(_2_) The other and more complex way is to arrange for selective
ringing, so that the person sending the call may ring the bell at the
station desired, allowing the bells at all the other stations to
remain quiet.

[Illustration: Fig. 163. Grounded-Circuit Series Line]

These two general classes of party-line systems may, therefore, be
termed "non-selective" and "selective" systems. Non-selective party
lines are largely used both on lines having connection with a central
office, and through the central office the privilege of connection
with other lines, and on isolated lines having no central-office
connection. The greatest field of usefulness of non-selective lines is
in rural districts and in connection with exchanges in serving rather
sparsely settled districts where the cost of individual lines or even
lines serving but a few subscribers, is prohibitive.

Non-selective telephone party lines most often employ magneto
telephones. The early forms of party lines employed the ordinary
series magneto telephone, the bells being of low resistance and
comparatively low impedance, while the generators were provided with
automatic shunting devices, so that their resistance would normally be
removed from the circuit of the line.

Series Systems. The general arrangement of a series party line
employing a ground return is shown in Fig. 163. In this three ordinary
series instruments are connected together in series, the end stations
being grounded, in order to afford a return path for the ringing and
voice currents.

[Illustration: Fig. 164. Metallic-Circuit Series Line]

In Fig. 164 there is shown a metallic-circuit series line on which
five ordinary series telephones are placed in series. In this no
ground is employed, the return being through a line wire, thus making
the circuit entirely metallic.

[Illustration: Fig. 165. Series Party Line]

The limitations of the ordinary series party line may be best
understood by reference to Fig. 165, in which the circuits of three
series telephones are shown connected with a single line. The receiver
of Station A is represented as being on its hook, while the receivers
of Stations B and C are removed from their hooks, as when the
subscribers at those two stations are carrying on a conversation. The
hook switches of Stations B and C being in raised positions, the
generators and ringers of those stations are cut out of the circuit,
and only the telephone apparatus proper is included, but the hook
switch of Station A being depressed by the weight of its receiver,
includes the ringer of that station in circuit, and through this
ringer, therefore, the voice currents of Stations B and C must pass.

The generator of Station A is not in the circuit of voice currents,
however, because of the automatic shunt with which the generator is
provided, as described in Chapter VIII.

A slight consideration of the series system as shown in this figure,
indicates that the voice currents of any two stations that are in use,
must pass (as indicated by the heavy lines) through the ringers of all
the stations that are not in use; and when a great number of stations
are placed upon a single line, as has been frequently the case, the
impedance offered by these ringers becomes a serious barrier to the
passage of the voice currents. This defect in the series party line is
fundamental, as it is obvious that the ringers must be left in the
circuit of the stations which are not in use, in order that those
stations may always be in such condition as to be able to receive a
call.

This defect may in some measure be reduced by making the ringers of
low impedance. This is the general practice with series telephones,
the ringers ordinarily having short cores and a comparatively small
number of turns, the resistance being as a rule about 80 ohms.

Bridging Systems. Very much better than the series plan of
party-line connections, is the arrangement by which the instruments
are placed in bridges across the line, such lines being commonly known
as bridged or bridging lines. This was first strongly advocated and
put into wide practical use by J.J. Carty, now the Chief Engineer of
the American Telephone and Telegraph Company.

A simple illustration of a bridging telephone line is shown in Fig.
166, where the three telephones shown are each connected in a bridge
path from the line wire to ground, a type known as a "grounded
bridging line." Its use is very common in rural districts.

A better arrangement is shown in Fig. 167, which represents a
metallic-circuit bridging line, three telephone instruments being
shown in parallel or bridge paths across the two line wires.

The actual circuit arrangements of a bridging party line are better
shown in Fig. 168. There are three stations and it will be seen that
at each station there are three possible bridges, or bridge paths,
across the two limbs of the line. The first of these bridges is
controlled by the hook switch and is normally open. When the hook is
raised, however, this path is closed through the receiver and
secondary of the induction coil, the primary circuit being also closed
so as to include the battery and transmitter. This constitutes an
ordinary local-battery talking set.

[Illustration: Fig. 166. Grounded Bridging Line]

[Illustration: Fig. 167. Metallic Bridging Line]

[Illustration: Fig. 168. Metallic Bridging Line]

A second bridge at each station is led through the ringer or
call-bell, and this, in most bridging telephones, is permanently
closed, the continuity of this path between the two limbs of the line
not being affected either by the hook switch or by the automatic
switch in connection with the generator.

A third bridge path at each station is led through the generator.
This, as indicated, is normally open, but the automatic cut-in switch
of the generator serves, when the generator is operated, to close its
path across the line, so that it may send its currents to the line and
ring the bells of all the stations.

When any generator is operated, its current divides and passes over
the line wires and through all of the ringers in multiple. It is seen,
therefore, that the requirements for a bridging generator are that it
shall be capable of generating a large current, sufficient when
divided up amongst all the bells to ring each of them; and that it
shall be capable of producing a sufficient voltage to send the
required current not only to the near-by stations, but to the stations
at the distant end of the line.

It might seem at first that the bridging system avoided one difficulty
only to encounter another. It clearly avoids the difficulty of the
series system in that the voice currents, in order to reach distant
stations, do not have to pass through all of the bells of the idle
stations in series. There is, however, presented at each station a
leakage path through the bell bridged across the line, through which
it would appear the voice currents might leak uselessly from one side
of the line to the other and not pass on in sufficient volume to the
distant station. This difficulty is, however, more apparent than real.
It is found that, by making the ringers of high impedance, the leakage
of voice currents through them from one side of the line to the other
is practically negligible.

It is obvious that in a heavily loaded bridged line, the bell at the
home station, that is at the station from which the call is being sent,
will take slightly more than its share of the current, and it is also
obvious that the ringing of the home bell performs no useful function.
The plan is frequently adopted, therefore, of having the operation of
the generator serve to cut its own bell out of the circuit. The
arrangement by which this is done is clearly shown in Fig. 169. The
circuit of the bell is normally complete across the line, while the
circuit of the generator is normally open. When, however, the generator
crank is turned these conditions are reversed, the bell circuit being
broken and the generator circuit closed, so as to allow its current all
to pass the line. This feature of having the local bell remain silent
upon the operation of its own generator is also of advantage because
other parties at the same station are not disturbed by the ringing of
the bell when a call is being made by that station.

A difficulty encountered on non-selective bridging party lines, which
at first seems amusing rather than serious, but which nevertheless is
often a vexatious trouble, is that due to the propensity of some
people to "listen in" on the line on hearing calls intended for other
than their own stations. People whose ethical standards would not
permit them to listen at, or peep through, a keyhole, often engage in
this telephonic eavesdropping.

Frequently, not only one but many subscribers will respond to a call
intended for others and will listen to the ensuing conversation. This
is disadvantageous in several respects: It destroys the privacy of
conversation between any two parties; it subjects the local batteries
to an unnecessary and useless drain; and it greatly impairs the
ringing efficiency of the line. The reason for this interference with
ringing is that the presence of the low-resistance receivers across
the line allows the current sent out by any of the generators to pass
in large measure through the receivers, thus depriving the ringers,
which are of comparatively high resistance and impedance, of the
energy necessary to operate them. As a result of this it is frequently
impossible for one party to repeat the call for another because,
during the interval between the first and second call, a number of
parties remove their receivers from their hooks in order to listen.
Ring-off or clearing-out signals are likewise interfered with.

[Illustration: Fig. 169. Circuits of Bridging Station]

A partial remedy for this interference with ringing, due to
eavesdropping, is to introduce a low-capacity condenser into the
receiver circuit at each station, as shown in Fig. 169. This does not
seriously interfere with the speech transmission since the condensers
will readily transmit the high-frequency voice currents. Such
condensers, however, have not sufficient capacity to enable them
readily to transmit the low-frequency ringing currents and hence
these are forced, in large measure, to pass through the bells for
which they are intended rather than leaking through the low-resistance
receiver paths.

The best condenser for this use is of about 1/2-microfarad capacity,
which is ample for voice-transmitting purposes, while it serves to
effectively bar the major portion of the generator currents. A higher
capacity condenser would carry the generator currents much more
readily and thus defeat the purpose for which it was intended.

In order that the requisite impedance may be given to the ringers
employed for bridging party lines, it is customary to make the cores
rather long and of somewhat larger diameter than in series ringers and
at the same time to wind the coils with rather fine wire so as to
secure the requisite number of turns. Bridging bells are ordinarily
wound to a resistance of 1,000 or 1,600 ohms, these two figures having
become standard practice. It is not, however, the high resistance so
much as the high impedance that is striven for in bridging bells; it
is the number of turns that is of principal importance.

As has already been stated, the generators used for bridging lines are
made capable of giving a greater current output than is necessary in
series instruments, and for this purpose they are usually provided with
at least four, and usually five, bar magnets. The armature is made
correspondingly long and is wound, as a rule, with about No. 33 wire.

Sometimes where a bridged party line terminates in a central-office
switchboard it is desired to so operate the line that the subscribers
shall not be able to call up each other, but shall, instead, be able
to signal only the central-office operator, who, in turn, will be
enabled to call the party desired, designating his station by a
suitable code ring. One common way to do this is to use biased bells
instead of the ordinary polarized bells. In order that the bells may
not be rung by the subscribers' generators, these generators are made
of the direct-current type and these are so associated with the line
that the currents which they send out will be in the wrong direction
to actuate the bells. On the other hand, the central-office generator
is of direct-current type and is associated with the line in the right
direction to energize the bells. Thus any subscriber on the line may
call the central office by merely turning his generator crank, which
action will not ring the bells of the subscribers on the line. The
operator will then be able to receive the call and in turn send out
currents of the proper direction to ring all the bells and, by code,
call the desired party to the telephone.

[Illustration: 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]

Signal Code. The code by which stations are designated on
non-selective party lines usually consists in combinations of long and
short rings similar to the dots and dashes in the Morse code. Thus,
one short ring may indicate Station No. 1; two short rings Station No.
2; and so on up to, say, five short rings, indicating Station No. 5.
It is not good practice to employ more than five successive short
rings because of the confusion which often arises in people's minds as
to the number of rings that they hear. When, therefore, the number of
stations to be rung by code exceeds five, it is better to employ
combinations of long and short rings, and a good way is to adopt a
partial decimal system, omitting the numbers higher than five in each
ten, and employing long rings to indicate the tens digits and short
rings to indicate the units digit, Table X.

TABLE X

Signal Code
+--------------+---------------+--------------+---------------+
|STATION NUMBER|RING           |STATION NUMBER|RING           |
|1             |1 short        |12            |1 long, 2 short|
|2             |2 short        |13            |1 long, 3 short|
|3             |3 short        |14            |1 long, 4 short|
|4             |4 short        |15            |1 long, 5 short|
|5             |5 short        |21            |2 long, 1 short|
|11            |1 long, 1 short|22            |2 long, 2 short|
+--------------+---------------+--------------+---------------+

Other arrangements are often employed and by almost any of them a
great variety of readily distinguishable signals may be secured. The
patrons of such lines learn to distinguish, with comparatively few
errors, between the calls intended for them and those intended for
others, but frequently they do not observe the distinction, as has
already been pointed out.

Limitations. With good telephones the limit as to the number of
stations that it is possible to operate upon a single line is usually
due more to limitations in ringing than in talking. As the number of
stations is increased indefinitely a condition will be reached at
which the generators will not be able to generate sufficient current
to ring all of the bells, and this condition is likely to occur before
the talking efficiency is seriously impaired by the number of bridges
across the line.

Neither of these considerations, however, should determine the maximum
number of stations to be placed on a line. The proper limit as to the
number of stations is not the number that can be rung by a single
generator, or the number with which it is possible to transmit speech
properly, but rather the number of stations that may be employed
without causing undue interference between the various parties who may
desire to use the line. Overloaded party lines cause much annoyance,
not only for the reason that the subscribers are often not able to use
the line when they want it, but also, in non-selective lines, because
of the incessant ringing of the bells, and the liability of confusion
in the interpretation of the signaling code, which of course becomes
more complex as the number of stations increases.

The amount of business that is done over a telephone line is usually
referred to as the "traffic." It will be understood, however, in
considering party-line working that the number of calls per day or per
hour, or per shorter unit, is not the true measure of the traffic and,
therefore, not the true measure of the amount of possible interference
between the various subscribers on the line.

An almost equally great factor is the average length of the
conversation. In city lines, that is, in lines in city exchanges, the
conversation is usually short and averages perhaps two minutes in
duration. In country lines, however, serving people in rural
districts, who have poor facilities for seeing each other,
particularly during the winter time, the conversations will average
very much longer. In rural communities the people often do much of
their visiting by telephone, and conversations of half an hour in
length are not unusual. It is obvious that under such conditions a
party line having a great many stations will be subject to very grave
interference between the parties, people desiring to use the line for
business purposes often being compelled to wait an undue time before
they may secure the use of the line.

It is obvious, therefore, that the amount of traffic on the line,
whether due to many short conversations or to a comparatively few
long ones, is the main factor that should determine the number of
stations that, economically, may be placed on a line. The facilities
also for building lines enter as a factor in this respect, since it is
obvious that in comparatively poor communities the money may not be
forthcoming to build as many lines as are needed to properly take care
of the traffic. A compromise is, therefore, often necessary, and the
only rule that may be safely laid down is to place as few parties on a
given line as conditions will admit.

No definite limit may be set to apply to all conditions but it may be
safely stated that under ordinary circumstances no more than ten
stations should be placed on a non-selective line. Twenty stations
are, however, common, and sometimes forty and even fifty have been
connected to a single line. In such cases the confusion which results,
even if the talking and the ringing efficiency are tolerable, makes
the service over such overloaded lines unsatisfactory to all
concerned.




CHAPTER XVI

SELECTIVE PARTY-LINE SYSTEMS


The problem which confronts one in the production of a system of
selective ringing on party lines is that of causing the bell of any
chosen one of the several parties on a circuit to respond to a signal
sent out from the central office without sounding any of the other
bells. This, of course, must be accomplished without interfering with
the regular functions of the telephone line and apparatus. By this is
meant that the subscribers must be able to call the central office and
to signal for disconnection when desired, and also that the
association of the selective-signaling devices with the line shall not
interfere with the transmission of speech over the line. A great many
ways of accomplishing selective ringing on party lines have been
proposed, and a large number of them have been used. All of these ways
may be classified under four different classes according to the
underlying principle involved.

Classification. (_1_) _Polarity_ systems are so called because they
depend for their operation on the use of bells or other responsive
devices so polarized that they will respond to one direction of
current only. These bells or other devices are so arranged in
connection with the line that the one to be rung will be traversed by
current in the proper direction to actuate it, while all of the others
will either not be traversed by any current at all, or by current in
the wrong direction to cause their operation.

(_2_) The _harmonic_ systems have for their underlying principle the
fact that a pendulum or elastic reed, so supported as to be capable of
vibrating freely, will have one particular rate of vibration which it
may easily be made to assume. This pendulum or reed is placed under
the influence of an electromagnet associated with the line, and owing
to the fact that it will vibrate easily at one particular rate of
vibration and with extreme difficulty at any other rate, it is clear
that for current impulses of a frequency corresponding to its natural
rate the reed will take up the vibration, while for other frequencies
it will fail to respond.

Selection on party lines by means of this system is provided for by
tuning all of the reeds on the line at different rates of vibration
and is accomplished by sending out on the line ringing currents of
proper frequency to ring the desired bell. The current-generating
devices for ringing these bells are capable of sending out different
frequencies corresponding respectively to the rates of vibration of
each of the vibrating reed tongues. To select any one station,
therefore, the current frequency corresponding to the rate of
vibration of the reed tongue at that station is sent and this, being
out of tune with the reed tongues at all of the other stations,
operates the tongue of the desired station, but fails to operate those
at all of the other stations.

(_3_) In the _step-by-step_ system the bells on the line are normally
not in operative relation with the line and the bell of the desired
party on the line is made responsive by sending over the line a
certain number of impulses preliminary to ringing it. These impulses
move step-by-step mechanisms at each of the stations in unison, the
arrangement being such that the bells at the several stations are each
made operative after the sending of a certain number of preliminary
impulses, this number being different for all the stations.

(_4_) The _broken-line_ systems are new in telephony and for certain
fields of work look promising. In these the line circuit is normally
broken up into sections, the first section terminating at the first
station out from the central office, the second section at the second
station, and so on. When the line is in its normal or inactive
condition only the bell at the first station is so connected with the
line circuit as to enable it to be rung, the line being open beyond.
Sending a single preliminary impulse will, however, operate a
switching device so as to disconnect the bell at the first station and
to connect the line through to the second station. This may be carried
out, by sending the proper number of preliminary impulses, so as to
build up the line circuit to the desired station, after which the
sending of the ringing current will cause the bell to ring at that
station only.

Polarity Method. The polarity method of selective signaling on party
lines is probably the most extensively used. The standard selective
system of the American Telephone and Telegraph Company operates on
this principle.

_Two-Party Line._ It is obvious that selection may be had between two
parties on a single metallic-circuit line without the use of biased
bells or current of different polarities. Thus, one limb of a metallic
circuit may be used as one grounded line to ring the bell at one of
the stations, and the other limb of the metallic circuit may be used
as another grounded line to ring the bell of the other station; and
the two limbs may be used together as a metallic circuit for talking
purposes as usual.

This is shown in Fig. 170, where the ringing keys at the central office
are diagrammatically shown in the left-hand portion of the figure as
_K_^{1} and _K_^{2}. The operation of these keys will be more fully
pointed out in a subsequent chapter, but a correct understanding will
be had if it be remembered that the circuits are normally maintained by
these keys in the position shown. When, however, either one of the keys
is operated, the two long springs may be considered as pressed apart so
as to disengage the normal contacts between the springs and to engage
the two outer contacts, with which they are shown in the cut to be
disengaged. The two outer contacts are connected respectively to an
ordinary alternating-current ringing generator and to ground, but the
connection is reversed on the two keys.

[Illustration: Fig. 170. Simple Two-Party Line Selection]

At Station A the ordinary talking set is shown in simplified form,
consisting merely of a receiver, transmitter, and hook switch in a
single bridge circuit across the line. An ordinary polarized bell is
shown connected in series with a condenser between the lower limb of
the line and ground. At Station B the same talking circuit is shown,
but the polarized bell and condenser are bridged between the upper
limb of the line and ground.

If the operator desires to call Station A, she will press key _K_^{1}
which will ground the upper side of the line and connect the lower
side of the line with the generator _G_^{1}, and this, obviously, will
cause the bell at Station A to ring. The bell at Station B will not
ring because it is not in the circuit. If, on the other hand, the
operator desires to ring the bell at Station B, she will depress key
_K_^{2}, which will allow the current from generator _G_^{2} to pass
over the upper side of the line through the bell and condenser at
Station B and return by the path through the ground. The object of
grounding the opposite sides of the keys at the central office is to
prevent cross-ringing, that is, ringing the wrong bell. Were the keys
not grounded this might occur when a ringing current was being sent
out while the receiver at one of the stations was off its hook; the
ringing current from, say, generator _G_^{1} then passing not only
through the bell at Station A as intended, but also through the bell
at Station B by way of the bridge path through the receiver that
happened to be connected across the line. With the ringing keys
grounded as shown, it is obvious that this will not occur, since the
path for the ringing current through the wrong bell will always be
shunted by a direct path to ground on the same side of the line.

In such a two-party-line selective system the two generators _G_^{1}
and _G_^{2} may be the same generator and may be of the ordinary
alternating-current type. The bells likewise may be of the ordinary
alternating-current type.

The two-party selective line just described virtually employs two
separate circuits for ringing. Now each of these circuits alone may be
employed to accomplish selective ringing between two stations by using
two biased bells oppositely polarized, and employing pulsating ringing
currents of one direction or the other according to which bell it is
desired to ring. One side of a circuit so equipped is shown in Fig.
171. In this the two biased bells are at Station A and Station B,
these being bridged to ground in each case and adapted to respond only
to positive and negative impulses respectively. At the central office
the two keys _K_^{1} and _K_^{2} are shown. A single
alternating-current generator _G_ is shown, having its brush _1_
grounded and brush _2_ connected to a commutator disk _3_ mounted on
the generator shaft so as to revolve therewith. One-half of the
periphery of this disk is of insulating material so that the brushes
_4_ and _5_, which bear against the disk, will be alternately
connected with the disk and, therefore, with the brush _2_ of the
generator. Now the brush _2_, being one terminal of an
alternating-current machine, is alternately positive and negative, and
the arrangement of the commutator is such that the disk, which is
always at the potential of the brush _2_, will be connected to the
brush _5_ only while it is positively charged and with the brush _4_
only while it is negatively charged. As a result, brush _5_ has a
succession of positive impulses and brush _4_ a succession of negative
ones. Obviously, therefore, when key _K_^{1} is depressed only the
bell at Station A will be rung, and likewise the depression of key
_K_^{2} will result only in the ringing of the bell at Station B.

[Illustration: Fig. 171. Principle of Selection by Polarity]

_Four-Party Line._ From the two foregoing two-party line systems it is
evident that a four-party line system may be readily obtained, that
is, by employing two oppositely polarized biased bells on each side of
the metallic circuit. The selection of any of the four bells may be
obtained, choosing between the pairs connected, respectively, with the
two limbs of the line, by choosing the limb on which the current is to
be sent, and choosing between the two bells of the pair on that side
of the line by choosing which polarity of current to send.

Such a four-party line system is shown in Fig. 172. In this the
generators are not shown, but the wires leading from the four keys are
shown marked plus or minus, according to the terminal of the generator
to which they are supposed to be connected. Likewise the two bells
connected with the lower side of the line are marked positive and
negative, as are the two bells connected with the upper side of the
line. From the foregoing description of Figs. 170 and 171, it is clear
that if key _K_^{1} is pressed the bell at Station A will be rung, and
that bell only, since the bells at Station C and Station _D_ are not
in the circuit and the positive current sent over the lower side of
the line is not of the proper polarity to ring the bell at Station B.

The system shown in Fig. 172 is subject to one rather grave defect. In
subsequent chapters it will be pointed out that in common-battery
systems the display of the line signal at the central office is
affected by any one of the subscribers merely taking his receiver off
its hook and thus establishing a connection between the two limbs of
the metallic circuit. Such common-battery systems should have the two
limbs of the line, normally, entirely insulated from each other. It is
seen that this is not the case in the system just described, since
there is a conducting path from one limb of the line through the two
bells on that side to ground, and thence through the other pair of
bells to the other limb of the line. This means that unless the
resistance of the bell windings is made very high, the path of the
signaling circuit will be of sufficiently low resistance to actuate
the line signal at the central office.

[Illustration: Fig. 172. Four-Party Polarity Selection]

It is not feasible to overcome this objection by the use of condensers
in series with the bells, as was done in the system shown in Fig. 170,
since the bells are necessarily biased and such bells, as may readily
be seen, will not work properly through condensers, since the placing
of a condenser in their circuit means that the current which passes
through the bell is alternating rather than pulsating, although the
original source may have been of pulsating nature only.

[Illustration: Fig 173. Standard Polarity System]

The remedy for this difficulty, therefore, has been to place in series
with each bell a very high non-inductive resistance of about 15,000 or
20,000 ohms, and also to make the windings of the bells of
comparatively high resistance, usually about 2,500 ohms. Even with
this precaution there is a considerable leakage of the central-office
battery current from one side of the line to the other through the two
paths to ground in series. This method of selective signaling has,
therefore, been more frequently used with magneto systems. An endeavor
to apply this principle to common-battery systems without the
objections noted above has led to the adoption of a modification,
wherein a relay at each station normally holds the ground connection
open. This is shown in Fig. 173 and is the standard four-party line
ringing circuit employed by the American Telephone and Telegraph
Company and their licensees.

In this system the biased bells are normally disconnected from the
line, and, therefore, the leakage path through them from one side of
the line to the other does not exist. At each station there is a relay
winding adapted to be operated by the ringing current bridged across
the line in series with a condenser. As a result, when ringing current
is sent out on the line all of the relays, _i.e._, one at each
station, are energized and attract their armatures. This establishes
the connection of all the bells to line and really brings about
temporarily a condition equivalent to that of Fig. 172. As a result,
the sending of a positive current on the lower line with a ground
return will cause the operation of the bell at Station A. It will not
ring the bell at Station B because of the wrong polarity. It will not
ring the bells of Station C and Station D because they are in the
circuit between the other side of the line and ground. As soon as the
ringing current ceases all of the relays release their armatures and
disconnect all the bells from the line.

By this very simple device the trouble, due to marginal working of
the line signal, is done away with, since normally there is no leakage
from one side of the line to the other on account of the presence of
the condensers in the bridge at each station.

[Illustration: Fig. 174. Ringing-Key Arrangement]

In Fig. 174, the more complete connections of the central-office
ringing keys are shown, by means of which the proper positive or
negative ringing currents are sent to line in the proper way to cause
the ringing of any one of the four bells on a party line of either of
the types shown in Figs. 172 and 173.

In this the generator _G_ and its commutator disk _3_, with the
various brushes, _1_, _2_, _4_, and _5_, are arranged in the same
manner as is shown in Fig. 171. It is evident from what has been said
that wire _6_ leading from generator brush _2_ and commutator disk _3_
will carry alternating potential; that wire _7_ will carry positive
pulsations of potential; and that wire _8_ will carry negative
pulsations of potential. There are five keys in the set illustrated in
Fig. 174, of which four, viz, _K_^{1}, _K_^{2}, _K_^{3}, and _K_^{4},
are connected in the same manner as diagrammatically indicated in
Figs. 172 and 173, and will, obviously, serve to send the proper
current over the proper limb of the line to ring one of the bells. Key
_K_^{5}, the fifth one in the set, is added so as to enable the
operator to ring an ordinary unbiased bell on a single party line when
connection is made with such line. As the two outside contacts of this
key are connected respectively to the two brushes of the
alternating-current dynamo _G_, it is clear that it will impress an
alternating current on the line when its contacts are closed.

_Circuits of Two-Party Line Telephones._ In Fig. 175 is shown in
detail the wiring of the telephone set usually employed in connection
with the party-line selective-ringing system illustrated in Fig. 170.
In the wiring of this set and the two following, it must be borne in
mind that the portion of the circuit used during conversation might
be wired in a number of ways without affecting the principle of
selective ringing employed; however, the circuits shown are those most
commonly employed with the respective selective ringing systems which
they are intended to illustrate. In connecting the circuits of this
telephone instrument to the line, the two line conductors are
connected to binding posts _1_ and _2_ and a ground connection is made
to binding post _3_. In practice, in order to avoid the necessity of
changing the permanent wiring of the telephone set in connecting it as
an A or B Station (Fig. 170), the line conductors are connected to the
binding posts in reverse order at the two stations; that is, for
Station A the upper conductor, Fig. 170, is connected to binding post
_1_ and the lower conductor to binding post _2_, while at Station B
the upper conductor is connected to binding post _2_ and the lower
conductor to binding post _1_. The permanent wiring of this telephone
set is the same as that frequently used for a set connected to a line
having only one station, the proper ringing circuit being made by the
method of connecting up the binding posts. For example, if this
telephone set were to be used on a single station line, the binding
posts _1_ and _2_ would be connected to the two conductors of the line
as before, while binding post _3_ would be connected to post _1_
instead of being grounded.

[Illustration: Fig. 175. Circuit of Two-Party Station]

_Circuits of Four-Party-Line Telephones._ The wiring of the telephone
set used with the system illustrated in Fig. 172 is shown in detail in
Fig. 176. The wiring of this set is arranged for local battery or
magneto working, as this method of selective ringing is more frequently
employed with magneto systems, on account of the objectionable features
which arise when applied to common-battery systems. In this figure the
line conductors are connected to binding posts _1_ and _2_, and a
ground connection is made to binding post _3_. In order that all sets
may be wired alike and yet permit the instrument to be connected for
any one of the various stations, the bell is not permanently wired to
any portion of the circuit but has flexible connections which will
allow of the set being properly connected for any desired station. The
terminals of the bell are connected to binding posts _9_ and _10_, to
which are connected flexible conductors terminating in terminals _7_
and _8_. These terminals may be connected to the binding posts _4_,
_5_, and _6_ in the proper manner to connect the set as an A, B, C, or
D station, as required. For example, in connecting the set for Station
A, Fig. 172, terminal _7_ is connected to binding post _6_ and _8_ to
_5_. For connecting the set for Station B terminal _7_ is connected to
binding post _5_ and _8_ to _6_. For connecting the set for Station C
terminal _7_ is connected to binding post _6_ and _8_ to _4_. For
connecting the set for Station D terminal _7_ is connected to binding
post _4_ and _8_ to _6_.

[Illustration: Fig. 176. Circuit of Four-Party Station without Relay]

[Illustration: Fig. 177. Circuit of Four-Party Station with Relay]

The detailed wiring of the telephone set employed in connection with
the system illustrated in Fig. 173 is shown in Fig. 177. The wiring of
this set is arranged for a common-battery system, inasmuch as this
arrangement of signaling circuit is more especially adapted for
common-battery working. However, this arrangement is frequently
adapted to magneto systems as even with magneto systems a permanent
ground connection at a subscriber's station is objectionable inasmuch
as it increases the difficulty of determining the existence or
location of an accidental ground on one of the line conductors. The
wiring of this set is also arranged so that one standard type of
wiring may be employed and yet allow any telephone set to be connected
as an A, B, C, or D station.

Harmonic Method. _Principles._ To best understand the principle of
operation of the harmonic party-line signaling systems, it is to be
remembered that a flexible reed, mounted rigidly at one end and having
its other end free to vibrate, will, like a violin string, have a
certain natural period of vibration; that is, if it be started in
vibration, as by snapping it with the fingers, it will take up a
certain rate of vibration which will continue at a uniform rate until
the vibration ceases altogether. Such a reed will be most easily
thrown into vibration by a series of impulses having a frequency
corresponding exactly to the natural rate of vibration of the reed
itself; it may be thrown into vibration by very slight impulses if
they occur at exactly the proper times.

It is familiar to all that a person pushing another in a swing may
cause a considerable amplitude of vibration with the exertion of but a
small amount of force, if he will so time his pushes as to conform
exactly to the natural rate of vibration of the swing. It is of course
possible, however, to make the swing take up other rates of vibrations
by the application of sufficient force. As another example, consider a
clock pendulum beating seconds. By gentle blows furnished by the
escapement at exactly the proper times, the heavy pendulum is kept in
motion. However, if a person grasps the pendulum weight and shakes it,
it may be made to vibrate at almost any desired rate, dependent on the
strength and agility of the individual.

The conclusion is, therefore, that a reed or pendulum may be made to
start and vibrate easily by the application of impulses at proper
intervals, and only with great difficulty by the application of
impulses at other than the proper intervals; and these facts form the
basis on which harmonic-ringing systems rest.

The father of harmonic ringing in telephony was Jacob B. Currier, an
undertaker of Lowell, Mass. His harmonic bells were placed in series
in the telephone line, and were considerably used in New England in
commercial practice in the early eighties. Somewhat later James A.
Lighthipe of San Francisco independently invented a harmonic-ringing
system, which was put in successful commercial use at Sacramento and a
few other smaller California towns. Lighthipe polarized his bells and
bridged them across the line in series with condensers, as in modern
practice, and save for some crudities in design, his apparatus closely
resembled, both in principle and construction, some of that in
successful use today.

Lighthipe's system went out of use and was almost forgotten, when
about 1903, Wm. W. Dean again independently redeveloped the harmonic
system, and produced a bell astonishingly like that of Lighthipe, but
of more refined design, thus starting the development which has
resulted in the present wide use of this system.

The signal-receiving device in harmonic-ringing systems takes the form
of a ringer, having its armature and striker mounted on a rather stiff
spring rather than on trunnions. By this means the moving parts of the
bell constitute in effect a reed tongue, which has a natural rate of
vibration at which it may easily be made to vibrate with sufficient
amplitude to strike the gongs. The harmonic ringer differs from the
ordinary polarized bell or ringer, therefore, in that its armature
will vibrate most easily at one particular rate, while the armature of
the ordinary ringer is almost indifferent, between rather wide limits,
as to the rate at which it vibrates.

As a rule harmonic party-line systems are limited to four stations on
a line. The frequencies employed are usually 16-2/3, 33-1/3, 50, and
66-2/3 cycles per second, this corresponding to 1,000, 2,000, 3,000,
and 4,000 cycles per minute. The reason why this particular set of
frequencies was chosen is that they represent approximately the range
of desirable frequencies, and that the first ringing-current machines
in such systems were made by mounting the armatures of four different
generators on a single shaft, these having, respectively, two poles,
four poles, six poles, and eight poles each. The two-pole generator
gave one cycle per revolution, the four-pole two, the six-pole three,
and the eight-pole four, so that by running the shaft of the machine
at exactly 1,000 revolutions per minute the frequencies before
mentioned were attained. This range of frequencies having proved
about right for general practice and the early ringers all having been
attuned so as to operate on this basis, the practice of adhering to
these numbers of vibrations has been kept up with one exception by all
the manufacturers who make this type of ringer.

_Tuning._ The process of adjusting the armature of a ringer to a
certain rate of vibration is called tuning, and it is customary to
refer to a ringer as being tuned to a certain rate of vibration, just
as it is customary to refer to a violin string as being tuned to a
certain pitch or rate of vibration.

The physical difference between the ringers of the various frequencies
consists mainly in the size of the weights at the end of the vibrating
reed, that is, of the weights which form the tapper for the bell. The
low-frequency ringers have the largest weights and the high-frequency
the smallest, of course. The ringers are roughly tuned to the desired
frequencies by merely placing on the tapper rod the desired weight and
then a more refined tuning is given them by slightly altering the
positions of the weights on the tapper rod. To make the reed have a
slightly lower natural rate of vibration, the weight is moved further
from the stationary end of the reed, while to give it a slightly
higher natural rate of vibration the weight is moved toward the
stationary. In this way very nice adjustments may be made, and the aim
of the various factories manufacturing these bells is to make the
adjustment permanent so that it will never have to be altered by the
operating companies. Several years of experience with these bells has
shown that when once properly assembled they maintain the same rate of
vibration with great constancy.

There are two general methods of operating harmonic bells. One of
these may be called the in-tune system and the other the under-tune
system. The under-tune system was the first employed.

[Illustration: OPERATING ROOM AT TOKYO, JAPAN]

_Under-Tune System._ The early workers in the field of
harmonic-selective signaling discovered that when the tapper of the
reed struck against gongs the natural rate of vibration of the reed
was changed, or more properly, the reed was made to have a different
rate of vibration from its natural rate. This was caused by the fact
that the elasticity of the gongs proved another factor in the set
of conditions causing the reeds to take up a certain rate of
vibration, and the effect of this added factor was always to
accelerate the rate of vibration which the reed had when it was not
striking the gongs. The rebound of the hammer from the gongs tended,
in other words, to accelerate the rate of vibration, which, as might
be expected, caused a serious difficulty in the practical operation of
the bells. To illustrate: If a reed were to have a natural rate of
vibration, when not striking the gongs, of 50 per second and a current
of 50 cycles per second were impressed on the line, the reed would
take up this rate of vibration easily, but when a sufficient amplitude
of vibration was attained to cause the tapper to strike the gongs, the
reed would be thrown out of tune, on account of the tendency of the
gongs to make the reed vibrate at a higher rate. This caused irregular
ringing and was frequently sufficient to make the bells cease ringing
altogether or to ring in an entirely unsatisfactory manner.

In order to provide for this difficulty the early bells of Currier and
Lighthipe were made on what has since been called the "under-tuned"
principle. The first bells of the Kellogg Switchboard and Supply
Company, developed by Dean, were based on this idea as their cardinal
principle. The reeds were all given a natural rate of vibration, when
not striking the gongs, somewhat below that of the current frequencies
to be employed; and yet not sufficiently below the corresponding
current frequency to make the bell so far out of tune that the current
frequency would not be able to start it. This was done so that when
the tapper began to strike the gongs the tapper would be accelerated
and brought practically into tune with the current frequency, and the
ringing would continue regularly as long as the current flowed. It
will be seen that the under-tuned system was, therefore, one involving
some difficulty in starting in order to provide for proper regularity
while actually ringing.

Ringers of this kind were always made with but a single gong, it being
found difficult to secure uniformity of ringing and uniformity of
adjustment when two gongs were employed. Although no ringers of this
type are being made at present, yet a large number of them are in use
and they will consequently be described. Their action is interesting
in throwing better light on the more improved types, if for no other
reason.

Figs. 178 and 179 show, respectively, side and front views of the
original Kellogg bell. The entire mechanism is self-contained, all
parts being mounted on the base plate _1_. The electromagnet is of the
two-coil type, and is supported on the brackets _2_ and _3_. The
bracket _2_ is of iron so as to afford a magnetic yoke for the field of
the electromagnet, while the bracket _3_ is of brass so as not to
short-circuit the magnetic lines across the air-gap. The reed
tongue--consisting of the steel spring _5_, the soft-iron armature
pieces _6_, the auxiliary spring _7_, and the tapper ball _8_, all of
which are riveted together, as shown in Fig. 178--constitutes the only
moving part of the bell. The steel spring _5_ is rigidly mounted in the
clamping piece _9_ at the upper part of the bracket _3_, and the reed
tongue is permitted to vibrate only by the flexibility of this spring.
The auxiliary spring _7_ is much lighter than the spring _5_ and has
for its purpose the provision of a certain small amount of flexibility
between the tapper ball and the more rigid portion of the armature
formed by the iron strips _6-6_. The front ends of the magnet pole
pieces extend through the bracket _3_ and are there provided with
square soft-iron pole pieces _10_ set at right angles to the magnet
cores so as to form a rather narrow air-gap in which the armature may
vibrate.

[Illustration: Fig. 178. Under-Tuned Ringer]

The cores of the magnet and also the reed tongue are polarized by
means of the =L=-shaped bar magnet _4_, mounted on the iron yoke _2_
at one end in such manner that its other end will lie quite close to
the end of the spring _5_, which, being of steel, will afford a path
for the lines of force to the armature proper. We see, therefore, that
the two magnet cores are, by this permanent magnet, given one
polarity, while the reed tongue itself is given the other polarity,
this being exactly the condition that has already been described in
connection with the regular polarized bell or ringer.

The electromagnetic action by which this reed tongue is made to
vibrate is, therefore, exactly the same as that of an ordinary
polarized ringer, but the difference between the two is that, in this
harmonic ringer, the reed tongue will respond only to one particular
rate of vibrations, while the regular polarized ringer will respond to
almost any.

As shown in Fig. 178, the tapper ball strikes on the inside surface of
the single gong. The function of the auxiliary spring _7_ between the
ball and the main portion of the armature is to allow some resilience
between the ball and the balance of the armature so as to counteract
in some measure the accelerating influence of the gong on the
armature. In these bells, as already stated, the natural rate of
vibration of the reed tongue was made somewhat lower than the rate at
which the bell was to be operated, so that the reed tongue had to be
started by a current slightly out of tune with it, and then, as the
tapper struck the gong, the acceleration due to the gong would bring
the vibration of the reed tongue, as modified by the gong, into tune
with the current that was operating it. In ether words, in this system
the ringing currents that were applied to the line had frequencies
corresponding to what may be called the _operative rates of vibration_
of the reed tongues, which operative rates of vibration were in each
case the resultant of the natural pitch of the reed as modified by the
action of the bell gong when struck.

[Illustration: Fig. 179. Under-Tuned Ringer]

_In-Tune System._ The more modern method of tuning is to make the
natural rate of vibration of the reed tongue, that is, the rate at
which it naturally vibrates when not striking the gongs, such as to
accurately correspond to the rate of vibration at which the bells are
to be operated--that is, the natural rate of vibration of the reed
tongues is made the same as the operative rate. Thus the bells are
attuned for easy starting, a great advantage over the under-tuned
system. In the under-tuned system, the reeds being out of tune in
starting require heavier starting current, and this is obviously
conducive to cross-ringing, that is, to the response of bells to other
than the intended frequency.

Again, easy starting is desirable because when the armature is at
rest, or in very slight vibration, it is at a maximum distance from
the poles of the electromagnet, and, therefore, subject to the weakest
influence of the poles. A current, therefore, which is strong enough
to start the vibration, will be strong enough to keep the bell ringing
properly.

[Illustration: Fig. 180. Dean In-Tune Ringer]

When with this "in-tune" mode of operation, the armature is thrown
into sufficiently wide vibration to cause the tapper to strike the
gong, the gong may tend to accelerate the vibration of the reed
tongue, but the current impulses through the electromagnet coils
continue at precisely the same rates as before. Under this condition
of vibration, when the reed tongue has an amplitude of vibration wide
enough to cause the tapper to strike the gongs, the ends of the
armature come closest to the pole pieces, so that the pole pieces have
their maximum magnetic effect on the armature, with the result that
even if the accelerating tendency of the gongs were considerable, the
comparatively large magnetic attractive impulses occurring at the same
rate as the natural rate of vibration of the reed tongue, serve wholly
to prevent any actual acceleration of the reed tongue. The magnetic
attractions upon the ends of the armature, continuing at the initial
rate, serve, therefore, as a check to offset any accelerating
tendency which the striking of the gong may have upon the vibrating
reed tongue.

It is obvious, therefore, that in the "in-tune" system the
electromagnetic effect on the armature should, when the armature is
closest to the pole pieces, be of such an overpowering nature as to
prevent whatever accelerating tendency the gongs may have from
throwing the armature out of its "stride" in step with the current.
For this reason it is usual in this type to so adjust the armature
that its ends will actually strike against the pole pieces of the
electromagnet when thrown into vibration. Sufficient flexibility is
given to the tapper rod to allow it to continue slightly beyond the
point at which it would be brought to rest by the striking of the
armature ends against the pole pieces and thus exert a whipping action
so as to allow the ball to continue in its movement far enough to
strike against the gongs. The rebound of the gong is then taken up by
the elasticity of the tapper rod, which returns to an unflexed
position, and at about this time the pole piece releases the armature
so that it may swing over in the other direction to cause the tapper
to strike the other gong.

[Illustration: Fig. 181. Tappers for Dean Ringers]

The construction of the "in-tune" harmonic ringer employed by the Dean
Electric Company, of Elyria, Ohio, is illustrated in Figs. 180, 181,
and 182. It will be seen from Fig. 180 that the general arrangement of
the magnet and armature is the same as that of the ordinary polarized
ringer; the essential difference is that the armature is
spring-mounted instead of pivoted. The armature and the tapper rod
normally stand in the normal central position with reference to the
pole pieces of the magnet and the gongs. Fig. 181 shows the complete
vibrating parts of four ringers, adapted, respectively, to the four
different frequencies of the system. The assembled armature, tapper
rod, and tapper are all riveted together and are non-adjustable. All
of the adjustment that is done upon them is done in the factory and
is accomplished, first, by choosing the proper size of weight, and
second, by forcing this weight into the proper position on the tapper
rod to give exactly the rate of vibration that is desired.

[Illustration: Fig. 182. Dean In-Tune Ringer]

An interesting feature of this Dean harmonic ringer is the gong
adjustment. As will be seen, the gongs are mounted on posts which are
carried on levers pivoted to the ringer frame. These levers have at
their outer end a curved rack provided with gear teeth adapted to
engage a worm or screw thread mounted on the ringer frame. Obviously,
by turning this worm screw in one direction or the other, the gongs
are moved slightly toward or from the armature or tapper. This affords
a very delicate means of adjusting the gongs, and at the same time one
which has no tendency to work loose or to get out of adjustment.

[Illustration: Fig. 183. Kellogg In-Tune Ringer]

In Fig. 183 is shown a drawing of the "in-tune" harmonic ringer
manufactured by the Kellogg Switchboard and Supply Company. This
differs in no essential respect from that of the Dean Company, except
in the gong adjustment, this latter being affected by a screw passing
through a nut in the gong post, as clearly indicated.

In both the Kellogg and the Dean in-tune ringers, on account of the
comparative stiffness of the armature springs and on account of the
normal position of the armature with maximum air gaps and consequent
minimum magnetic pull, the armature will practically not be affected
unless the energizing current is accurately attuned to its own natural
rate. When the proper current is thrown on to the line, the ball will
be thrown into violent vibration, and the ends of the armature brought
into actual contact with the pole pieces, which are of bare iron and
shielded in no way. The armature in this position is very strongly
attracted and comes to a sudden stop on the pole pieces. The gongs are
so adjusted that the tapper ball will have to spring about one
thirty-second of an inch in order to hit them. The armature is held
against the pole piece while the tapper ball is engaged in striking
the gong and in partially returning therefrom, and so strong is the
pull of the pole piece on the armature in this position that the
accelerating influence of the gong has no effect in accelerating the
rate of vibration of the reed.

[Illustration: Fig. 184. Circuits of Dean Harmonic System]

_Circuits_. In Fig. 184 are shown in simplified form the circuits of a
four-station harmonic party line. It is seen that at the central
office there are four ringing keys, adapted, respectively, to impress
on the line ringing currents of four different frequencies. At the
four stations on the line, lettered A, B, C, and D, there are four
harmonic bells tuned accordingly. At Station A there is shown the
talking apparatus employing the Wheatstone bridge arrangement. The
talking apparatus at all of the other stations is exactly the same,
but is omitted for the sake of simplicity. A condenser is placed in
series with each of the bells in order that there may be no
direct-current path from one side of the line to the other when all of
the receivers are on their hooks at the several stations.

In Fig. 185 is shown exactly the same arrangement, with the exception
that the talking apparatus illustrated in detail at Station A is that
of the Kellogg Switchboard and Supply Company. Otherwise the circuits
of the Dean and the Kellogg Company, and in fact of all the other
companies manufacturing harmonic ringing systems, are the same.

_Advantages_. A great advantage of the harmonic party-line system is
the simplicity of the apparatus at the subscriber's station. The
harmonic bell is scarcely more complex than the ordinary polarized
ringer, and the only difference between the harmonic-ringing telephone
and the ordinary telephone is in the ringer itself. The absence of all
relays and other mechanism and also the absence of the necessity for
ground connections at the telephone are all points in favor of the
harmonic system.

[Illustration: Fig. 185. Circuits of Kellogg Harmonic System]

_Limitations_. As already stated, the harmonic systems of the various
companies, with one exception, are limited to four frequencies. The
exception is in the case of the North Electric Company, which sometimes
employs four and sometimes five frequencies and thus gets a selection
between five stations. In the four-party North system, the frequencies,
unlike those in the Dean and Kellogg systems, wherein the higher
frequencies are multiples of the lower, are arranged so as to be
proportional to the whole numbers 5, 7, 9, and 11, which, of course,
have no common denominator. The frequencies thus employed in the North
system are, in cycles per second, 30.3, 42.4, 54.5, and 66.7. In the
five-party system, the frequency of 16.7 is arbitrarily added.

While all of the commercial harmonic systems on the market are
limited to four or five frequencies, it does not follow that a greater
number than four or five stations may not be selectively rung. Double
these numbers may be placed on a party line and selectively actuated,
if the first set of four or five is bridged across the line and the
second set of four or five is connected between one limb of the line
and ground. The first set of these is selectively rung, as already
described, by sending the ringing currents over the metallic circuit,
while the second set may be likewise selectively rung by sending the
ringing currents over one limb of the line with a ground return. This
method is frequently employed with success on country lines, where it
is desired to place a greater number of instruments on a line than
four or five.

Step-by-Step Method. A very large number of step-by-step systems
have been proposed and reduced to practice, but as yet they have not
met with great success in commercial telephone work, and are nowhere
near as commonly used as are the polarity and harmonic systems.

_Principles_. An idea of the general features of the step-by-step
systems may be had by conceiving at each station on the line a ratchet
wheel, having a pawl adapted to drive it one step at a time, this pawl
being associated with the armature of an electromagnet which receives
current impulses from the line circuit. There is thus one of these
driving magnets at each station, each bridged across the line so that
when a single impulse of current is sent out from the central office
all of the ratchet wheels will be moved one step. Another impulse will
move all of the ratchet wheels another step, and so on throughout any
desired number of impulses. The ratchet wheels, therefore, are all
stepped in unison.

Let us further conceive that all of these ratchet wheels are provided
with a notch or a hole or a projection, alike in all respects at all
stations save in the position which this notch or hole or projection
occupies on the wheel. The thing to get clear in this part of the
conception is that all of these notches, holes, or projections are
alike on all of the wheels, but they occupy a different position on
the wheel for each one of the stations.

Consider further that the bell circuit at each of the stations is
normally open, but that in each case it is adapted to be closed when
the notch, hole, or projection is brought to a certain point by the
revolution of the wheel.

Let us conceive further that this distinguishing notch, hole, or
projection is so arranged on the wheel of the first station as to
close the bell circuit when one impulse has been sent, that that on
the second station will close the bell circuit after the second
impulse has been sent, and so on throughout the entire number of
stations. It will, therefore, be apparent that the bell circuits at
the various stations will, as the wheels are rotated in unison, be
closed one after the other. In order to call a given station,
therefore, it is only necessary to rotate all of the wheels in unison,
by sending out the proper stepping impulses until they all occupy such
a position that the one at the desired station is in such position as
to close the bell circuit at that station. Since all of the notches,
holes, or projections are arranged to close the bell circuits at their
respective stations at different times, it follows that when the bell
circuit at the desired station is closed those at all of the other
stations will be open. If, therefore, after the proper number of
stepping impulses has been sent to the line to close the bell circuit
of the desired station, ringing current be applied to the line, it is
obvious that the bell of that one station will be rung to the
exclusion of all others. It is, of course, necessary that provision be
made whereby the magnets which furnish the energy for stepping the
wheels will not be energized by the ringing current. This is
accomplished in one of several ways, the most common of which is to
have the stepping magnets polarized or biased in one direction and the
bells at the various stations oppositely biased, so that the ringing
current will not affect the stepping magnet and the stepping current
will not affect the ringer magnets.

After a conversation is finished, the line may be restored to its
normal position in one of several ways. Usually so-called release
magnets are employed, for operating on the releasing device at each
station. These, when energized, will withdraw the holding pawls from
the ratchets and allow them all to return to their normal positions.
Sometimes these release magnets are operated by a long impulse of
current, being made too sluggish in their action to respond to the
quick-stepping impulses; sometimes the release magnets are tapped from
one limb of the line to ground, so as not to be affected by the
stepping or ringing currents sent over the metallic circuit; and
sometimes other expedients are used for obtaining the release of the
ratchets at the proper time, a large amount of ingenuity having been
spent to this end.

As practically all step-by-step party-line systems in commercial use
have also certain other features intended to assure privacy of
conversation to the users, and, therefore, come under the general
heading of lock-out party-line systems, the discussion of commercial
examples of these systems will be left for the next chapter, which is
devoted to such lock-out systems.

Broken-Line Method. The broken-line system, like the step-by-step
system, is also essentially a lock-out system and for that reason only
its general features, by which the selective ringing is accomplished,
will be dealt with here.

_Principles_. In this system there are no tuned bells, no positively
and negatively polarized bells bridged to ground on each side of the
line, and no step-by-step devices in the ordinary sense, by which
selective signaling has ordinarily been accomplished on party lines.
Instead of this, each instrument on the line is exclusively brought
into operative relation with the line, and then removed from such
operative relation until the subscriber wanted is connected, at which
time all of the other instruments are locked out and the line is not
encumbered by any bridge circuits at any of the instruments that are
not engaged in the conversation. Furthermore, in the selecting of a
subscriber or the ringing of his bell there is no splitting up of
current among the magnets at the various stations as in ordinary
practice, but the operating current goes straight to the station
desired and to that station alone where its entire strength is
available for performing its proper work.

In order to make the system clear it may be stated at the outset that
one side of the metallic circuit line is continued as in ordinary
practice, passing through all of the stations as a continuous
conductor. The other side of the line, however, is divided into
sections, its continuity being broken at each of the subscriber's
stations. Fig. 186 is intended to show in the simplest possible way
how the circuit of the line may be extended from station to station in
such manner that only the ringer of one station is in circuit at a
time. The two sides of the line are shown in this figure, and it will
be seen that limb _L_ extends from the central office on the left to
the last station on the right without a break. The limb _R_, however,
extends to the first station, at which point it is cut off from the
extension _R_{x}_ by the open contacts of a switch. For the purpose of
simplicity this switch is shown as an ordinary hand switch, but as a
matter of fact it is a part of a relay, the operating coil of which is
shown at _6_, just above it, in series with the ringer.

[Illustration: Fig. 186. Principle of Broken-Line System]

Obviously, if a proper ringing current is sent over the metallic
circuit from the central office, only the bell at Station A will
operate, since the bells at the other stations are not in the circuit.
If by any means the switch lever _2_ at Station A were moved out of
engagement with contact _1_ and into engagement with contact _3_, it
is obvious that the bell of Station A would no longer be in circuit,
but the limb _R_ of the line would be continued to the extension
_R_{x}_ and the bell of Station B would be in circuit. Any current
then sent over the circuit of the line from the central office would
ring the bell of this station. In Fig. 187 the switches of both
Station A and Station B have been thus operated, and Station C is thus
placed in circuit. Inspection of this figure will show that the bells
of Station A, Station B, and Station D are all cut out of circuit, and
that, therefore, no current from the central office can affect them.
This general scheme of selection is a new-comer in the field, and for
certain classes of work it is of undoubted promise.

[Illustration: Fig. 187. Principle of Broken-Line System]




CHAPTER XVII

LOCK-OUT PARTY-LINE SYSTEMS


The party-line problem in rural districts is somewhat different from
that within urban limits. In the latter cases, owing to the closer
grouping of the subscribers, it is not now generally considered
desirable, even from the standpoint of economy, to place more than
four subscribers on a single line. For such a line selective ringing
is simple, both from the standpoint of apparatus and operation; and
moreover owing to the small number of stations on a line, and the
small amount of traffic to and from such subscribers as usually take
party-line service, the interference between parties on the same line
is not a very serious matter.

For rural districts, particularly those tributary to small towns,
these conditions do not exist. Owing to the remoteness of the stations
from each other it is not feasible from the standpoint of line cost to
limit the number of stations to four. A much greater number of
stations is employed and the confusion resulting is distressing not
only to the subscribers themselves but also to the management of the
company. There exists then the need of a party-line system which will
give the limited user in rural districts a service, at least
approaching that which he would get if served by an individual line.

The principal investment necessary to provide facilities for telephone
service is that required to produce the telephone line. In many cases
the cost of instruments and apparatus is small in comparison with the
cost of the line. By far the greater number of subscribers in rural
districts are those who use their instruments a comparatively small
number of times a day, and to maintain an expensive telephone line for
the exclusive use of one such subscriber who will use it but a few
minutes each day is on its face an economic waste. As a result, where
individual line service is practiced exclusively one of two things
must be true: either the average subscriber pays more for his service
than he should, or else the operating company sells the service for
less than it costs, or at best for an insufficient profit. Both of
these conditions are unnatural and cannot be permanent.

The party-line method of giving service, by which a single line is
made to serve a number of subscribers, offers a solution to this
difficulty, but the ordinary non-selective or even selective party
line has many undesirable features if the attempt is made to place on
it such a large number of stations as is considered economically
necessary in rural work. These undesirable features work to the
detriment of both the user of the telephone and the operating company.

Many attempts have been made to overcome these disadvantages of the
party line in sparsely settled communities, by producing what are
commonly called lock-out systems. These, as their name implies, employ
such an arrangement of parts that when the line is in use by any two
parties, all other parties are locked out from the circuit and cannot
gain access to it until the parties who are using it are through.
System after system for accomplishing this purpose has been announced
but for the most part these have involved such a degree of complexity
and have introduced so many undesirable features as to seriously
affect the smooth operation of the system and the reliability of the
service.

We believe, however, in spite of numerous failures, that the lock-out
selective-signaling party line has a real field of usefulness and that
operating companies as well as manufacturing companies are beginning
to appreciate this need, and as a result that the relief of the rural
subscriber from the almost intolerable service he has often had to
endure is at hand. A few of the most promising lock-out party-line
systems now before the public will, therefore, be described in some
detail.

Poole System. The Poole system is a lock-out system pure and simple,
its devices being in the nature of a lock-out attachment for
selective-signaling lines, either of the polarity or of the harmonic
type wherein common-battery transmission is employed. It will be here
described as employed in connection with an ordinary harmonic-ringing
system.

In Fig. 188 there is shown a four-station party line equipped with
Poole lock-out devices, it being assumed that the ringers at each
station are harmonic and that the keys at the central office are the
ordinary keys adapted to impress the proper frequency on the line for
ringing any one of the stations. In addition to the ordinary talking
and ringing apparatus at each subscriber's station, there is a relay
of special form and also a push-button key.

[Illustration: Fig. 188. Poole Lock-Out System]

Each of the relays has two windings, one of high resistance and the
other of low resistance. Remembering that the system to which this
device is applied is always a common-battery system, and that,
therefore, the normal condition of the line will be one in which there
is a difference of potential between the two limbs, it will be evident
that whenever any subscriber on a line that is not in use raises his
receiver from its hook, a circuit will be established from the upper
contact of the hook through the lever of the hook to the
high-resistance winding _1_ of the relay and thence to the other side
of the line by way of wire _6_. This will result in current passing
through the high-resistance winding of the relay and the relay will
pull up its armature. As soon as it does so it establishes two other
circuits by the closure of the relay armature against the contacts _4_
and _5_.

The closing of the contact _4_ establishes a circuit from the upper
side of the line through the upper contact of the switch hook, thence
through the contacts of the push button _3_, thence through the
low-resistance winding _2_ of the relay to the terminal _4_, thence
through the relay armature and the transmitter to the lower side of
the line. This low-resistance path across the line serves to hold the
relay armature attracted and also to furnish current to the
transmitter for talking. The establishment of this low-resistance path
across the line does another important thing, however; it practically
short-circuits the line with respect to all the high-resistance relay
windings, and thus prevents any of the other high-resistance relay
windings from receiving enough current to actuate them, should the
subscriber at any other station remove his receiver from the hook in
an attempt to listen in or to make a call while the line is in use. As
a subscriber can only establish the proper conditions for talking and
listening by the attraction of this relay armature at his station, it
is obvious that unless he can cause the pulling up of his relay
armature he can not place himself in communication with the line.

The second thing that is accomplished by the pulling up of the relay
armature is the closure of the contacts _5_, and that completes the
talking circuit through the condenser and receiver across the line in
an obvious fashion. The result of this arrangement is that it is the
first party who raises his receiver from its hook who is enabled to
successfully establish a connection with the line, all subsequent
efforts, by other subscribers, failing to do so because of the fact
that the line is short-circuited by the path through the
low-resistance winding and the transmitter of the station that is
already connected with the line.

A little target is moved by the action of the relay so that a visual
indication is given to the subscriber in making a call to show whether
or not he is successful in getting the use of the line. If the relay
operates and he secures control of the line, the target indicates the
fact by its movement, while if someone else is using the line and the
relay does not operate, the target, by its failure to move, indicates
that fact.

When one party desires to converse with another on the same line, he
depresses the button _3_ at his station until after the called party
has been rung and has responded. This holds the circuit of his
low-resistance winding open, and thus prevents the lock-out from
becoming effective until the called party is connected with the line.
The relay armature of the calling party does not fall back with the
establishment of the low-resistance path at the called station,
because, even though shunted, it still receives sufficient current to
hold its armature in its attracted position. After the called party
has responded, the button at the calling station is released and both
low-resistance holding coils act in multiple.

[Illustration: 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.]

No induction coil is used in this system and the impedance of the
holding coil is such that incoming voice currents flow through the
condenser and the receiver, which, by reference to the figure, will be
seen to be in shunt with the holding coil. The holding coil is in
series with the local transmitter, thus making a circuit similar to
that of the Kellogg common-battery talking circuit already discussed.

A possible defect in the use of this system is one that has been common
to a great many other lock-out systems, depending for their operation
on the same general plan of action. This appears when the instruments
are used on a comparatively long line. Since the locking-out of all the
instruments that are not in use by the one that is in use depends on
the low-resistance shunt that is placed across the line by the
instrument that is in use, it is obvious that, in the case of a long
line, the resistance of the line wire will enter into the problem in
such a way as to tend to defeat the locking-out function in some cases.
Thus, where the first instrument to use the line is at the remote end
of the line, the shunting effect that this instrument can exert with
respect to another instrument near the central office is that due to
the resistance of the line plus the resistance of the holding coil at
the end instrument. The resistance of the line wire may be so high as
to still allow a sufficient current to flow through the high-resistance
coil at the nearer station to allow its operation, even though the more
remote instrument is already in use.

Coming now to a consideration of the complete selective-signaling
lock-out systems, wherein the selection of the party and the locking
out of the others are both inherent features, a single example of the
step-by-step, and of the broken-line selective lock-out systems will
be discussed.

Step-by-Step System. The so-called K.B. system, manufactured by the
Dayton Telephone Lock-out Manufacturing Company of Dayton, Ohio,
operates on the step-by-step principle. The essential feature of the
subscriber's telephone equipment in this system is the step-by-step
actuating mechanism which performs also the functions of a relay. This
device consists of an electromagnet having two cores, with a permanent
polarizing magnet therebetween, the arrangement in this respect being
the same as in an ordinary polarized bell. The armature of this magnet
works a rocker arm, which, besides stepping the selector segment
around, also, under certain conditions, closes the bell circuit and
the talking circuit, as will be described.

[Illustration: Fig. 189. K.B. Lock-Out System]

Referring first to Fig. 189, which shows in simplified form a
four-station K.B. lock-out line, the electromagnet is shown at _1_ and
the rocker arm at _2_. The ratchet _3_ in this case is not a complete
wheel but rather a segment thereof, and it is provided with a series
of notches of different depths. It is obvious that the depth of the
notches will determine the degree of movement which the upper end of
the rocker arm may have toward the left, this being dependent on the
extent to which the pawl _6_ is permitted to enter into the segment.
The first or normal notch, _i.e._, the top notch, is always of such a
depth that it will allow the rocker-arm lever _2_ to engage the
contact lever _4_, but will not permit the rocker arm to swing far
enough to the left to cause that contact to engage the bell contact
_5_. As will be shown later, the condition for the talking circuit to
be closed is that the rocker arm _2_ shall rest against the contact
_4_; and from this we see that the normal notch of each of the
segments _3_ is of such a depth as to allow the talking circuit at
each station to be closed. The next notch, _i.e._, the second one in
each disk, is always shallow, as are all of the other notches except
one. A deep notch is placed on each disk anywhere from the third to
the next to the last on the segment. This deep notch is called the
_selective notch_, and it is the one that allows of contact being made
with the ringer circuit of that station when the pawl _6_ drops into
it. The position of this notch differs on all of the segments on a
line, and obviously, therefore, the ringer circuit at any station may
be closed to the exclusion of all the others by stepping all of the
segments in unison until the deep notch on the segment of the desired
station lies opposite to the pawl _6_, which will permit the rocker
arm _2_ to swing so far to the left as to close not only the circuit
between _2_ and _4_, but also between _2_, _4_, and _5_. In this
position the talking and the ringing circuits are both closed.

The position of the deepest notch, _i.e._, the selective notch, on the
circumference of the segment at any station depends upon the number of
that station; thus, the segment of Station 4 will have a deep notch in
the sixth position; the segment for Station 9 will have a deep notch
in the eleventh position; the segment for any station will have a deep
notch in the position corresponding to the number of that station plus
two.

From what has been said, therefore, it is evident that the first, or
normal, notch on each segment is of such a depth as to allow the
moving pawl _6_ to fall to such a depth in the segment as to permit
the rocker arm _2_ to close the talking circuit only. All of the other
notches, except one, are comparatively shallow, and while they permit
the moving pawl _6_ under the influence of the rocker arm _2_ to move
the segment _3_, yet they do not permit the rocker arm _2_ to move so
far to the left as to close even the talking circuit. The exception is
the deep notch, or selective notch, which is of such depth as to
permit the pawl _6_ to fall so far into the segment as to allow the
rocker arm _2_ to close both the talking and the ringing circuits.
Besides the moving pawl _6_ there is a detent pawl _7_. This always
holds the segment _3_ in the position to which it has been last moved
by the moving pawl _6_.

The actuating magnet _1_, as has been stated, is polarized and when
energized by currents in one direction, the rocker arm moves the pawl
_6_ so as to step the segment one notch. When this relay is energized
by current in the opposite direction, the operation is such that both
the moving pawl _6_ and the detent pawl _7_ will be pulled away from
the segment, thus allowing the segment to return to its normal position
by gravity. This is accomplished by the following mechanism: An
armature stop is pivoted upon the face of the rocker arm so as to swing
in a plane parallel to the pole faces of the relay, and is adapted,
when the relay is actuated by selective impulses of one polarity, to be
pulled towards one of the pole faces where it acts, through impact with
a plate attached to the pole face of the relay, as a limiting means
for the motion of the rocker arm when the rocker arm is actuated by the
magnet. When, however, the relay is energized by current in the
opposite direction, as on a releasing impulse, the armature stop swings
upon its pivot towards the opposite pole face, in which position the
lug on the end of the armature stop registers with a hole in the plate
on the relay, thus allowing the full motion of the rocker arm when it
is attracted by the magnet. This motion of the rocker arm withdraws the
detent pawl from engagement with the segment as well as the moving
pawl, and thereby permits the segment to return to its normal position.
As will be seen from Fig. 189, each of the relay magnets _1_ is
permanently bridged across the two limbs of the line.

Each station is provided with a push button, not shown, by means of
which the subscriber who makes a call may prevent the rocker arm of
his instrument from being actuated while selective impulses are being
sent over the line. The purpose of this is to enable one party to make
a call for another on the same line, depressing his push button while
the operator is selecting and ringing the called party. The segment at
his own station, therefore, remains in its normal position, in which
position, as we have already seen, his talking circuit is closed; all
of the other segments are, however, stepped up until the ringing and
talking circuits of the desired station are in proper position, at
which time ringing current is sent over the line. The segments in Fig.
189, except at Station C, are shown as having been stepped up to the
sixth position, which corresponds to the ringing position of the
fourth station, or Station D. The condition shown in this figure
corresponds to that in which the subscriber at Station C originated
the call and pressed his button, thus retaining his own segment in its
normal position so that the talking circuits would be established with
Station D.

When the line is in normal position any subscriber may call central by
his magneto generator, not shown in Fig. 189, which will operate the
drop at central, but will not operate any of the subscribers' bells,
because all bell circuits are normally open. When a subscriber desires
connection with another line, the operator sends an impulse back on
the line which steps up and locks out all instruments except that of
the calling subscriber.

[Illustration: Fig. 190. K.B. Lock-Out Station]

A complete K.B. lock-out telephone is shown in Fig. 190. This is the
type of instrument that is usually furnished when new equipment is
ordered. If, however, it is desired to use the K.B. system in
connection with telephones of the ordinary bridging type that are
already in service, the lock-out and selective mechanism, which is
shown on the upper inner face of the door in Fig. 190, is furnished
separately in a box that may be mounted close to the regular telephone
and connected thereto by suitable wires, as shown in Fig. 191. It is
seen that this instrument employs a local battery for talking and also
a magneto generator for calling the central office.

The central-office equipment consists of a dial connected with an
impulse wheel, together with suitable keys by which the various
circuits may be manipulated. This dial and its associated mechanism
may be mounted in the regular switchboard cabinet, or it may be
furnished in a separate box and mounted alongside of the cabinet in
either of the positions shown at _1_ or _2_ of Fig. 192.

In order to send the proper number of impulses to the line to call a
given party, the operator places her finger in the hole in the dial
that bears the number corresponding to the station wanted and rotates
the dial until the finger is brought into engagement with the fixed
stop shown at the bottom of the dial in Fig. 192. The dial is then
allowed to return by the action of a spring to its normal position,
and in doing so it operates a switch within the box to make and break
the battery circuit the proper number of times.

_Operation._ A complete description of the operation may now be had in
connection with Fig. 193, which is similar to Fig. 189, but contains
the details of the calling arrangement at the central office and also
of the talking circuits at the various subscribers' stations.

[Illustration: Fig. 191. K.B. Lock-Out Station]

Referring to the central-office apparatus the usual ringing key is
shown, the inside contacts of which lead to the listening key and to
the operator's telephone set as in ordinary switchboard practice.
Between the outside contact of this ringing key and the ringing
generator there is interposed a pair of contact springs _8-8_ and
another pair _9-9_. The contact springs _8_ are adapted to be moved
backward and forward by the impulse wheel which is directly controlled
by the dial under the manipulation of the operator. When these springs
_8_ are in their normal position, the ringing circuit is continued
through the release-key springs _9_ to the ringing generator. These
springs _8_ occupy their normal position only when the dial is in its
normal position, this being due to the notch _10_ in the contact wheel.
At all other times, _i.e._, while the impulse wheel is out of its
normal position, the springs _8-8_ are either depressed so as to engage
the lower battery contacts, or else held in an intermediate position so
as to engage neither the battery contacts nor the generator contacts.

[Illustration: Fig. 192. Calling Apparatus K.B. System]

When it is desired to call a given station, the operator pulls the
subscriber's number on the dial and holds the ringing key closed,
allowing the dial to return to normal. This connects the impulse
battery to the subscriber's line as many times as is required to move
the subscriber's sectors to the proper position, and in such direction
as to cause the stepping movement of the various relays. As the
impulse wheel comes to its normal position, the springs _8_,
associated with it, again engage their upper contacts, by virtue of
the notch _10_ in the impulse wheel, and this establishes the
connection between the ringing generator and the subscriber's line,
the ringing key being still held closed. The pulling of the
transmitter dial and holding the ringing key closed, therefore, not
only sends the stepping impulses to line, but also follows it by the
ringing current. The sending of five impulses to line moves all of the
sectors to the sixth notch, and this corresponds to the position
necessary to make the fourth station operative. Such a condition is
shown in Fig. 193, it being assumed that the subscriber at Station C
originated the call and pressed his own button so as to prevent his
sector from being moved out of its normal position. As a result of
this, the talking circuit at Station C is left closed, and the talking
and the ringing circuit of Station D, the called station, are closed,
while both the talking and the ringing circuits of all the other
stations are left open. Station D may, therefore, be rung and may
communicate with Station C, while all of the other stations on the
line are locked out, because of the fact that both their talking and
ringing circuits are left open.

[Illustration: Fig. 193. Circuit K.B. System]

When conversation is ended, the operator is notified by the usual
clearing-out signal, and she then depresses the release button, which
brings the springs _9_ out of engagement with the generator contact
but into engagement with the battery contact in such relation as to
send a battery current on the line in the reverse direction from that
sent out by the impulse wheel. This sends current through all of the
relays in such direction as to withdraw both the moving and the
holding pawls from the segments and thus allow all of the segments to
return to their normal positions. Of course, in thus establishing the
release current, it is necessary for the operator to depress the
ringing key as well as the release key.

A one-half microfarad condenser is placed in the receiver circuit at
each station so that the line will not be tied up should some
subscriber inadvertently leave his receiver off its hook. This permits
the passage of voice currents, but not of the direct currents used in
stepping the relays or in releasing them.

The circuit of Fig. 193 is somewhat simplified from that in actual
practice, and it should be remembered that the hook switch, which is
not shown in this figure, controls in the usual way the continuity of
the receiver and the transmitter circuits as well as of the generator
circuits, the generator being attached to the line as in an ordinary
telephone.

Broken-Line System. The broken-line method of accomplishing
selective signaling and locking-out on telephone party lines is due to
Homer Roberts and his associates.

[Illustration: Fig. 194. Roberts Latching Relay]

To understand just how the principles illustrated in Figs. 186 and 187
are put into effect, it will be necessary to understand the latching
relay shown diagrammatically in its two possible positions in Fig. 194,
and in perspective in Fig. 195. Referring to Fig. 194, the left-hand
cut of which shows the line relay in its normal position, it is seen
that the framework of the device resembles that of an ordinary
polarized ringer. Under the influence of current in one direction
flowing through the left-hand coil, the armature of this device
depresses the hard rubber stud _4_, and the springs _1_, _2_, and _3_
are forced downwardly until the spring _2_ has passed under the latch
carried on the spring _5_. When the operating current through the coil
_6_ ceases, the pressure of the armature on the spring _1_ is relieved,
allowing this spring to resume its normal position and spring _3_ to
engage with spring _2_. The spring _2_ cannot rise, since it is held by
the latch _5_, and the condition shown in the right-hand cut of Fig.
194 exists. It will be seen that the spring _2_ has in this operation
carried out just the same function as the switch lever performed as
described in connection with Figs. 186 and 187. An analysis of this
action will show that the normal contact between the springs _1_ and
_2_, which contact controls the circuit through the relay coil and the
bell, is not broken until the coil _6_ is de-energized, which means
that the magnet is effective until it has accomplished its work. It is
impossible, therefore, for this relay to cut itself out of circuit
before it has caused the spring _2_ to engage under the latch _5_. If
current of the proper direction were sent through the coil _7_ of the
relay, the opposite end of the armature would be pulled down and the
hard rubber stud at the left-hand end of the armature would bear
against the bent portion of the spring _5_ in such manner as to cause
the latch of this spring to release the spring _2_ and thus allow the
relay to assume its normal, or unlatched, position.

A good idea of the mechanical construction of this relay may be
obtained from Fig. 195. The entire selecting function of the Roberts
system is performed by this simple piece of apparatus at each station.

[Illustration: Fig. 195. Roberts Latching Relay]

The diagram of Fig. 196 shows, in simplified form, a four-station
line, the circuits being given more in detail than in the diagrams of
Chapter XVI.

It will be noticed that the ringer and the relay coil _6_ at the
first station are bridged across the sides of the line leading to the
central office. In like manner the bell and the relay magnets are
bridged across the two limbs of the line leading into each succeeding
station, but this bridge at each of the stations beyond Station A is
ineffective because the line extension _R__{x} is open at the next
station nearest the central office.

[Illustration: Fig. 196. Simplified Circuits of Roberts System]

In order to ring Station A it is only necessary to send out ringing
current from the central office. This current is in such direction as
not to cause the operation of the relay, although it passes through
the coil _6_. If, on the other hand, it is desired to ring Station B,
a preliminary impulse would be sent over the metallic circuit from the
central office, which impulse would be of such direction as to operate
the relay at Station A, but not to operate the bell at that station.
The operation of the relay at Station A causes the spring _2_ of this
relay to engage the spring _3_, thus extending the line on to the
second station. After the spring _2_ at Station A has been forced into
contact with the spring _3_, it is caught by the latch of the spring
_5_ and held mechanically. When the impulse from the central office
ceases, the spring _1_ resumes its normal position, thus breaking the
bridge circuit through the bell at that station. It is apparent now
that the action of coil _6_ at Station A has made the relay powerless
to perform any further action, and at the same time the line has been
extended on to the second station. A second similar impulse from the
central office will cause the relay at Station B to extend the line on
to Station C, and at the same time break the circuit through the
operating coil and the bell at Station B. In this way any station may
be picked out by sending the proper number of impulses to operate the
line relays of all the stations between the station desired and the
central office, and having picked out a station it is only necessary
to send out ringing current, which current is in such direction as to
ring the bell but not to operate the relay magnet at that station.

In Fig. 197, a four-station line, such as is shown in Fig. 196, is
illustrated, but the condition shown in this is that existing when two
preliminary impulses have been sent over the line, which caused the
line relays at Station A and Station B to be operated. The bell at
Station C is, therefore, the only one susceptible to ringing current
from the central office.

[Illustration: Fig. 197. Simplified Circuits of Roberts System]

Since only one bell and one relay are in circuit at any one time, it
is obvious that all of the current that passes over the line is
effective in operating a single bell or relay only. There is no
splitting up of the current among a large number of bells as in the
bridging system of operating step-by-step devices, which method
sometimes so greatly reduces the effective current for each bell that
it is with great difficulty made to respond. All the energy available
is applied directly to the piece of apparatus at the time it is being
operated. This has a tendency toward greater surety of action, and the
adjustment of the various pieces of apparatus may be made with less
delicacy than is required where many pieces of apparatus, each having
considerable work to do, must necessarily be operated in multiple.

The method of unlatching the relays has been briefly referred to.
After a connection has been established with a station in the manner
already described, the operator may clear the line when it is proper
to do so by sending impulses of such a nature as to cause the line
relays of the stations beyond the one chosen to operate, thus
continuing the circuit to the end of the line. The operation of the
line relay at the last station brings into circuit the coil _8_, Figs.
196 and 197, of a grounding device. This is similar to the line relay,
but it holds its operating spring in a normally latched position so as
to maintain the two limbs of the line disconnected from the ground.
The next impulse following over the metallic circuit passes through
the coil _8_ and causes the operation of this grounding device which,
by becoming unlatched, grounds the limb _L_ of the line through the
coil _8_. This temporary ground at the end of the line makes it
possible to send an unlocking or restoring current from the central
office over the limb _L_, which current passes through all of the
unlocking coils _7_, shown in Figs. 194, 196, and 197, thus causing
the simultaneous unlocking of all of the line relays and the
restoration of the line to its normal condition, as shown in Fig. 196.

[Illustration: Fig. 198. Details of Latching Relay Connections]

As has been stated, the windings _7_ on the line relays are the
unlatching windings. In Figs. 196 and 197, for the purpose of
simplicity, these windings are not shown connected, but as a matter of
fact each of them is included in series in the continuous limb _L_ of
the line. This would introduce a highly objectionable feature from the
standpoint of talking over the line were it not for the balancing
coils _7_^{1}, each wound on the same core as the corresponding
winding _7_, and each included in series in the limb _R_ of the line,
and in such direction as to be differential thereto with respect to
currents passing in series over the two limbs of the line.

The windings _7_ are the true unlocking windings, while the windings
_7_^{1} have no other function than to neutralize the inductive
effects of these unlocking windings necessarily placed in series in
the talking circuit. All of these windings are of low ohmic
resistance, a construction which, as has previously been noted, brings
about the desired effect without introducing any self-induction in
the line, and without producing any appreciable effect upon the
transmission. A study of Fig. 198 will make clear the connections of
these unlocking and balancing windings at each station.

The statement of operation so far given discloses the general method
of building up the line in sections in order to choose any party and
of again breaking it up into sections when the conversation is
finished. It has been stated that the same operation which selects the
party wanted also serves to give that party the use of the line and to
lock the others off. That this is true will be understood when it is
stated that the ringer is of such construction that when operated to
ring the subscriber wanted, it also operates to unlatch a set of
springs similar to those shown in Fig. 194, this unlatching causing
the proper connection of the subscriber's talking circuit across the
limbs of the line, and also closing the local circuit through his
transmitter. The very first motion of the bell armature performs this
unlatching operation after which the bell behaves exactly as an
ordinary polarized biased ringer.

[Illustration: Fig. 199. Broken-Back Ringer]

The construction of this ringer is interesting and is shown in its two
possible positions in Fig. 199. The group of springs carried on its
frame is entirely independent of the movement of the armature during
the ringing operation. With reversed currents, however, the armature
is moved in the opposite direction from that necessary to ring the
bells, and this causes the latching of the springs into their normal
position. In order that this device may perform the double function
of ringer and relay the tapper rod of the bell is hinged on the
armature so as to partake of the movements of the armature in one
direction only. This has been called by the inventor and engineers of
the Roberts system a _broken-back ringer_, a name suggestive of the
movable relation between the armature and the tapper rod. The
construction of the ringer is of the same nature as that of the
standard polarized ringer universally employed, but a hinge action
between the armature and the tapper rod, of such nature as to make the
tapper partake positively of the movements of the armature in one
direction, but to remain perfectly quiescent when the armature moves
in the other direction, is provided.

[Illustration: Fig. 200. Details of Ringer Connection]

How this broken-back ringer controls the talking and the locking-out
conditions may best be understood in connection with Fig. 200. The
ringer springs are normally latched at all stations. Under these
conditions the receiver is short-circuited by the engagement of
springs _10_ and _11_, the receiver circuit is open between springs
_10_ and _12_, and the local-battery circuit is open between springs
_9_ and _12_. The subscribers whose ringers are latched are,
therefore, locked out in more ways than one.

When the bell is rung, the first stroke it makes unlatches the springs,
which assume the position shown in the right-hand cut of Fig. 199, and
this, it will be seen from Fig. 200, establishes proper conditions for
enabling the subscriber to transmit and to receive speech.

The hook switch breaks both transmitter and receiver circuits when down
and in raising it establishes a momentary circuit between the ground
and the limb _L_ of the line, both upper and lower hook contacts
engaging the hook lever simultaneously during the rising of the hook.

The mechanism at the central office by which selection of the proper
station is made in a rapid manner is shown in Fig. 201. It has already
been stated that the selection of the proper subscriber is brought
about by the sending of a predetermined number of impulses from the
central office, these impulses passing in one direction only and over
the metallic circuit. After the proper party has been reached, the
ringing current is put on in the reverse direction.

[Illustration: Fig. 201. Central-Office Impulse Transmitter]

The operator establishes the number of impulses to be sent by placing
the pointer opposite the number on the dial corresponding to the
station wanted. The ratchet wheel is stepped around automatically by
each impulse of current from an ordinary pole changer such as is
employed in ringing biased bells. When the required number of impulses
has been sent, a projection, carried on a group of springs, drops into
a notch on the drum of the selector shaft, which operation instantly
stops the selecting current impulses and at the same time throws on
the ringing current which consists of impulses in the reverse
direction. So rapidly does this device operate that it will readily
follow the impulses of an ordinary pole changer, even when this is
adjusted to its maximum rate of vibration.

[Illustration: VIEW OF A LARGE FOREIGN MULTIPLE SWITCHBOARD]

_Operation._ Space will not permit a full discussion of the details of
the central-office selective apparatus, but a general resumé of the
operation of the system may now be given, with the aid of Fig. 202,
which shows a four-station line with the circuits of three of the
stations somewhat simplified. In this figure Station A, Station B,
and Station D are shown in their locked-out positions, A and B having
been passed by the selection and ringing of Station C, while Station D
is inoperative because it was not reached in the selection and the
line is still broken at Station C. Station C, therefore, has
possession of the line.

When the subscriber at Station C raised his receiver in order to call
central, a "flash" contact was made as the hook moved up, which
momentarily grounded the limb _L_ of the line. (See Fig. 200.) This
"flash" contact is produced by the arrangement of the hook which
assures that the lower contact shall, by virtue of its flexibility,
follow up the hook lever until the hook lever engages the upper
contact, after which the lower contact breaks. This results in the
momentary connection of both the upper and the lower contacts of the
hook with the lever, and, therefore, the momentary grounding of the
limb _L_ of the line. This limb always being continuous serves, when
this "flash" contact is made, to actuate the line signal at the
central office.

[Illustration: Fig. 202. Circuits of Roberts Line]

Since, however, all parties on the line are normally locked out of
talking circuits, some means must be provided whereby the operator may
place the signaling party in talking connection and leave all the
other instruments on the line in their normally locked-out position.
In fact, the operator must be able automatically to pick out the
station that signaled in, and operate the ringer to unlatch the
springs controlling the talking circuit of that station. Accordingly
the operator sends impulses on the line, from a grounded battery,
which are in the direction to operate the line relays and to continue
the line circuit to the station calling. When, after a sufficient
number of impulses, this current reaches that station it finds a path
to ground from the limb _L_. This path is made possible by the fact
that the subscriber's receiver is off its hook at that station. In
order to understand just how this ground connection is made, it must
be remembered that each of the ringer magnets is energized with each
selecting impulse, but in such a direction as not to ring the bells,
it being understood that all of the ringer mechanisms are normally
latched. When the selecting impulse for Station C arrives, it passes
through the ringer and the selecting relay coils at that station and
starts to operate the remainder of the ringers sufficiently to cause
the spring _12_ to engage the spring _13_. This establishes the ground
connection from the limb _L_ of the line, the circuit being traced
through limb _L_ through the upper contact of the switch, thence
through springs _12_ and _13_ to ground, and this, before the line
relay has time to latch, operates the quick-acting relay at the
central office, which acts to cut off further impulses, and thus
automatically stops at the calling station. Ringing current in the
opposite direction is then sent to line; this unlatches the ringer
springs and places the calling subscriber in talking circuit. When the
operator has communicated with the calling subscriber, and found, for
example, that another party on another similar line is desired, she
turns the dial pointer on the selector to the number corresponding to
the called-for party's number on that line, and presses the signal
key. Pressing this key causes impulses to "run down the line,"
selecting the proper party and ringing his bell in the manner already
described. The connection between the two parties is then established,
and no one else can in any possible way, except by permission of the
operator, obtain access to the line.

It is obvious that some means must be provided for restoring the
selecting relays to normal after a conversation is finished. By
referring to Fig. 194 it will be seen that the upper end of the latch
spring _5_ is bent over in such a manner that when the armature is
attracted by current flowing through the coil _7_, the knob on the
left-hand end of the armature on rising engages with the bent cam
surface and forces back the latch, permitting spring _2_ to return to
its normal position.

To restore the line the operator sends out sufficient additional
selective impulses to extend the circuit to the end of the line, and
thus brings the grounder into circuit. The winding of the grounder is
connected in such a manner that the next passing impulse throws off
its latch, permitting the long spring to contact with the ground
spring. The operator now sends a grounded impulse over the continuous
limb _L_ of the line which passes through the restoring coils _7_ at
all the stations and through the right-hand coil of the grounding
device to ground. The selecting relays are, therefore, simultaneously
restored to normal. The grounder is also energized and restored to its
normal position by the same current.

If a party in calling finds that his own line is busy and he cannot
get central, he may leave his receiver off its hook. When the party
who is using the line hangs up his receiver the fact that another
party desires a connection is automatically indicated to the operator,
who then locks out the instrument of the party who has just finished
conversation and passes his station by. When the operator again throws
the key, the waiting subscriber is automatically selected in the same
manner as was the first party. If there are no subscribers waiting for
service, the stop relay at central will not operate until the grounder
end of the line is unlatched, the selecting relays being then restored
automatically to normal.

The circuits are so organized that at all times whether the line is
busy or not, the movement up and down of the switch hook, at any
sub-station, operates a signal before the operator. Such a movement,
when made slowly and repeatedly, indicates to the operator that the
subscriber has an emergency call and she may use her judgment as to
taking the line away from the parties who are using it, and finding
out what the emergency call is for. If the operator finds that the
subscriber has misused this privilege of making the emergency call,
she may restore the connection to the parties previously engaged in
conversation.

One of the salient points of this Roberts system is that the operator
always has control of the line. A subscriber is not able even to use
his own battery till permitted to do so. A subscriber who leaves his
receiver off its hook in order that he may be signaled by the operator
when the line is free, causes no deterioration of the local battery
because the battery circuit is held open by the switch contacts
carried on the ringer. It cannot be denied, however, that this system
is complicated, and that it has other faults. For instance, as
described herein, both sides of the line must be looped into each
subscriber's station, thus requiring four drop, or service, wires
instead of two. It is possible to overcome this objection by placing
the line relays on the pole in a suitably protected casing, in which
case it is sufficient to run but two drop wires from the nearer line
to station. There are undoubtedly other objections to this system, and
yet with all its faults it is of great interest, and although radical
in many respects, it teaches lessons of undoubted value.




CHAPTER XVIII

ELECTRICAL HAZARDS


All telephone systems are exposed to certain electrical hazards. When
these hazards become actively operative as causes, harmful results
ensue. The harmful results are of two kinds: those causing damage to
property and those causing damage to persons. The damage to persons
may be so serious as to result in death. Damage to property may
destroy the usefulness of a piece of apparatus or of some portion of
the wire plant. Or the property damage may initiate itself as a harm
to apparatus or wiring and may result in greater and extending damage
by starting a fire.

Electrical currents which endanger life and property may be furnished
by natural or artificial causes. Natural electricity which does such
damage usually displays itself as lightning. In rare cases, currents
tending to flow over grounded lines because of extraordinary
differences of potential between sections of the earth's surface have
damaged apparatus in such lines, or only have been prevented from
causing such damage by the operation of protective devices.

Telegraph and telephone systems have been threatened by natural
electrical hazards since the beginning of the arts and by artificial
electrical hazards since the development of electric light and power
systems. At the present time, contrary to the general supposition, it
is in the artificial, and not in the natural electrical hazards that
the greater variety and degree of danger lies.

Of the ways in which artificial electricity may injure a telephone
system, the entrance of current from an external electrical power
system is a greater menace than an abnormal flow of current from a
source belonging to the telephone system itself. Yet modern practice
provides opportunities for a telephone system to inflict damage upon
itself in that way. Telephone engineering designs need to provide
means for protecting _all_ parts of a system against damage, from
external ("foreign") as well as internal ("domestic") hazards, and to
cause this protection to be inclusive enough to protect persons
against injury and property from damage by any form of overheating or
electrolytic action.

A part of a telephone system for which there is even a remote
possibility of contact with an external source of electrical power,
whether natural or artificial, is said to be _exposed_ to electrical
hazard. The degree or character of possible contact or other
interference often is referred to in relative terms of _exposure_. The
same terms are used concerning inductive relations between circuits.
The whole tendency of design, particularly of wire plants, is to
arrange the circuits in such a way as to limit the exposure as greatly
as possible, the intent being to produce a condition in which all
parts of the system will be _unexposed_ to hazards.

Methods of design are not yet sufficiently advanced for any plant to
be formed of circuits wholly unexposed, so that protective means are
required to safeguard apparatus and circuits in case the hazard,
however remote, becomes operative.

Lightning discharges between the clouds and earth frequently charge
open wires to potentials sufficiently high to damage apparatus; and
less frequently, to destroy the wires of the lines themselves.
Lightning discharges between clouds frequently induce charges in lines
sufficient to damage apparatus connected with the lines. Heavy rushes
of current in lines, from lightning causes, occasionally induce
damaging currents in adjacent lines not sufficiently exposed to the
original cause to have been injured without this induction. The
lightning hazard is least where the most lines are exposed. In a small
city with all of the lines formed of exposed wires and all of them
used as grounded circuits, a single lightning discharge may damage
many switchboard signals and telephone ringers if there be but 100 or
200 lines, while the damage might have been nothing had there been 800
to 1,000 lines in the same area.

Means of protecting lines and apparatus against damage by lightning
are little more elaborate than in the earliest days of telegraph
working. They are adequate for the almost entire protection of life
and of apparatus.

Power circuits are classified by the rules of various governing bodies
as high-potential and low-potential circuits. The classification of
the National Board of Fire Underwriters in the United States defines
low-potential circuits as having pressures below 550 volts;
high-potential circuits as having pressures from 550 to 3,500 volts,
and extra high-potential circuits as having pressures above 3,500
volts. Pressures of 100,000 volts are becoming more common. Where
power is valuable and the distance over which it is to be transmitted
is great, such high voltages are justified by the economics of the
power problem. They are a great hazard to telephone systems, however.
An unprotected telephone system meeting such a hazard by contact will
endanger life and property with great certainty. A very common form of
distribution for lighting and power purposes is the three-wire system
having a grounded neutral wire, the maximum potential above the earth
being about 115 volts.

Telephone lines and apparatus are subject to damage by any power
circuit whether of high or low potential. The cause of property damage
in all cases is the flow of current. Personal damage, if it be death
from shock, ordinarily is the result of a high potential between two
parts of the body. The best knowledge indicates that death uniformly
results from shock to the heart. It is believed that death has
occurred from shock due to pressure as low as 100 volts. The critical
minimum voltage which can not cause death is not known. A good rule is
never willingly to subject another person to personal contact with any
electrical pressure whatever.

Electricity can produce actions of four principal kinds:
physiological, thermal, chemical, and magnetic. Viewing electricity as
establishing hazards, the physiological action may injure or kill
living things; the thermal action may produce heat enough to melt
metals, to char things which can be burned, or to cause them actually
to burn, perhaps with a fire which can spread; the chemical action may
destroy property values by changing the state of metals, as by
dissolving them from a solid state where they are needed into a state
of solution where they are not needed; the magnetic action introduces
no direct hazard. The greatest hazard to which property values are
exposed is the electro-thermal action; that is, the same useful
properties by which electric lighting and electric heating thrive may
produce heat where it is not wanted and in an amount greater than can
safely be borne.

The tendency of design is to make all apparatus capable of carrying
without overheating any current to which voltage within the telephone
system may subject it, and to provide the system so designed with
specific devices adapted to isolate it from currents originating
without. Apparatus which is designed in this way, adapted not only to
carry its own normal working currents but to carry the current which
would result if a given piece of apparatus were connected directly
across the maximum pressure within the telephone system itself, is
said to be self-protecting. Apparatus amply able to carry its maximum
working current but likely to be overheated, to be injured, or perhaps
to destroy itself and set fire to other things if subjected to the
maximum pressure within the system, is not self-protecting apparatus.

To make all electrical devices self-protecting by surrounding them
with special arrangements for warding off abnormal currents from
external sources, is not as simple as might appear. A lamp, for
example, which can bear the entire pressure of a central-office
battery, is not suitable for direct use in a line several miles long
because it would not give a practical signal in series with that line
and with the telephone set, as it is required to do. A lamp suitable
for use in series with such a line and a telephone set would burn out
by current from its own normal source if the line should become
short-circuited in or near the central office. The ballast referred to
in the chapter on "Signals" was designed for the very purpose of
providing rapidly-rising resistance to offset the tendency toward
rapidly-rising current which could burn out the lamp.

As another example, a very small direct-current electric motor can be
turned on at a snap switch and will gain speed quickly enough so that
its armature winding will not be overheated. A larger motor of that
kind can not be started safely without introducing resistance into the
armature circuit on starting, and cutting it out gradually as the
armature gains speed. Such a motor could be made self-protecting by
having the armature winding of much larger wire than really is
required for mere running, choosing its size great enough to carry the
large starting current without overheating itself and its insulation.
It is better, and for long has been standard practice, to use starting
boxes, frankly admitting that such motors are not self-protecting
until started, though they are self-protecting while running at normal
speeds. Such a motor, once started, may be overloaded so as to be
slowed down. So much more current now can pass through the armature
that its winding is again in danger. Overload circuit-breakers are
provided for the very purpose of taking motors out of circuit in cases
where, once up to speed, they are mechanically brought down again and
into danger. Such a circuit-breaker is a device for protecting against
an _internal_ hazard; that is, internal to the power system of which
the motor is a part.

Another example: In certain situations, apparatus intended to operate
under impulses of large current may be capable of carrying its normal
impulses successfully but incapable of carrying currents from the same
pressure continuously. Protective means may be provided for detaching
such apparatus from the circuit whenever the period in which the
current acts is not short enough to insure safety. This is cited as a
case wherein a current, normal in amount but abnormal in duration,
becomes a hazard.

The last mentioned example of damage from internal hazards brings us
to the law of the electrical generation of heat. _The greater the
current or the greater the resistance of the conductor heated or the
longer the time, the greater will he the heat generated in that
conductor._ But this generated heat varies directly as the resistance
and as the time and as the square of the current, that is, the law is

Heat generated = _C^{2}Rt_

in which _C_ = the current; _R_=the resistance of the conductor; and
_t_ = the time.

It is obvious that a protective device, such as an overload
circuit-breaker for a motor, or a protector for telephone apparatus,
needs to operate more quickly for a large current than for a small
one, and this is just what all well-designed protective devices are
intended to do. The general problem which these heating hazards
present with relation to telephone apparatus and circuits is: _To
cause all parts of the telephone system to be made so as to carry
successfully all currents which may flow in them because of any
internal or external pressure, or to supplement them by devices which
will stop or divert currents which could overheat them._

Electrolytic hazards depend not on the heating effects of currents but
on their chemical effects. The same natural law which enables primary
and secondary batteries to be useful provides a hazard which menaces
telephone-cable sheaths and other conductors. When a current leaves a
metal in contact with an electrolyte, the metal tends to dissolve into
the electrolyte. In the processes of electroplating and electrotyping,
current enters the bath at the anode, passes from the anode through
the solution to the cathode, removing metal from the former and
depositing it upon the latter. In a primary battery using zinc as the
positive element and the negative terminal, current is caused to pass,
within the cell, from the zinc to the negative element and zinc is
dissolved. Following the same law, any pipe buried in the earth may
serve to carry current from one region to another. As single-trolley
traction systems with positive trolley wires constantly are sending
large currents through the earth toward their power stations, such a
pipe may be of positive potential with relation to moist earth at some
point in its length. Current leaving it at such a point may cause its
metal to dissolve enough to destroy the usefulness of the pipe for its
intended purpose.

Lead-sheathed telephone cables in the earth are particularly exposed
to such damage by electrolysis. The reasons are that such cables often
are long, have a good conductor as the sheath-metal, and that metal
dissolves readily in the presence of most aqueous solutions when
electrolytic differences of potential exist. The length of the cables
enables them to connect between points of considerable difference of
potential. It is lack of this length which prevents electrolytic
damage to masses of structural metal in the earth.

Electrical power is supplied to single-trolley railroads principally
in the form of direct current. Usually all the trolley wires of a city
are so connected to the generating units as to be positive to the
rails. This causes current to flow from the cars toward the power
stations, the return path being made up jointly of the rails, the
earth itself, actual return wires which may supplement the rails, and
also all other conducting things in the earth, these being principally
lead-covered cables and other pipes. These conditions establish
definite areas in which the currents tend to leave the cables and
pipes, _i.e._, in which the latter are positive to other things. These
positive areas usually are much smaller than the negative areas, that
is, the regions in which currents tend _to enter_ the cables form a
larger total than the regions in which the currents tend _to leave_
the cables. These facts simplify the ways in which the cables may be
protected against damage by direct currents leaving them and also they
reduce the amount, complication, and cost of applying the corrective
and preventive measures.

All electric roads do not use direct current. Certain simplifications
in the use of single-phase alternating currents in traction motors
have increased the number of roads using a system of
alternating-current power supply. Where alternating current is used,
the electrolytic conditions are different and a new problem is set,
for, as the current flows in recurrently different directions, an area
which at one instant is positive to others, is changed the next
instant into a negative area. The protective means, therefore, must be
adapted to the changed requirements.




CHAPTER XIX

PROTECTIVE MEANS


Any of the heating hazards described in the foregoing chapter may
cause currents which will damage apparatus. All devices for the
protection of apparatus from such damage, operate either to stop the
flow of the dangerous current, or to send that flow over some other
path.

Protection Against High Potentials. Lightning is the most nearly
universal hazard. All open wires are exposed to it in some degree.
Damaging currents from lightning are caused by extraordinarily high
potentials. Furthermore, a lightning discharge is oscillatory; that
is, alternating, and of very high frequency. Drops, ringers,
receivers, and other devices subject to lightning damage suffer by
having their windings burned by the discharge. The impedance these
windings offer to the high frequency of lightning oscillations is
great. The impedance of a few turns of heavy wire may be negligible to
alternating currents of ordinary frequencies because the resistance of
the wire is low, its inductance small, and the frequency finite. On
the other hand, the impedance of such a coil to a lightning discharge
is much higher, due to the very high frequency of the discharge.

Were it not for the extremely high pressure of lightning discharges,
their high frequency of oscillation would enable ordinary coils to be
self-protecting against them. But a discharge of electricity can take
place through the air or other insulating medium if its pressure be
high enough. A pressure of 70,000 volts can strike across a gap in air
of one inch, and lower pressures can strike across smaller distances.
When lightning encounters an impedance, the discharge seldom takes
place through the entire winding, as an ordinary current would flow,
usually striking across whatever short paths may exist. Very often
these paths are across the insulation between the outer turns of a
coil. It is not unusual for a lightning discharge to plow its way
across the outer layer of a wound spool, melting the copper of the
turns as it goes. Often the discharge will take place from inner turns
directly to the core of the magnet. This is more likely when the core
is grounded.

_Air-Gap Arrester_. The tendency of a winding to oppose lightning
discharges and the ease with which such discharge may strike across
insulating gaps, points the way to protection against them. Such
devices consist of two conductors separated by an air space or other
insulator and are variously known as lightning arresters, spark gaps,
open-space cutouts, or air-gap arresters. The conductors between which
the gap exists may be both of metal, may be one of metal and one of
carbon, or both of carbon. One combination consists of carbon and
mercury, a liquid metal. The space between the conductors may be
filled with either air or solid matter, or it may be a vacuum.
Speaking generally, the conductors are separated by some insulator.
Two conductors separated by an insulator form a condenser. The
insulator of an open-space arrester often is called the dielectric.

[Illustration Fig. 203. Saw Tooth Arrester]

Discharge Across Gaps:--Electrical discharges across a given distance
occur at lower potentials if the discharge be between points than if
between smooth surfaces. Arresters, therefore, are provided with
points. Fig. 203 shows a device known as a "saw-tooth" arrester
because of its metal plates being provided with teeth. Such an
arrester brings a ground connection close to plates connected with the
line and is adapted to protect apparatus either connected across a
metallic circuit or in series with a single wire circuit.

Fig. 201 shows another form of metal plate air-gap arrester having the
further possibility of a discharge taking place from one line wire to
the other. Inserting a plug in the hole between the two line plates
connects the line wires directly together at the arrester. This
practice was designed for use with series lines, the plug
short-circuiting the telephone set when in place.

A defect of most ordinary types of metal air-gap lightning arresters
is that heavy discharges tend to melt the teeth or edges of the
plates and often to weld them together, requiring special attention
to re-establish the necessary gap.

Advantages of Carbon:--Solid carbon is found to be a much better
material than metal for the reasons that a discharge will not melt it
and that its surface is composed of multitudes of points from which
discharges take place more readily than from metals.

[Illustration Fig. 204. Saw-Tooth Arrester]

[Illustration Fig. 205. Carbon Block Arrester]

Carbon arresters now are widely used in the general form shown in Fig.
205. A carbon block connected with a wire of the line is separated
from a carbon block connected to ground by some form of insulating
separator. Mica is widely used as such a separator, and holes of some
form in a mica slip enable the discharge to strike freely from block
to block, while preventing the blocks from touching each other.
Celluloid with many holes is used as a separator between carbon
blocks. Silk and various special compositions also have their uses.

[Illustration Fig. 206. Arrester Separators]

Dust Between Carbons:--Discharges between the carbon blocks tend to
throw off particles of carbon from them. The separation between the
blocks being small--from .005 to .015 inch--the carbon particles may
lodge in the air-gap, on the edges of the separator, or otherwise, so
as to leave a conducting path between the two blocks. Slight moisture
on the separator may help to collect this dust, thus placing a ground
on that wire of the line. This ground may be of very high resistance,
but is probably one of many such--one at each arrester connected to
the line. In special forms of carbon arresters an attempt has been
made to limit this danger of grounding by the deposit of carbon dust.
The object of the U-shaped separator of Fig. 206 is to enable the
arrester to be mounted so that this opening in the separator is
downward, in the hope that loosened carbon particles may fall out of
the space between the blocks. The deposit of carbon on the inside
edges of the U-shaped separator often is so fine and clings so tightly
as not to fall out. The separator projects beyond the blocks so as to
avoid the collection of carbon on the outer edges.

Commercial Types:--Fig. 207 is a commercial form of the arrangement
shown in Fig. 205 and is one of the many forms made by the American
Electric Fuse Company. Line wires are attached to outside binding
posts shown in the figure and the ground wire to the metal binding
post at the front. The carbon blocks with their separator slide
between clips and a ground plate. The air-gap is determined by the
thickness of the separator between the carbon blocks.

[Illustration: Fig. 207. Carbon Block Arrester]

[Illustration: Fig. 208 Roberts "Self-Cleaning" Arrester]

The Roberts carbon arrester is designed with particular reference to
the disposal of carbon dust and is termed self-cleaning for that
reason. The arrangement of carbons and dielectric in this device is
shown in Fig. 208; mica is cemented to the line carbon and is large
enough to provide a projecting margin all around. The spark gap is not
uniform over the entire surface of the block but is made wedge-shaped
by grinding away the line carbon as shown. It is claimed that a
continuous arcing fills the wedge-shaped chamber with heated air or
gas, converting the whole of the space into a field of low resistance
to ground, and that this gas in expanding drives out every particle of
carbon that may be thrown off. It seems obvious that the wedge-shaped
space offers greater freedom for carbon dust to fall out than in the
case of the parallel arrangement of the block faces.

An outdoor arrester for metallic circuits, designed by F.B. Cook, is
shown in Fig. 209. The device is adapted to mount on a pole or
elsewhere and to be covered by a protecting cap. The carbons are large
and are separated by a special compound intended to assist the
self-cleaning feature. The three carbons being grouped together as a
unit, the device has the ability to care for discharges from one
terminal to either of the others direct, without having to pass
through two gaps. In this particular, the arrangement is the same as
that of Fig. 204.

[Illustration: Fig. 209. Cook Air-Gap Arrester]

A form of Western Electric arrester particularly adapted for outside
use on railway lines is shown with its cover in Fig. 210.

[Illustration: Fig. 210. Western Electric Air-Gap Arrester]

The Kellogg Company regularly equips its magneto telephones with
air-gap arresters of the type shown in Fig. 211. The two line plates
are semicircular and of metal. The ground plate is of carbon,
circular in form, covering both line plates with a mica separator.
This is mounted on the back board of the telephone and permanently
wired to the line and ground binding posts.

[Illustration: OLD SWITCHBOARD OF BELL EXCHANGE SERVING CHINATOWN,
SAN FRANCISCO, CALIFORNIA]

[Illustration: Fig. 211. Kellogg Air-Gap Arrester]

Vacuum Arresters:--All of the carbon arresters so far mentioned depend
on the discharge taking place through air. A given pressure will
discharge further in a fairly good vacuum than in air. The National
Electric Specialty Company mounts three conductors in a vacuum of the
incandescent lamp type, Fig. 212. A greater separation and less
likelihood of short-circuiting can be provided in this way. Either
carbon or metal plates are adapted for use in such vacuum devices. The
plates may be further apart for a given discharge pressure if the
surfaces are of carbon.

[Illustration: Fig. 212. Vacuum Arrester]

Introduction of Impedance:--It has been noted that the existence of
impedance tends to choke back the passage of lightning discharge
through a coil. Fig. 213 suggests the relation between such an
impedance and air-gap arrester. If the coil shown therein be considered
an arrangement of conductors having inductance, it will be seen that a
favorable place for an air-gap arrester is between that impedance and
the line. This fact is made known in practice by frequent damage to
aërial cables by electricity brought into them over long open wires,
the discharge taking place at the first turn or bend in the aërial
cable; this discharge often damages both core and sheath. It is well to
have such bends as near the end of the cable as possible, and turns or
goosenecks at entrances to terminals have that advantage.

[Illustration: Fig. 213. Impedance and Air-Gap]

This same principle is utilized in some forms of arresters, such as
the one shown in Fig. 214, which provides an impedance of its own
directly in the arrester element. In this device an insulating base
carries a grounded carbon rod and two impedance coils. The impedance
coils are wound on insulating rods, which hold them near, but not
touching, the ground carbon. The coils are arranged so that they may
be turned when discharges roughen the surfaces of the wires.

[Illustration: Fig. 214. Holtzer-Cabot Arrester]

Metallic Electrodes:--Copper or other metal blocks with roughened
surfaces separated by an insulating slip may be substituted for the
carbon blocks of most of the arresters previously described. Metal
blocks lack the advantage of carbon in that the latter allows
discharges at lower potentials for a given separation, but they have
the advantage that a conducting dust is not thrown off from them.

[Illustration: Fig. 215. Carbon Air-Gap Arrester]

Provision Against Continuous Arc:--For the purpose of short-circuiting
an arc, a globule of low-melting alloy may be placed in one carbon
block of an arrester. This feature is not essential in an arrester
intended solely to divert lightning discharges. Its purpose is to
provide an immediate path to ground if an arc arising from artificial
electricity has been maintained between the blocks long enough to melt
the globule. Fig. 215 is a plan and section of the Western Electric
Company's arrester used as the high potential element in conjunction
with others for abnormal currents and sneak currents; the latter are
currents too small to operate air-gap arresters or substantial fuses.

Protection Against Strong Currents. _Fuses._ A fuse is a metal
conductor of lower carrying capacity than the circuit with which it is
in series at the time it is required to operate. Fuses in use in
electrical circuits generally are composed of some alloy of lead,
which melts at a reasonably low temperature. Alloys of lead have lower
conductivity than copper. A small copper wire, however, may fuse at
the same volume of current as a larger lead alloy wire.

Proper Functions:--A fuse is not a good lightning arrester. As
lightning damage is caused by current and as it is current which
destroys a fuse, a lightning discharge _can_ open a circuit over which
it passes by melting the fuse metal. But lightning may destroy a fuse
and at the same discharge destroy apparatus in series with the fuse.
There are two reasons for this: One is that lightning discharges act
very quickly and may have destroyed apparatus before heating the fuse
enough to melt it; the other reason is that when a fuse is operated
with enough current even to vaporize it, the vapor serves as a
conducting path for an instant after being formed. This conducting
path may be of high resistance and still allow currents to flow
through it, because of the extremely high pressure of the lightning
discharge. A comprehensive protective system may include fuses, but it
is not to be expected that they always will arrest lightning or even
assist other things in arresting lightning. They should be considered
as of no value for that purpose. Furthermore, fuses are best adapted
to be a part of a general protective system when they do all that they
must do in stopping abnormal currents and yet withstand lightning
discharges which may pass through them. Other things being equal, that
system of protection is best in which all lightning discharges are
arrested by gap arresters and in which no fuses ever are operated by
lightning discharges.

Mica Fuse:--A convenient and widely used form of fuse is that shown in
Fig. 216. A mica slip has metal terminals at its ends and a fuse wire
joins these terminals. The fuse is inserted in the circuit by clamping
the terminals under screws or sliding them between clips as in Figs.
217 and 218. Advantages of this method of fuse mounting for protecting
circuits needing small currents are that the fuse wire can be seen, the
fuses are readily replaced when blown, and their mountings may be made
compact. As elements of a comprehensive protective system, however, the
ordinary types of mica-slip fuses are objectionable because too short,
and because they have no means of their own for extinguishing an arc
which may follow the blowing of the fuses. As protectors for use in
distributing low potential currents from central-office power plants
they are admirable. By simple means, they may be made to announce
audibly or visibly that they have operated.

[Illustration: Fig. 216. Mica Slip Fuse]

[Illustration: Fig. 217. Postal Type Mica Fuse]

[Illustration: Fig. 218. Western Union Type Mica Fuse]

Enclosed Fuses:--If a fuse wire within an insulating tube be made to
connect metal caps on that tube and the space around the tube be
filled with a non-conducting powder, the gases of the vaporized fuse
metal will be absorbed more quickly than when formed without such
imbedding in a powder. The filling of such a tubular fuse also muffles
the explosion which occurs when the fuse is vaporized.

[Illustration: Fig. 219. Pair of Enclosed Fuses]

Fuses of the enclosed type, with or without filling, are widely used
in power circuits generally and are recommended by fire insurance
bodies. Fig. 219 illustrates an arrester having a fuse of the enclosed
type, this example being that of the H. W. Johns-Manville Company.

[Illustration Fig. 220. Bank of Enclosed Fuses]

In telephony it is frequently necessary to mount a large number of
fuses or other protective devices together in a restricted space. In
Fig. 220 a group of Western Electric tubular fuses, so mounted, is
shown. These fuses have ordinarily a carrying capacity of 6 or 7
amperes. It is not expected that this arrester will blow because 6 or
7 amperes of abnormal currents are flowing through it and the
apparatus to be protected. What is intended is that the fuse shall
withstand lightning discharges and when a foreign current passes
through it, other apparatus will increase that current enough to blow
the fuse. It will be noticed that the fuses of Fig. 220 are open at
the upper end, which is the end connected to the exposed wire of the
line The fuses are closed at the lower end, which is the end connected
to the apparatus. When the fuse blows, its discharge is somewhat
muffled by the lining of the tube, but enough explosion remains so
that the heated gases, in driving outward, tend to break the arc which
is established through the vaporized metal.

A pair of Cook tubular fuses in an individual mounting is shown in
Fig. 221. Fuses of this type are not open at one end like a gun, but
opportunity for the heated gases to escape exists at the caps. The
tubes are made of wood, of lava, or of porcelain.

Fig. 222 is another tubular fuse, the section showing the arrangement
of asbestos lining which serves the two purposes of muffling the sound
of the discharge and absorbing and cooling the resulting gases.

[Illustration: Fig. 221. Pair of Wooden Tube Fuses]

_Air-Gap vs. Fuse Arresters._ It is hoped that the student grasps
clearly the distinction between the purposes of air-gap and fuse
arresters. The air-gap arrester acts in response to high voltages,
either of lightning or of high-tension power circuits. The fuse acts in
response to a certain current value flowing through it and this minimum
current in well-designed protectors for telephone lines is not very
small. Usually it is several times larger than the maximum current
apparatus in the line can safely carry. Fuses _can_ be made so delicate
as to operate on the very smallest current which could injure apparatus
and the earlier protective systems depended on such an arrangement. The
difficulty with such delicate fuses is that they are not robust enough
to be reliable, and, worse still, they change their carrying capacity
with age and are not uniform in operation in different surroundings and
at different temperatures. They are also sensitive to lightning
discharges, which they have no power to stop or to divert.

Protection Against Sneak Currents. For these reasons, a system
containing fuses and air-gap arresters only, does not protect against
abnormal currents which are continuous and small, though large enough
to injure apparatus _because_ continuous. These currents have come to
be known as sneak currents, a term more descriptive than elegant.
Sneak currents though small, may, when allowed to flow for a long time
through the winding of an electromagnet for instance, develop enough
heat to char or injure the insulation. They are the more dangerous
because insidious.

[Illustration: Fig. 222. Tubular Fuse with Asbestos Filling]

_Sneak-Current Arresters._ As typical of sneak-current arresters,
Fig. 223 shows the principle, though not the exact form, of an
arrester once widely used in telephone and signal lines. The normal
path from the line to the apparatus is through a small coil of fine
wire imbedded in sealing wax. A spring forms a branch path from the
line and has a tension which would cause it to bear against the ground
contact if it were allowed to do so. It is prevented from touching
that contact normally by a string between itself and a rigid support.
The string is cut at its middle and the knotted ends as thus cut are
imbedded in the sealing wax which contains the coil.

[Illustration: Fig. 223. Principle of Sneak-Current Arrester]

A small current through the little coil will warm the wax enough to
allow the string to part. The spring then will ground the line. Even
so simple an apparatus as this operates with considerable accuracy.
All currents below a certain critical amount may flow through the
heating coil indefinitely, the heat being radiated rapidly enough to
keep the wax from softening and the string from parting. All currents
above this critical amount will operate the arrester; the larger the
current, the shorter the time of operating. It will be remembered that
the law of these heating effects is that the heat generated =
_C^{2}Rt_, so that if a certain current operates the arrester in, say
40 seconds, twice as great a current should operate the arrester in 10
seconds. In other words, the time of operation varies inversely as the
square of the current and inversely as the resistance. To make the
arrester more sensitive for a given current--_i.e._, to operate in a
shorter time--one would increase the resistance of the coil in the wax
either by using more turns or finer wire, or by making the wire of a
metal having higher specific resistance.

The present standard sneak-current arrester embodies the two elements
of the devices of Fig. 223: a _resistance_ material to transform the
dangerous sneak current into localized heat; and a _fusible_ material
softened by this heat to release some switching mechanism.

The resistance material is either a resistance wire or a bit of
carbon, the latter being the better material, although both are good.
The fusible material is some alloy melting at a low temperature. Lead,
tin, bismuth, and cadmium can be combined in such proportions as will
enable the alloy to melt at temperatures from 140° to 180° F. Such an
alloy is a solder which, at ordinary temperatures, is firm enough to
resist the force of powerful springs; yet it will melt so as to be
entirely fluid at a temperature much less than that of boiling water.

[Illustration: Fig. 224. Heat Coil]

_Heat Coil._ Fig. 224 shows a practical way of bringing the heating and
to-be-heated elements together. A copper spool is wound with resistance
wire. A metal pin is soldered in the bore of the spool by an easily
melting alloy. When current heats the spool enough, the pin may slide
or turn in the spool. It may slide or turn in many ways and this
happily enables many types of arresters to result. For example, the pin
may pull out, or push in, or push through, or rotate like a shaft in a
bearing, or the spool may turn on it like a hub on an axle. Messrs.
Hayes, Rolfe, Cook, McBerty, Kaisling, and many other inventors have
utilized these combinations and motions in the production of
sneak-current arresters. All of them depend on one action: the
softening of a low-melting alloy by heat generated in a resistance.

When a heat coil is associated with the proper switching springs, it
becomes a sneak-current arrester. The switching springs always are
arranged to ground the line wire. In some arresters, the line wire is
cut off from the wire leading toward the apparatus by the same
movement which grounds it. In others, the line is not broken at all,
but merely grounded. Each method has its advantages.

Complete Line Protection. Fig. 225 shows the entire scheme of protectors
in an exposed line and their relation to apparatus in the central-office
equipment and at the subscriber's telephone. The central-office
equipment contains heat coils, springs, and carbon arresters. At some
point between the central office and the subscriber's premises, each
wire contains a fuse. At the subscriber's premises each wire contains
other fuses and these are associated with carbon arresters. The figure
shows a central battery equipment, in which the ringer of the telephone
is in series with a condenser. A sneak-current arrester is not required
at the subscriber's station with such equipment.

Assume the line to meet an electrical hazard at the point _X_. If this
be lightning, it will discharge to ground at the central office or at
the subscriber's instrument or at both through the carbon arresters
connected to that side of the line. If it be a high potential from a
power circuit and of more than 350 volts, it will strike an arc at the
carbon arrester connected to that wire of the line in the central
office or at the subscriber's telephone or at both, if the separation
of the carbons in those arresters is .005 inch or less. If the carbon
arresters are separated by celluloid, it will burn away and allow the
carbons to come together, extinguishing the arc. If they are separated
by mica and one of the carbons is equipped with a globule of
low-melting alloy, the heat of the arc will melt this,
short-circuiting the gap and extinguishing the arc. The passage of
current to ground at the arrester, however, will be over a path
containing nothing but wire and the arrester. The resulting current,
therefore, may be very large. The voltage at the arrester having been
350 volts or more, in order to establish the arc, short-circuiting the
gap will make the current 7 amperes or more, unless the applied
voltage miraculously falls to 50 volts or less. The current through
the fuse being more than 7 amperes, it will blow promptly, opening the
line and isolating the apparatus. It will be noted that this
explanation applies to equipment at either end of the line, as the
fuse lies between the point of contact and the carbon arrester.

[Illustration: Fig. 225. Complete Line Protection]

Assume, on the other hand, that the contact is made at the point _Y_.
The central-office carbon arrester will operate, grounding the line and
increasing the amount of current flowing. There being no fuse to blow,
a worse thing will befall, in the overheating of the line wire and the
probable starting of a fire in the central office. It is obvious,
therefore, that a fuse must be located between the carbon arrester and
any part of the line which is subject to contact with a potential which
can give an abnormal current when the carbon arrester acts.

Assume, as a third case, that the contact at the point _X_ either is
with a low foreign potential or is so poor a contact that the
difference of potential across the gap of the carbon arrester is lower
than its arcing point. Current will tend to flow by the carbon
arrester without operating it, but such a current must pass through
the winding of the heat coil if it is to enter the apparatus. The
sneak current may be large enough to overheat the apparatus if allowed
to flow long enough, but before it has flowed long enough it will have
warmed the heat-coil winding enough to soften its fusible alloy and to
release springs which ground the line, just as did the carbon arrester
in the case last assumed. Again the current will become large and will
blow the fuse which lies between the sneak-current arrester and the
point of contact with the source of foreign current. In this case,
also, contact at the point _Y_ would have operated mechanism to ground
the line at the central office, and, no fuse interposing, the wiring
would have been overheated.

_Exposed and Unexposed Wiring._ Underground cables, cables formed of
rubber insulated wires, and interior wiring which is properly done,
all may be considered to be wiring which is unexposed, that is, not
exposed to foreign high potentials, discharges, sneak, or abnormal
currents. _All other wiring_, such as bare wires, aërial cables, etc.,
should be considered as _exposed_ to such hazards and a fuse should
exist in each wire between its exposed portion and the central office
or subscriber's instrument. The rule of action, therefore, becomes:

_The proper position of the fuse is between exposed and unexposed
wiring._

It may appear to the student that wires in an aërial cable with a lead
sheath--that sheath being either grounded or ungrounded--are not
exposed to electrical hazards; in the case of the grounded sheath,
this would presume that a contact between the cable and a high
potential wire would result merely in the foreign currents going to
ground through the cable sheath, the arc burning off the
high-potential wire and allowing the contact to clear itself by the
falling of the wire. If the assumption be that the sheath is not
grounded, then the student may say that no current at all would flow
from the high-potential wire.

Both assumptions are wrong. In the case of the grounded sheath, the
current flows to it at the contact with the high-potential wire; the
lead sheath is melted, arcs strike to the wires within, and currents
are led directly to the central office and to subscribers' premises.
In the case of the ungrounded sheath, the latter charges at once
through all its length to the voltage of the high-potential wire; at
some point, a wire within the cable is close enough to the sheath for
an arc to strike across, and the trouble begins. All the wires in the
cable are endangered if the cross be with a wire of the primary
circuit of a high-tension transmission line. Any series arc-light
circuit is a high-potential menace. Even a 450-volt trolley wire or
feeder can burn a lead-covered cable entirely in two in a few seconds.
The authors have seen this done by the wayward trolley pole of a
street car, one side of the pole touching the trolley wire and the
extreme end just touching the telephone cable.

The answer lies in the foregoing rule. Place the fuse between the wires
which _can_ and the wires which _can not_ get into contact with high
potentials. In application, the rule has some flexibility. In the case
of a cable which is aërial as soon as it leaves the central office,
place the fuses in the central office; in a cable wholly underground,
from central office to subscriber--as, for example, the feed for an
office building--use no fuses at all; in a cable which leaves the
central office underground and becomes aërial, fuse the wires just
where they change from underground to aërial. The several branches of
an underground cable into aërial ones should be fused as they branch.

Wires properly installed in subscribers' premises are considered
unexposed. The position of the fuse thus is at or near the point of
entrance of the wires into that building if the wires of the
subscriber's line outside the premises are exposed, as determined by
the definitions given. If the line is unexposed, by those definitions,
no protector is required. If one is indicated, it should be used, as
compliance with the best-known practice is a clear duty. Less than
what is known to be best is not honest practice in a matter which
involves life, limb, and indefinite degrees of property values.

Protectors in central-battery subscribers' equipments need no
sneak-current arresters, as the condenser reduces that hazard to a
negligible amount. Magneto subscribers' equipments usually lack
condensers in ringer circuits, though they may have them in talking
circuits on party lines. The ringer circuit is the only path through
the telephone set for about 98 per cent of the time. Sneak-current
arresters, therefore, should be a part of subscribers' station
protectors in magneto equipment, except in such rural districts as may
have no lighting or power wires. When sneak-current arresters are so
used the arrangement of the parts then is the same as in the
central-office portion of Fig. 225.

Types of Central-Office Protectors. A form of combined heat coil and
air-gap arrester, widely used by Bell companies for central-office
protection, is shown in Fig. 226. The two inner springs form the
terminals for the two limbs of the metallic-circuit line, while the
two outside springs are terminals for the continuation of the line
leading to the switchboard. The heat coils, one on each side, are
supported between the inner and outer springs. High-tension currents
jump to ground through the air-gap arrester, while sneak currents
permit the pin of the heat coil to slide within the sleeve, thus
grounding the outside line and the line to the switchboard.

[Illustration: Fig. 226. Sneak-Current and Air-Gap Arrester]

_Self-Soldering Heat Coils._ Another form designed by Kaisling and
manufactured by the American Electric Fuse Company is shown in Fig.
227. In this the pin in the heat coil projects unequally from the ends
of the coil, and under the action of a sneak current the melting of
the solder which holds it allows the outer spring to push the pin
through the coil until it presses the line spring against the ground
plate and at the same time opens the path to the switchboard. When the
heat-coil pin assumes this new position it cools off, due to the
cessation of the current, and _resolders_ itself, and need only be
turned end for end by the attendant to be reset. Many are the
variations that have been made on this self-soldering idea, and there
has been much controversy as to its desirability. It is certainly a
feature of convenience.

[Illustration: Fig. 227. Self-Soldering Heat-Coil Arrester]

Instead of using a wire-wound resistance element in heat-coil
construction some manufacturers employ a mass of high-resistance
material, interposed in the path of the current. The Kellogg Company
has long employed for its sneak-current arrester a short graphite rod,
which forms the resistance element. The ends of this rod are
electroplated with copper to which the brass terminal heads are
soldered. These heads afford means for making the connection with the
proper retaining springs.

[Illustration: Fig. 228. Cook Arrester]

Another central-office protector, which uses a mass of special metal
composition for its heat producing element is that designed by Frank B.
Cook and shown in Fig. 228. In this the carbon blocks are cylindrical
in form and specially treated to make them "self-cleaning." Instead of
employing a self-soldering feature in the sneak-current arrester of
this device, Cook provides for electrically resoldering them after
operation, a clip being designed for holding the elements in proper
position and passing a battery current through them to remelt the
solder.

In small magneto exchanges it is not uncommon to employ combined fuse
and air-gap arresters for central-office line protection, the fuses
being of the mica-mounted type already referred to. A group of such
arresters, as manufactured by the Dean Electric Company, is shown in
Fig. 229.

[Illustration: Fig. 229. Mica Fuse and Air-Gap Arresters]

Types of Subscribers' Station Protectors. Figs. 230 and 231 show types
of subscribers' station protectors adapted to the requirements of
central-battery and magneto systems. These, as has been said, should be
mounted at or near the point of entrance of the subscriber's line into
the premises, if the line is exposed outside of the premises. It is
possible to arrange the fuses so that they will be safe and suitable
for their purposes if they are mounted out-of-doors near the point of
entrance to the premises. The sneak-current arrester, if one exists,
and the carbon arrester also, must be mounted inside of the premises or
in a protecting case, if outside, on account of the necessity of
shielding both of these devices from the weather. Speaking generally,
the wider practice is to put all the elements of the subscriber's
station protector inside of the house. It is nearer to the ideal
arrangement of conditions if the protector be placed immediately at the
point of entrance of the outside wires into the building.

[Illustration: Fig. 230. Western Electric Station Arrester]

[Illustration: Fig. 231. Cook Arrester for Magneto Stations]

_Ribbon Fuses_. A point of interest with relation to tubular fuses is
that in some of the best types of such fuses, the resistance material
is not in the form of a round wire but in the form of a flat ribbon.
This arrangement disposes the necessary amount of fusible metal in a
form to give the greatest amount of surface, while a round wire offers
the least surface for a given weight of metal--a circle encloses its
area with less periphery than any other figure. The reason for giving
the fuse the largest possible surface area is to decrease the
likelihood of the fuse being ruptured by lightning. The fact that such
fuses do withstand lightning discharges much more thoroughly than
round fuses of the same rating is an interesting proof of the
oscillating nature of lightning discharges, for the density of the
current of those discharges is greater on and near the surface of the
conductor than within the metal and, therefore, flattening the fuse
increases its carrying capacity for high-frequency currents, without
appreciably changing its carrying capacity for direct currents. The
reason its capacity for direct currents is increased at all by
flattening it, is that the surface for the radiation of heat is
increased. However, when enclosed in a tube, radiation of heat is
limited, so that for direct currents the carrying capacity of fuses
varies closely with the area of cross-section.

City-Exchange Requirements. The foregoing has set down the
requirements of good practice in an average city-exchange system.
Nothing short of the general arrangement shown in Fig. 225 meets the
usual assortment of hazards of such an exchange. It is good modern
practice to distribute lines by means of cables, supplemented in part
by short insulated drop wires twisted in pairs. Absence of bare wires
reduces electrical hazards enormously. Nevertheless, hazards remain.

Though no less than the spirit of this plan of protection should be
followed, additional hazards may exist, which may require additional
elements of protection. At the end of a cable, either aërial or
underground, long open wires may extend into the open country as rural
or long-distance circuits. If these be longer than a mile or two, in
most regions they will be subjected to lightning discharges. These may
be subjected to high-potential contacts as well.

If a specific case of such exposure indicates that the cables may be
in danger, the long open lines then are equipped with additional
air-gap arresters at the point of junction of those open lines with
the cable. Practice varies as to the type. Maintenance charges are
increased if carbon arresters separated .005 inch are used, because of
the cost of sending to the end of the long cable to clear the blocks
from carbon dust after each slight discharge. Roughened metal blocks
do not become grounded as readily as do carbon blocks. The occasions
of visit to the arresters, therefore, usually follow actual heavy
discharges through them.

The recommendations and the practice of the American Telephone and
Telegraph Company differ on this point, while the practice of other
companies varies with the temperaments of the engineers. The American
Company specifies copper-block arresters where long country lines
enter cables, if those lines are exposed to lightning discharges only.
The exposed line is called _long_ if more than one-half mile in
length. If it is exposed to high-potential hazards, carbon blocks are
specified instead of copper. Other specifications of that company have
called for the use of copper-block arresters on lines exposed to
hazards above 2,500 volts.

[Illustration: 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.]

The freedom of metal-block arresters from dust troubles gives them a
large economical advantage over carbon. For similar separations, the
ratio of striking voltages between carbon blocks and metal blocks
respectively is as 7 to 16. In certain regions of the Pacific Coast
where the lightning hazard is negligible and the high tension hazard
is great, metal-block arresters at the outer ends of cables give
acceptable protection.

High winds which drive snow or dust against bare wires of a long line,
create upon or place upon those wires a charge of static electricity
which makes its way from the line in such ways as it can. Usually it
discharges across arresters and when this discharge takes place, the
line is disturbed in its balance and loud noises are heard in the
telephones upon it.

[Fig. 232. Drainage Coils]

A telephone line which for a long distance is near a high-tension
transmission line may have electrostatic or electromagnetic
potentials, or both, induced upon it. If the line be balanced in its
properties, including balance by transposition of its wires, the
electrostatic induction may neutralize itself. The electromagnetic
induction still may disturb it.

_Drainage Coils_. The device shown in Fig. 232, which amounts merely
to an inductive leak to earth, is intended to cure both the snowstorm
and electromagnetic induction difficulties. It is required that its
impedance be high enough to keep voice-current losses low, while being
low enough to drain the line effectively of the disturbing charges.
Such devices are termed "drainage coils."

Electrolysis. The means of protection against the danger due to
chemical action, set forth in the preceding chapter, form such a
distinct phase of the subject of guarding property against electrical
hazards as to warrant treatment in a separate chapter devoted to the
subject of electrolysis.

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




CHAPTER XX

GENERAL FEATURES OF THE TELEPHONE EXCHANGE


Up to this point only those classes of telephone service which could be
given between two or more stations on a single line have been
considered. Very soon after the practical conception of the telephone,
came the conception of the telephone exchange; that is, the conception
of centering a number of lines at a common point and there terminating
them in apparatus to facilitate their interconnection, so that any
subscriber on any line could talk with any subscriber on any other
line.

The complete equipment of lines, telephone instruments, and switching
facilities by which the telephone stations of the community are given
telephone service is called a telephone exchange.

The building where a group of telephone lines center for
interconnection is called a central office, and its telephonic
equipment the central-office equipment. The terms telephone office and
telephone exchange are frequently confused. Although a telephone office
building may be properly referred to as a telephone exchange building,
it is hardly proper to refer to the telephone office as a telephone
exchange, as is frequently done. In modern parlance the telephone
exchange refers not only to the central office and its equipment but to
the lines and instruments connected therewith as well; furthermore, a
telephone exchange may embrace a number of telephone offices that are
interconnected by means of so-called trunk lines for permitting the
communication of subscribers whose lines terminate in one office with
those subscribers whose lines terminate in any other office.

Since a given telephone exchange may contain one or more central
offices, it is proper to distinguish between them by referring to an
exchange which contains but a single central office as a single office
exchange, and to an exchange which contains a plurality of central
offices as a multi-office exchange.

In telephone exchange working, three classes of lines are dealt
with--subscribers' lines, trunk lines, and toll lines.

Subscribers' Lines. The term subscriber is commonly applied to the
patron of the telephone service. His station is, therefore, referred
to as a subscriber's station, and the telephone equipment at any
subscriber's station is referred to as a subscriber's station
equipment. Likewise, a line leading from a central office to one or
more subscribers' stations is called a subscriber's line. A
subscriber's line may, as has been shown in a previous chapter, be an
individual line if it serves but one station, or a party line if it
serves to connect more than one station with the central office.

Trunk Lines. A trunk line is a line which is not devoted to the
service of any particular subscriber, but which may form a connecting
link between any one of a group of subscribers' lines which terminate
in one place and any one of a group of subscribers' lines which
terminate in another place. If the two groups of subscribers' lines
terminate in the same building or in the same switchboard, so that the
trunk line forming the connecting link between them is entirely within
the central-office building, it is called a local trunk line, or a
local trunk. If, on the other hand, the trunk line is for connecting
groups of subscribers' lines which terminate in different central
offices, it is called an inter-office trunk.

Toll Lines. A toll line is a telephone line for the use of which a
special fee or toll is charged; that is, a fee that is not included in
the charges made to the subscriber for his regular local exchange
service. Toll lines extend from one exchange district to another, more
or less remote, and they are commonly termed _local_ toll and
_long-distance_ toll lines according to the degree of remoteness. A
toll line, whether local or long-distance, may be looked upon in the
nature of an inter-exchange trunk.

Districts. The district in a given community which is served by a
single central office is called an office district. Likewise, the
district which is served by a complete exchange is called an exchange
district. An exchange district may, therefore, consist of a number of
central-office districts, just as an exchange may comprise a number of
central offices. To illustrate, the entire area served by the exchange
of the Chicago Telephone Company in Chicago, embracing the entire city
and some of its suburbs, is the Chicago exchange district. The area
served by one of the central offices, such as the Hyde Park office,
the Oakland office, the Harrison office, or any of the others, is an
office district.

Switchboards. The apparatus at the central office by which the
telephone lines are connected for conversation and afterwards
disconnected, and by which the various other functions necessary to
the giving of complete telephone service are performed, is called a
switchboard. This may be simple in the case of small exchanges, or of
vast complexity in the case of the larger exchanges.

Sometimes the switchboards are of such nature as to require the
presence of operators, usually girls, to connect and disconnect the
line and perform the other necessary functions, and such switchboards,
whether large or small, are termed _manual_.

Sometimes the switchboards are of such a nature as not to require the
presence of operators, the various functions of connection,
disconnection, and signaling being performed by the aid of special
forms of apparatus which are under the control of the subscriber who
makes the call. Such switchboards are termed _automatic_.

Of recent years there has appeared another class of switchboards,
employing in some measure the features of the automatic and in some
measure those of the manual switchboard. These boards are commonly
referred to as _semi-automatic_ switchboards, presumably because they
are supposed to be half automatic and half manual.

_Manual_. Manual switchboards may be subdivided into two classes
according to the method of distributing energy for talking purposes.
Thus we may have _magneto_ switchboards, which are those capable of
serving lines equipped with magneto telephones, local batteries being
used for talking purposes. On the other hand, we may have
_common-battery_ switchboards, adapted to connect lines employing
common-battery telephones in which all the current for both talking
and signaling is furnished from the central office. In still another
way we may classify manual switchboards if the method of distributing
the energy for talking and signaling purposes is ignored. Thus,
entirely irrespective of whether the switchboards are adapted to serve
common-battery or local-battery lines, we may have non-multiple
switchboards and multiple switchboards.

The term _multiple_ switchboard is applied to that class of
switchboards in which the connection terminals or jacks for all the
lines are repeated at intervals along the face of the switchboard, so
that each operator may have within her reach a terminal for each line
and may thus be able to complete by herself any connection between two
lines terminating in the switchboard.

The term _non-multiple_ switchboard is applied to that class of boards
where the provision for repeating the line terminals at intervals along
the face of the board is not employed, but where, as a consequence,
each line has but a single terminal on the face of the board.
Non-multiple switchboards have their main use in small exchanges where
not more than a few hundred lines terminate. Where such is the case, it
is an easy matter to handle all the traffic by one, two, or three
operators, and as all of these operators may reach all over the face of
the switchboard, there is no need for giving any line any more than one
connection terminal. Such boards may be called _simple_ switchboards.

There is another type of non-multiple switchboard adaptable for use in
larger exchanges than the simple switchboard. A correct idea of the
fundamental principle involved in these may be had by imagining a row
of simple switchboards each containing terminals or jacks for its own
group of lines. In order to provide for the connection of a line in
one of these simple switchboards with a line in another one, out of
reach of the operator at the first, short connecting lines extending
between the two switchboards are provided, these being called
_transfer_ or _trunk_ lines. In order that connections may be made
between any two of the simple boards, a group of transfer lines is run
from each board to every other one.

In such switchboards an operator at one of the boards or positions may
complete the connection herself between any two lines terminating at
her own board. If, however, the line called for terminates at another
one of the boards, the operator makes use of the transfer or trunk line
extending to that board, and the operator at this latter board
completes the connection, so that the two subscribers' lines are
connected through the trunk or transfer line. A distinguishing feature,
therefore, in the operation of so-called transfer switchboards, is that
an operator can not always complete a connection herself, the
connection frequently requiring the attention of two operators.

Transfer systems are not now largely used, the multiple switchboard
having almost entirely supplanted them in manual exchanges of such size
as to be beyond the limitation of the simple switchboard. At
multi-office manual exchanges, however, where there are a number of
multiple switchboards employed at various central offices, the same
sort of a requirement exists as that which was met by the provision of
trunk lines between the various simple switchboards in a transfer
system. Obviously, the lines in one central office must be connected to
those of another in order to give universal service in the community in
which the exchange operates. For this purpose inter-office trunk lines
are used, the arrangement being such that when an operator at one
office receives a call for a subscriber in another office, she will
proceed to connect the calling subscriber's line, not directly with the
line of the called subscriber because that particular line is not
within her reach, but rather with a trunk line leading to the office in
which the called-for subscriber's line terminates; having done this she
will then inform an operator at that second office of the connection
desired, usually by means of a so-called order-wire circuit. The
connection between the trunk line so used and the line of the
called-for subscriber will then be completed by the connecting link or
trunk line extending between the two offices.

In such cases the multiple switchboard at each office is divided into
two portions, termed respectively the _A_ board and the _B_ board.
Each of these boards, with the exception that will be pointed out in a
subsequent chapter, is provided with a full complement of multiple
jacks for all of the lines entering that office. At the _A_ board are
located operators, called _A_ operators, who answer all the calls from
the subscribers whose lines terminate in that office. In the case of
calls for lines in that same office, they complete the connection
themselves without the assistance of the other operators. On the other
hand, the calls for lines in another office are handled through trunk
lines leading to that other office, as before described, and these
trunk lines always terminate in the _B_ board at that office. The _B_
operators are, therefore, those operators who receive the calls over
trunk lines and complete the connection with the line of the
subscriber desired.

To define these terms more specifically, an _A_ board is a multiple
switchboard in which the subscriber's lines of a given office district
terminate. For this reason the _A_ board is frequently referred to as
a subscribers' board, and the operators who work at these boards and
who answer the calls of the subscribers are called _A_ operators or
subscribers' operators. _B_ boards are switchboards in which terminate
the incoming ends of the trunk lines leading from other offices in the
same exchange. These boards are frequently called incoming trunk
boards, or merely trunk boards, and the operators who work at them and
who receive the directions from the _A_ operators at the other boards
are called _B_ operators, or incoming trunk operators.

The circuits which are confined wholly to the use of operators and
over which the instructions from one operator to another are sent, as
in the case of the _A_ operator giving an order for a connection to a
_B_ operator at another switchboard, are designated _call circuits_ or
_order wire circuits_.

Sometimes trunk lines are so arranged that connections may be
originated at either of their ends. In other cases they are so arranged
that one group of trunk lines connecting two offices is for the traffic
in one direction only, while another group leading between the same two
offices is for handling only the traffic in the other direction. Trunk
lines are called _one-way_ or _two-way_ trunks, according to whether
they handle the traffic in one direction or in two. A trunking system,
where the same trunks handle traffic both ways, is called a
_single-track system_; and, on the other hand, a system in which there
are two groups of trunks, one handling traffic in one direction and the
other in the other, is called a _double-track system_. This
nomenclature is obviously borrowed from railroad practice.

There is still another class of manual switchboards called the _toll
board_ of which it will be necessary to treat. Telephone calls made by
one person for another within the limits of the same exchange district
are usually charged for either by a flat rate per month, or by a
certain charge for each call. This is usually regardless of the
duration of the conversation following the call. On the other hand,
where a call is made by one party for another outside of the limits of
the exchange district and, therefore, in some other exchange district,
a charge is usually made, based on the time that the connecting
long-distance line is employed. Such calls and their ensuing
conversations are charged for at a very much higher rate than the
purely local calls, this rate depending on the distance between the
stations involved. The making up of connections between a
long-distance and a local line is usually done by means of operators
other than those employed in handling the local calls, who work either
by means of special equipment located on the local board, or by means
of a separate board. Such equipments for handling long-distance or
toll traffic are commonly termed toll switchboards.

They differ from local boards (a) in that they are arranged for a very
much smaller number of lines; (b) in that they have facilities by
which the toll operator may make up the connections with a minimum
amount of labor on the part of the assisting local operators; and (c)
in that they have facilities for recording the identification of the
parties and timing the conversations taking place over the toll lines,
so that the proper charge may be made to the proper subscriber.




CHAPTER XXI

THE SIMPLE MAGNETO SWITCHBOARD


Definitions. As already stated those switchboards which are adapted
to work in conjunction with magneto telephones are called magneto
switchboards. The signals on such switchboards are electromagnetic
devices capable of responding to the currents of the magneto
generators at the subscribers' stations. Since, as a rule, magneto
telephones are equipped with local batteries, it follows that the
magneto switchboard does not need to be arranged for supplying the
subscribers' stations with talking current. This fact is accountable
for magneto switchboards often being referred to as local-battery
switchboards, in contradistinction to common-battery switchboards
which are equipped so as to supply the connected subscribers' stations
with talking current.

The term _simple_ as applied in the headings of this and the next
chapter, is employed to designate switchboards adapted for so small a
number of lines that they may be served by a single or a very small
group of operators; each line is provided with but a single connection
terminal and all of them, without special provision, are placed
directly within the reach of the operator, or operators if there are
more than one. This distinction will be more apparent under the
discussion of transfer and multiple switchboards.

Mode of Operation. The cycle of operation of any simple manual
switchboard may be briefly outlined as follows: The subscriber desiring
a connection transmits a signal to the central office, the operator
seeing the signal makes connection with the calling line and places
herself in telephonic communication with the calling subscriber to
receive his orders; the operator then completes the connection with the
line of the called subscriber and sends ringing current out on that
line so as to ring the bell of that subscriber; the two subscribers
then converse over the connected lines and when the conversation is
finished either one or both of them may send a signal to the central
office for disconnection, this signal being called a clearing-out
signal; upon receipt of the clearing-out signal, the operator
disconnects the two lines and restores all of the central-office
apparatus involved in the connection to its normal position.

Component Parts. Before considering further the operation of manual
switchboards it will be well to refer briefly to the component pieces
of apparatus which go to make up a switchboard.

_Line Signal._ The line signal in magneto switchboards is practically
always in the form of an electromagnetic annunciator or drop. It
consists in an electromagnet adapted to be included in the line
circuit, its armature controlling a latch, which serves to hold the
drop or shutter or target in its raised position when the magnet is not
energized, and to release the drop or shutter or target so as to permit
the display of the signal when the magnet is energized. The symbolic
representation of such an electromagnetic drop is shown in Fig. 233.

[Illustration: Fig. 233. Drop Symbol]

_Jacks and Plugs._ Each line is also provided with a connection
terminal in the form of a switch socket. This assumes many forms, but
always consists in a cylindrical opening behind which are arranged one
or more spring contacts. The opening forms a receptacle for plugs
which have one or more metallic terminals for the conductors in the
flexible cord in which the plug terminates. The arrangement is such
that when a plug is inserted into a jack the contacts on the plug will
register with certain of the contacts in the jack and thus continue
the line conductors, which terminate in the jack contacts, to the cord
conductors, which terminate in the plug contacts. Usually also when a
plug is inserted certain of the spring contacts in the jack are made
to engage with or disengage other contacts in the jack so as to make
or break auxiliary circuits.

[Illustration: Fig. 234. Spring Jack]

A simple form of spring jack is shown in section in Fig. 234. In Fig.
235 is shown a sectional view of a plug adapted to co-operate with
the jack of Fig. 234. In Fig. 236 the plug is shown inserted into the
jack. The cylindrical portion of the jack is commonly called the
_sleeve_ or _thimble_ and it usually forms one of the main terminals
of the jack; the spring, forming the other principal terminal, is
called the _tip spring_, since it engages the tip of the plug. The tip
spring usually rests on another contact which may be termed the
_anvil_. When the plug is inserted into the jack as shown in Fig. 236,
the tip spring is raised from contact with this anvil and thus breaks
the circuit leading through it. It will be understood that spring
jacks are not limited to three contacts such as shown in these figures
nor are plugs limited to two contacts. Sometimes the plugs have three,
and even more, contacts, and frequently the jacks corresponding to
such plugs have not only a contact spring adapted to register with
each of the contacts of the plug, but several other auxiliary contacts
also, which will be made or broken according to whether the plug is
inserted or withdrawn from the jack. Symbolic representations of plugs
and jacks are shown in Fig. 237. These are employed in diagrammatic
representations of circuits and are supposed to represent the
essential elements of the plugs and jacks in such a way as to be
suggestive of their operation. It will be understood that such symbols
may be greatly modified to express the various peculiarities of the
plugs and jacks which they represent.

[Illustration: Fig. 235. Plug]

[Illustration: Fig. 236. Plug and Jack]

[Illustration: Fig. 237. Jack and Plug Symbols]

_Keys_. Other important elements of manual switchboards are ringing
and listening keys. These are the devices by means of which the
operator may switch the central-office generator or her telephone set
into or out of the circuit of the connected lines. The details of a
simple ringing and listening key are shown in Fig. 238. This consists
of two groups of springs, one of four and one of six, the springs in
each group being insulated from each other at their points of
mounting. Two of these springs _1_ and _2_ in one group--the ringing
group--are longer than the others, and act as movable levers engaging
the inner pair of springs _3_ and _4_ when in their normal positions,
and the outer pair _5_ and _6_ when forced into their alternate
positions. Movement is imparted to these springs by the action of a
cam which is mounted on a lever, manipulated by the operator. When
this lever is moved in one direction the cam presses the two springs
_1_ and _2_ apart, thus causing them to disengage the springs _3_ and
_4_ and to engage the springs _5_ and _6_.

[Illustration: Fig. 238. Ringing and Listening Key]

The springs of the other group constitute the switching element of the
listening key and are very similar in their action to those of the
ringing key, differing in the fact that they have no inner pair of
springs such as _3_ and _4_. The two long springs _7_ and _8_,
therefore, normally do not rest against anything, but when the key
lever is pressed, so as to force the cam between them, they are made
to engage the two outer springs _9_ and _10_.

[Illustration: Fig. 239. Ringing-and Listening-Key Symbols]

The design and construction of ringing and listening keys assume many
different forms. In general, however, they are adapted to do exactly
the same sort of switching operations as that of which the device of
Fig. 238 is capable. Easily understood symbols of ringing and
listening keys are shown in Fig. 239; the cam member which operates on
the two long springs is usually omitted for ease of illustration. It
will be understood in considering these symbols, therefore, that the
two long curved springs usually rest against a pair of inner contacts
in case of the ringing key or against nothing at all in case of the
listening key, and that when the key is operated the two springs are
assumed to be spread apart so as to engage the outer pair of contacts
with which they are respectively normally disconnected.

_Line and Cord Equipments._ The parts of the switchboard that are
individual to the subscriber's line are termed the _line equipment;_
this, in the case of a magneto switchboard, consists of the line drop
and the jack together with the associated wiring necessary to connect
them properly in the line circuit. The parts of the switchboard that
are associated with a connecting link--consisting of a pair of plugs
and associated cords with their ringing and listening keys and
clearing-out drop--are referred to as a _cord equipment_. The circuit
of a complete pair of cords and plugs with their associated apparatus
is called a _cord circuit_. In order that there may be a number of
simultaneous connections between different pairs of lines terminating
in a switchboard, a number of cord circuits are provided, this number
depending on the amount of traffic at the busiest time of the day.

_Operator's Equipment._ A part of the equipment that is not individual
to the lines or to the cord circuits, but which may, as occasion
requires, be associated with any of them is called the _operator's
equipment_. This consists of the operator's transmitter and receiver,
induction coil, and battery connections together with the wiring and
other associated parts necessary to co-ordinate them with the rest of
the apparatus. Still another part of the equipment that is not
individual to the lines nor to the cord circuits is the
calling-current generator. This may be common to the entire office or
a separate one may be provided for each operator's position.

Operation in Detail. With these general statements in mind we may
take up in some detail the various operations of a telephone system
wherein the lines center in a magneto switchboard. This may best be
done by considering the circuits involved, without special regard to
the details of the apparatus.

The series of figures showing the cycle of operations of the magneto
switchboard about to be discussed are typical of this type of
switchboard almost regardless of make. The apparatus is in each case
represented symbolically, the representations indicating type rather
than any particular kind of apparatus within the general class to
which it belongs.

_Normal Condition of Line._ In Fig. 240 is shown the circuit of an
ordinary magneto line. The subscriber's sub-station apparatus, shown
at the left, consists of the ordinary bridging telephone but might
with equal propriety be indicated as a series telephone. The
subscriber's station is shown connected with the central office by the
two limbs of a metallic-circuit line. One limb of the line terminates
in the spring _1_ of the jack, and the other limb in the sleeve or
thimble _2_ of the jack. The spring _1_ normally rests on the third
contact or anvil _3_ in the jack, its construction being such that
when a plug is inserted this spring will be raised by the plug so as
to break contact with the anvil _3_. It is understood, of course, that
the plug associated with this jack has two contacts, referred to
respectively as the tip and the sleeve; the tip makes contact with the
tip spring _1_ and the sleeve with the sleeve or thimble _2_.

[Illustration: Fig. 240. Normal Condition of Line]

The drop or line signal is permanently connected between the jack
sleeve and the anvil _3_. As a result, the drop is normally bridged
across the circuit of the line so as to be in a receptive condition to
signaling current sent out by the subscriber. It is evident, however,
that when the plug is inserted into the jack this connection between
the line and the drop will be broken.

In this normal condition of the line, therefore, the drop stands
ready at the central office to receive the signal from the subscriber
and the generator at the sub-station stands ready to be bridged across
the circuit of the line as soon as the subscriber turns its handle.
Similarly the ringer--the call-receiving device at the sub-station--is
permanently bridged across the line so as to be responsive to any
signal that may be sent out from the central office in order to call
the subscriber. The subscriber's talking apparatus is, in this normal
condition of the line, cut out of the circuit by the switch hook.

_Subscriber Calling._ Fig. 241 shows the condition of the line when
the subscriber at the sub-station is making a call. In turning his
generator the two springs which control the connection of the
generator with the line are brought into engagement with each other so
that the generator currents may pass out over the line. The condition
at the central office is the same as that of Fig. 240 except that the
drop is shown with its shutter fallen so as to indicate a call.

[Illustration: Fig. 241. Subscriber Calling]

[Illustration: A SPECIALLY FORMED CABLE FOR KEY SHELF OF MONARCH
SWITCHBOARD]

_Operator Answering._ The next step is for the operator to answer the
call and this is shown in Fig. 242. The subscriber has released the
handle of his generator and the generator has, therefore, been
automatically cut out of the circuit. He also has removed his receiver
from its hook, thus bringing his talking apparatus into the line
circuit. The operator on the other hand has inserted one of the plugs
_P__{a} into the jack. This action has resulted in the breaking of the
circuit through the drop by the raising of the spring _1_ from the
anvil _3_, and also in the continuance of the line circuit through the
conductors of the cord circuits. Thus, the upper limb of the line is
continued by means of the engagement of the tip spring _1_ with the
tip _4_ of the plug to the conducting strand _6_ of the cord circuit;
likewise the lower limb of the line is continued by the engagement
of the thimble _2_ of the jack with the sleeve contact _5_ of the plug
_P__{a} to the strand _7_ of the cord circuit. The operator has also
closed her listening key _L.K._ In doing so she has brought the
springs _8_ and _9_ into engagement with the anvils _10_ and _11_ and
has thus bridged her head telephone receiver with the secondary of her
induction coil across the two strands _6_ and _7_ of the cord.
Associated with the secondary winding of her receiver is a primary
circuit containing a transmitter, battery, and the primary of the
induction coil. It will be seen that the conditions are now such as to
permit the subscriber at the calling station to converse with the
operator and this conversation consists in the familiar "Number
Please" on the part of the operator and the response of the subscriber
giving the number of the line that is desired. Neither the plug
_P__{c}, nor the ringing key _R.K._, shown in Fig. 242, is used in
this operation. The clearing-out drop _C.O._ is bridged permanently
across the strands _6-7_ of the cord, but is without function at this
time; the fact that it is wound to a high resistance and impedance
prevents its having a harmful effect on the transmission.

[Illustration: Fig. 242. Operator Answering]

It may be stated at this point that the two plugs of an associated
pair are commonly referred to as the answering and calling plugs. The
answering plug is the one which the operator always uses in answering
a call as just described in connection with Fig. 242. The calling plug
is the one which she next uses in connecting with the line of the
called subscriber. It lies idle during the answering of a call and is
only brought into play after the order of the calling subscriber has
been given, in which case it is used in establishing connection with
the called subscriber.

[Illustration: Fig. 243. Operator Calling]

_Operator Calling._ We may now consider how the operator calls the
called subscriber. The condition existing for this operation is shown
in Fig. 243. The operator after receiving the order from the calling
subscriber inserts the calling plug _P__{c} into the jack of the line
of the called station. This act at once connects the limbs of the line
with the strands _6_ and _7_ of the cord circuit, and also cuts out the
line drop of the called station, as already explained. The operator is
shown in this figure as having opened her listening key _L.K._ and
closed her ringing key _R.K._ As a result, ringing current from the
central-office generator will flow out over the two ringing key springs
_12_ and _13_ to the tip and sleeve contacts of the calling plug
_P__{c}, then to the tip spring _1_ and the sleeve or thimble _2_ of
the jack, and then to the two sides of the metallic-circuit line to the
sub-station and through the bell there. This causes the ringing of the
called subscriber's bell, after which the operator releases the ringing
key and thereby allows the two springs _12_ and _13_ of that key to
again engage their normal contacts _14_ and _15_, thus making the two
strands _6_ and _7_ of the cord circuit continuous from the contacts of
the answering plug _P__{a} to the contacts of the calling plug
_P__{c}. This establishes the condition at the central office for
conversation between the two subscribers.

[Illustration: Fig. 244. Subscribers Connected for Conversation.]

_Subscribers Conversing._ The only other thing necessary to establish a
complete set of talking conditions between the two subscribers is for
the called subscriber to remove his receiver from its hook, which he
does as soon as he responds to the call. The conditions for
conversation between the two subscribers are shown in Fig. 244. It is
seen that the two limbs of the calling line are connected respectively
to the two limbs of the called line by the two strands of the cord
circuit, both the operator's receiver and the central-office generator
being cut out by the listening and ringing keys, respectively. Likewise
the two line drops are cut out of circuit and the only thing left
associated with the circuit at the central office is the clearing-out
drop _C. O._, which remains bridged across the cord circuit. This, like
the two ringers at the respective connected stations, which also remain
bridged across the circuit when bridging instruments are used, is of
such high resistance and impedance that it offers practically no path
to the rapidly fluctuating voice currents to leak from one side of the
line circuit to the other. Fluctuating currents generated by the
transmitter at the calling station, for instance, are converted by
means of the induction coil into alternating currents flowing in the
secondary of the induction coil at that station. Considering a
momentary current as passing up through the secondary winding of the
induction coil at the calling station, it passes through the receiver
of that station through the upper limb of the line to the spring _1_ of
the line jack belonging to that line at the central office; thence
through the tip _4_ of the answering plug to the conductor _6_ of the
cord; thence through the pair of contacts _14_ and _12_ forming one
side of the ringing key to the tip _4_ of the calling plug; thence to
the tip spring _1_ of the jack of the called subscriber's line; thence
over the upper limb of his line through his receiver and through the
secondary of the induction to one of the upper switch-hook contacts;
thence through the hook lever to the lower side of the line, back to
the central office and through the sleeve contact _2_ of the jack and
the sleeve contact _5_ of the plug; thence through the other ringing
key contacts _13_ and _15_; thence through the strand _7_ of the cord
to the sleeve contact _5_ and the sleeve contact _2_ of the answering
plug and jack, respectively; thence through the lower limb of the
calling subscriber's line to the hook lever at his station; thence
through one of the upper contacts of this hook to the secondary of the
induction coil, from which point the current started.

[Illustration: Fig. 245. Clearing-Out Signal]

Obviously, when the called subscriber is talking to the calling
subscriber the same path is followed. It will be seen that at any time
the operator may press her listening key _L.K._, bridge her telephone
set across the circuit of the two connected lines, and listen to the
conversation or converse with either of the subscribers in case of
necessity.

_Clearing Out_. At the close of the conversation, either one or both
of the subscribers may send a clearing-out signal by turning their
generators after hanging up their receivers. This condition is shown
in Fig. 245. The apparatus at the central office remains in exactly
the same position during conversation as that of Fig. 244, except that
the clearing-out drop shutter is shown as having fallen. The two
subscribers are shown as having hung up their receivers, thus cutting
out their talking apparatus, and as operating their generators for the
purpose of sending the clearing-out signals. In response to this act
the operator pulls down both the calling and the answering plug, thus
restoring them to their normal seats, and bringing both lines to the
normal condition as shown in Fig. 240. The line drops are again
brought into operative relation with their respective lines so as to
be receptive to subsequent calls and the calling generators at the
sub-stations are removed from the bridge circuits across the line by
the opening of the automatic switch contacts associated with those
generators.

_Essentials of Operation_. The foregoing sequence of operations while
described particularly with respect to magneto switchboards is, with
certain modifications, typical of the operation of nearly all manual
switchboards. In the more advanced types of manual switchboards,
certain of the functions described are sometimes done automatically,
and certain other functions, not necessary in connection with the
simple switchboard, are added. The essential mode of operation,
however, remains the same in practically all manual switchboards, and
for this reason the student should thoroughly familiarize himself with
the operation and circuits of the simple switchboard as a foundation
for the more complex and consequently more-difficult-to-understand
switchboards that will be described later on.

Commercial Types of Drops and Jacks. _Early Drops_. Coming now to
the commercial types of switchboard apparatus, the first subject that
presents itself is that of magneto line signals or drops. The very
early forms of switchboard drops had, in most cases, two-coil magnets,
the cores of which were connected at their forward ends by an iron
yoke and the armature of which was pivoted opposite the rear end of
the two cores. To the armature was attached a latch rod which
projected forwardly to the front of the device and was there adapted
to engage the upper edge of the hinged shutter, so as to hold it in
its raised or undisplayed position when the armature was unattracted.
Such a drop, of Western Electric manufacture, is shown in Fig. 246.

[Illustration: Fig. 246 Old-Style Drop]

Liability to Cross-Talk:--This type of drop is suitable for use only
on small switchboards where space is not an important consideration,
and even then only when the drop is entirely cut out of the circuit
during conversation. The reason for this latter requirement will be
obvious when it is considered that there is no magnetic shield around
the winding of the magnet and no means for preventing the stray field
set up by the talking currents in one of the magnets from affecting by
induction the windings of adjacent magnets contained in other talking
circuits. Unless the drops are entirely cut out of the talking
circuit, therefore, they are very likely to produce cross-talk between
adjacent circuits. Furthermore, such form of drop is obviously not
economical of space, two coils placed side by side consuming
practically twice as much room as in the case of later drops wherein
single magnet coils have been made to answer the purpose.

_Tubular Drops._ In the case of line drops, which usually can readily
be cut out of the circuit during conversation, this cross-talk feature
is not serious, but sometimes the line drops, and always the
clearing-out drops must be left in connection with the talking circuit.
On account of economy in space and also on account of this cross-talk
feature, there has come into existence the so-called tubular or
iron-clad drop, one of which is shown in section in Fig. 247. This was
developed a good many years ago by Mr. E.P. Warner of the Western
Electric Company, and has since, with modifications, become standard
with practically all the manufacturing companies. In this there is but
a single bobbin, and this is enclosed in a shell of soft Norway iron,
which is closed at its front end and joined to the end of the core as
indicated, so as to form a complete return magnetic path for the lines
of force generated in the coil. The rear end of the shell and core are
both cut off in the same plane and the armature is made in such form as
to practically close this end of the shell. The armature carries a
latch rod extending the entire length of the shell to the front portion
of the structure, where it engages the upper edge of the pivoted
shutter; this, when released by the latch upon the attraction of the
armature, falls so as to display a target behind it.

[Illustration: Fig. 247. Tubular Drop]

[Illustration: Fig. 248. Strip of Tubular Drops]

These drops may be mounted individually on the face of the
switchboard, but it is more usual to mount them in strips of five or
ten. A strip of five drops, as manufactured by the Kellogg Switchboard
and Supply Company, is shown in Fig. 248. The front strip on which
these drops are mounted is usually of brass or steel, copper plated,
and is sufficiently heavy to provide a rigid support for the entire
group of drops that are mounted on it. This construction greatly
facilitates the assembling of the switchboard and also serves to
economize space--obviously, the thing to economize on the face of a
switchboard is space as defined by vertical and horizontal dimensions.
These tubular drops, having but one coil, are readily mounted on
1-inch centers, both vertically and horizontally. Sometimes even
smaller dimensions than this are secured. The greatest advantage of
this form of construction, however, is in the absolute freedom from
cross-talk between two adjacent drops. So completely is the magnetic
field of force kept within the material of the shell, that there is
practically no stray field and two such drops may be included in two
different talking circuits and the drops mounted immediately adjacent
to each other without producing any cross-talk whatever.

_Night Alarm._ Switchboard drops in falling make but little noise, and
during the day time, while the operator is supposed to be needed
continually at the board, the visual signal which they display is
sufficient to attract her attention. In small exchanges, however, it
is frequently not practicable to keep an operator at the switchboard
at night or during other comparatively idle periods, and yet calls
that do arrive during such periods must be attended to. For this
reason some other than a visual signal is necessary, and this need is
met by the so-called night-alarm attachment. This is merely an
arrangement by which the shutter in falling closes a pair of contacts
and thus completes the circuit of an ordinary vibrating bell or buzzer
which will sound until the shutter is restored to its normal position.
Such contacts are shown in Fig. 249 at _1_ and _2_. Night-alarm
contacts have assumed a variety of forms, some of which will be
referred to in the discussion of other types of drops and jacks.

[Illustration: Fig. 249. Drop with Night-Alarm Contacts]

_Jack Mounting._ Jacks, like drops, though frequently individually
mounted are more often mounted in strips. An individually mounted jack
is shown in Fig. 250, and a strip of ten jacks in Fig. 251. In such a
strip of jacks, the strips supporting the metallic parts of the
various jacks are usually of hard rubber reinforced by brass so as to
give sufficient strength. Various forms of supports for these strips
are used by different manufacturers, the means for fastening them in
the switchboard frame usually consisting of brass lugs on the end of
the jack strip adapted to be engaged by screws entering the stationary
portion of the iron framework; or sometimes pins are fixed in the
framework, and the jack is held in place by nuts engaging
screw-threaded ends on such pins.

[Illustration: Fig. 250. Individual Jack]

[Illustration: Fig. 251. Strip of Jacks]

_Methods of Associating Jacks and Drops._ There are two general
methods of arranging the drops and jacks in a switchboard. One of
these is to place all of the jacks in a group together at the lower
portion of the panel in front of the operator and all of the drops
together in another group above the group of jacks. The other way is
to locate each jack in immediate proximity to the drop belonging to
the same line so that the operator's attention will always be called
immediately to the jack into which she must insert her plug in
response to the display of a drop. This latter practice has several
advantages over the former. Where the drops are all mounted in one
group and the jacks in another, an operator seeing a drop fall must
make mental note of it and pick out the corresponding jack in the
group of jacks. On the other hand, where the jacks and drops are
mounted immediately adjacent to each other, the falling of a drop
attracts the attention of the operator to the corresponding jack
without further mental effort on her part.

The immediate association of the drops and jacks has another
advantage--it makes possible such a mechanical relation between the
drop and its associated jack that the act of inserting the plug into
the jack in making the connection will automatically and mechanically
restore the drop to its raised position. Such drops are termed
_self-restoring drops_, and, since a drop and jack are often made
structurally a unitary piece of apparatus, they are frequently called
_combined_ drops and jacks.

_Manual vs. Automatic Restoration._. There has been much difference of
opinion on the question of manual versus automatic restoration of
drops. Some have contended that there is no advantage in having the
drops restored automatically, claiming that the operator has plenty of
time to restore the drops by hand while receiving the order from the
calling subscriber or performing some of her other work. Those who
think this way have claimed that the only place where an automatically
restored drop is really desirable is where, on account of the lack of
space on the front of the switchboard, the drops are placed on such a
portion of the board as to be not readily reached by the operator.
This resulted in the electrically restored drop, mention of which will
be made later.

Others have contended that even though the drop is mounted within easy
reach of the operator, it is advantageous that the operator should be
relieved of the burden of restoring it, claiming that even though
there are times in the regular performance of the operator's duties
when she may without interfering with other work restore the drops
manually, such requirement results in a double use of her attention
and in a useless strain on her which might better be devoted to the
actual making of connections.

Until recently the various Bell operating companies have adhered, in
their small exchange work, to the manual restoring method, while most
of the so-called independent operating companies have adhered to the
automatic self-restoring drops.

Methods of Automatic Restoration. Two general methods present
themselves for bringing about the automatic restoration of the drop.
First, the mechanical method, which is accomplished by having some
moving part of the jack or of the plug as it enters the jack force the
drop mechanically into its restored position. This usually means the
mounting of the drop and the corresponding jack in juxtaposition, and
this, in turn, has usually resulted in the unitary structure
containing both the drop and the jack. Second, the electrical method
wherein the plug in entering the jack controls a restoring circuit,
which includes a battery or other source of energy and a restoring
coil on the drop, the result being that the insertion of the plug into
the jack closes this auxiliary circuit and thus energizes the
restoring magnet, the armature of which pulls the shutter back into
its restored position. This practice has been followed by Bell
operating companies whenever conditions require the drop to be mounted
out of easy reach of the operator; not otherwise.

_Mechanical--Direct Contact with Plug._ One widely used method of
mechanical restoration of drops, once employed by the Western
Telephone Construction Company with considerable success, was to hang
the shutter in such position that it would fall immediately in front
of the jack so that the operator in order to reach the jack with the
plug would have to push the plug directly against the shutter and thus
restore it to its normal or raised position. In this construction the
coil of the drop magnet was mounted directly behind the jack, the
latch rod controlled by the armature reaching forward, parallel with
the jack, to the shutter, which, as stated, was hung in front of the
jack. This resulted in a most compact arrangement so far as the space
utilization on the front of the board was concerned and such combined
drops and jacks were mounted on about 1-inch centers, so that a bank
of one hundred combined drops and jacks occupied a space only a little
over 10 inches square.

A modification of this scheme, as used by the American Electric
Telephone Company, was to mount the drop immediately over the jack so
that its shutter, when down, occupied a position almost in front of,
but above, the jack opening. The plug was provided with a collar,
which, as it entered the jack, engaged a cam on the base of the
shutter and forced the latter mechanically into its raised position.

Neither of these methods of restoring--_i.e._, by direct contact
between the shutter or part of it and the plug or part of it--is now
as widely used as formerly. It has been found that there is no real
need in magneto switchboards for the very great compactness which the
hanging of the shutter directly in front of the drop resulted in, and
the tendency in later years has been to make the combined drops and
jacks more substantial in construction at the expense of some space on
the face of the switchboard.

[Illustration: Fig. 252. Kellogg Drop and Jack]

Kellogg Type:--A very widely used scheme of mechanical restoration is
that employed in the Miller drop and jack manufactured by the Kellogg
Switchboard and Supply Company, the principles of which may be
understood in connection with Fig. 252. In this figure views of one of
these combined drops and jacks in three different positions are shown.
The jack is composed of the framework _B_ and the hollow screw _A_,
the latter forming the sleeve or thimble of the jack and being
externally screw-threaded so as to engage and bind in place the front
end of the framework _B_. The jack is mounted on the lower part of the
brass mounting strip _C_ but insulated therefrom. The tip spring of
the jack is bent down as usual to engage the tip of the plug, as
better shown in the lower cut of Fig. 252, and then continues in an
extension _D_, which passes through a hole in the mounting plate _C_.
This tip spring in its normal position rests against another spring as
shown, which latter spring forms one terminal of the drop winding.

The drop or annunciator is of tubular form, and the shutter is so
arranged on the front of the mounting strip _C_ as to fall directly
above the extension _D_ of the tip spring. As a result, when the plug
is inserted into the jack, the upward motion of the tip spring forces
the drop into its restored position, as indicated in the lower cut of
the figure. These drops and jacks are usually mounted in banks of
five, as shown in Fig. 253.

[Illustration: Fig. 253. Strip of Kellogg Drops and Jacks]

Western Electric Type:--The combined drop and jack of the Western
Electric Company recently put on the market to meet the demands of the
independent trade, differs from others principally in that it employs
a spherical drop or target instead of the ordinary flat shutter. This
piece of apparatus is shown in its three possible positions in Fig.
254. The shutter or target normally displays a black surface through a
hole in the mounting plate. The sphere forming the target is out of
balance, and when the latch is withdrawn from it by the action of the
electromagnet it falls into the position shown in the middle cut of
Fig. 254, thus displaying a red instead of a black surface to the view
of the operator. When the operator plugs in, the plug engages the
lower part of an =S=-shaped lever which acts on the pivoted sphere to
restore it to its normal position. A perspective view of one of these
combined line signals and jacks is shown in Fig. 255.

A feature that is made much of in recently designed drops and jacks
for magneto service is that which provides for the ready removal of the
drop coil, from the rest of the structure, for repair. The drop and
jack of the Western Electric Company, just described, embodies this
feature, a single screw being so arranged that its removal will permit
the withdrawal of the coil without disturbing any of the other parts or
connections. The coil windings terminate in two projections on the
front head of the spool, and these register with spring clips on the
inside of the shell so that the proper connections for the coil are
automatically made by the mere insertion of the coil into the shell.

[Illustration: Fig. 254. Western Electric Drop and Jack]

[Illustration: Fig. 255. Western Electric Drop and Jack]

Dean Type:--The combined drop and jack of the Dean Electric Company is
illustrated in Figs. 256 and 257. The two perspective views show the
general features of the drop and jack and the method by which the
magnet coil may be withdrawn from the shell. As will be seen the
magnet is wound on a hollow core which slides over the iron core, the
latter remaining permanently fixed in the shell, even though the coil
be withdrawn.

Fig. 258 shows the structural details of the jack employed in this
combination and it will be seen that the restoring spring for the drop
is not the tip spring itself, but another spring located above and
insulated from it and mechanically connected therewith.

[Illustration: Fig. 256. Dean Drop and Jack]

[Illustration: Fig. 257. Dean Drop and Jack]

[Illustration: Fig. 258. Details of Dean Jack]

Monarch Type:--Still another combined drop and jack is that of the
Monarch Telephone Manufacturing Company of Chicago, shown in sectional
view in Fig. 259. This differs from the usual type in that the
armature is mounted on the front end of the electromagnet, its latch
arm retaining the shutter in its normal position when raised, and
releasing it when depressed by the attraction of the armature. As is
shown, there is within the core of the magnet an adjustable spiral
spring which presses forward against the armature and which spring is
compressed by the attraction of the armature of the magnet. The
night-alarm contact is clearly shown immediately below the strip which
supports the drop, this consisting of a spring adapted to be engaged
by a lug on the shutter and pressed upwardly against a stationary
contact when the shutter falls. The method of restoration of the
shutter in this case is by means of an auxiliary spring bent up so as
to engage the shutter and restore it when the spring is raised by the
insertion of a plug into the jack.

[Illustration: Fig. 259. Monarch Drop and Jack]

_Code Signaling._ On bridging party lines, where the subscribers
sometimes call other subscribers on the same line and sometimes call
the switchboard so as to obtain a connection with another line, it is
not always easy for the operator at the switchboard to distinguish
whether the call is for her or for some other party on the line. On
such lines, of course, code ringing is used and in most cases the
operator's only way of distinguishing between calls for her and those
for some sub-station parties on the line is by listening to the
rattling noise which the drop armature makes. In the case of the
Monarch drop the adjustable spring tension on the armature is intended
to provide for such an adjustment as will permit the armature to give
a satisfactory buzz in response to the alternating ringing currents,
whether the line be long or short.

[Illustration: Fig. 260. Code Signal Attachment]

The Monarch Company provides in another way for code signaling at the
switchboard. In some cases there is a special attachment, shown in
Fig. 260, by means of which the code signals are repeated on the
night-alarm bell. This is in the nature of a special attachment
placed on the drop, which consists of a light, flat spring attached to
the armature and forming one side of a local circuit. The other side
of the circuit terminates in a fixture which is mounted on the drop
frame and is provided with a screw, having a platinum point forming
the other contact point; this allows of considerable adjustment. At
the point where the screw comes in contact with the spring there is a
platinum rivet. When an operator is not always in attendance, this
code-signaling attachment has some advantages over the drop as a
signal interpreter, in that it permits the code signals to be heard
from a distance. Of course, the addition of spring contacts to the
drop armature tends to complicate the structure and perhaps to cut
down the sensitiveness of the drop, which are offsetting
disadvantages.

[Illustration: Fig. 261. Combined Drop and Ringer]

For really long lines, this code signaling by means of the drop is
best provided for by employing a combined drop and ringer, although in
this case whatever advantages are secured by the mechanical
restoration of the shutter upon plugging in are lost. Such a device as
manufactured by the Dean Electric Company is shown in Fig. 261. In
this the ordinary polarized ringer is used, but in addition the tapper
rod carries a latch which, when vibrated by the ringing of the bell,
releases a shutter and causes it to fall, thus giving a visual as well
as an audible signal.

_Electrical_. Coming now to the electrical restoration of drop
shutters, reference is made to Fig. 262, which shows in side section
the electrical restoring drop employed by the Bell companies and
manufactured by the Western Electric Company. In this the coil _1_ is
a line coil, and it operates on the armature _2_ to raise the latch
lever _3_ in just the same manner as in the ordinary tubular drop.
The latch lever _3_ acts, however, to release another armature _4_
instead of a shutter. This armature _4_ is pivoted at its lower end at
the opposite end of the device from the armature _2_ and, by falling
outwardly when released, it serves to raise the light shutter _5_. The
restoring coil of this device is shown at _6_, and when energized it
attracts the armature _4_ so as to pull it back under the catch of the
latch lever _3_ and also so as to allow the shutter _5_ to fall into
its normal position. The method of closing the restoring circuit is by
placing coil _6_ in circuit with a local battery and with a pair of
contacts in the jack, which latter contacts are normally open but are
bridged across by the plug when it enters the jack, thus energizing
the restoring coil and restoring the shutter.

[Illustration: Fig. 262. Electrically Restored Drop]

A perspective view of this Western Electric electrical restoring drop
is shown in Fig. 263, a more complete mention being made of this
feature under the discussion of magneto multiple switchboards, wherein
it found its chief use. It is mentioned here to round out the methods
that have been employed for accomplishing the automatic restoration of
shutters by the insertion of the plug.

[Illustration: Fig. 263. Electrically Restored Drop]

Switchboard Plugs. A switchboard plug such as is commonly used in
simple magneto switchboards is shown in Fig. 264 and also in Fig. 235.
The tip contact is usually of brass and is connected to a slender
steel rod which runs through the center of the plug and terminates
near the rear end of the plug in a connector for the tip conductor of
the cord. This central core of steel is carefully insulated from the
outer shell of the plug by means of hard rubber bushings, the parts
being forced tightly together. The outer shell, of course, forms the
other conductor of the plug, called the sleeve contact. A handle of
tough fiber tubing is fitted over the rear end of the plug and this
also serves to close the opening formed by cutting away a portion of
the plug shell, thus exposing the connector for the tip conductor.

[Illustration: Fig. 264. Switchboard Plug]

_Cord Attachment._ The rear end of the plug shell is usually bored out
just about the size of the outer covering of the switchboard cord, and
it is provided with a coarse internal screw thread, as shown. The cord
is attached by screwing it tightly into this screw-threaded chamber,
the screw threads in the brass being sufficiently coarse and of
sufficiently small internal diameter to afford a very secure
mechanical connection between the outer braiding of the cord and the
plug. The connection between the tip conductor of the cord and the tip
of the plug is made by a small machine screw connection as shown,
while the connection between the sleeve conductor of the plug and the
sleeve conductor of the cord is made by bending back the latter over
the outer braiding of the cord before it is screwed into the shank of
the plug. This results in the close electrical contact between the
sleeve conductor of the cord and the inner metal surface of the shank
of the plug.

Switchboard Cords. A great deal of ingenuity has been exerted toward
the end of producing a reliable and durable switchboard cord. While
great improvement has resulted, the fact remains that the cords of
manual switchboards are today probably the most troublesome element,
and they need constant attention and repairs. While no two
manufacturers build their cords exactly alike, descriptions of a few
commonly used and successful cords may be here given.

_Concentric Conductors._ In one the core is made from a double strand
of strong lock stitch twine, over which is placed a linen braid. Then
the tip conductor, which is of stranded copper tinsel, is braided on.
This is then covered with two layers of tussah silk, laid in reverse
wrappings, then there is a heavy cotton braid, and over the latter a
linen braid. The sleeve conductor, which is also of copper tinsel, is
then braided over the structure so formed, after which two reverse
wrappings of tussah silk are served on, and this is covered by a cotton
braid and this in turn by a heavy linen or polished cotton braid. The
plug end of the cord is reinforced for a length of from 12 to 18 inches
by another braiding of linen or polished cotton, and the whole cord is
treated with melted beeswax to make it moisture-proof and durable.

[Illustration: Fig. 265. Switchboard Cord]

_Steel Spiral Conductors._ In another cord that has found much favor
the two conductors are formed mainly by two concentric spiral
wrappings of steel wire, the conductivity being reinforced by adjacent
braidings of tinsel. The structure of such a cord is well shown in
Fig. 265. Beginning at the right, the different elements shown are, in
the order named, a strand of lock stitch twine, a linen braiding, into
the strands of which are intermingled tinsel strands, the inner spiral
steel wrapping, a braiding of tussah silk, a linen braiding, a loose
tinsel braiding, the outer conductor of round spiral steel, a cotton
braid, and an outside linen or polished cotton braid. The inner tinsel
braiding and the inner spiral together form the tip conductor while
the outer braiding and spiral together form the sleeve conductor. The
cord is reinforced at the plug end for a length of about 14 inches by
another braiding of linen. The tinsel used is, in each case, for the
purpose of cutting down the resistance of the main steel conductor.
These wrappings of steel wire forming the tip and sleeve conductors
respectively, have the advantage of affording great flexibility, and
also of making it certain that whatever strain the cord is subjected
to will fall on the insulated braiding rather than on the spiral
steel which has in itself no power to resist tensile strains.

_Parallel Tinsel Conductors._ Another standard two-conductor
switchboard cord is manufactured as follows: One conductor is of very
heavy copper tinsel insulated with one wrapping of sea island cotton,
which prevents broken ends of the tinsel or knots from piercing
through and short-circuiting with the other conductor. Over this is
placed one braid of tussah silk and an outer braid of cotton. This
combines high insulation with considerable strength. The other
conductor is of copper tinsel, not insulated, and this is laid
parallel to the thrice insulated conductor already described. Around
these two conductors is placed an armor of spring brass wire in spiral
form, and over this a close, stout braid of glazed cotton. This like
the others is reinforced by an extra braid at the plug end.

Ringing and Listening Keys. The general principles of the ringing
key have already been referred to. Ringing keys are of two general
types, one having horizontal springs and the other vertical.

[Illustration: Fig. 266. Horizontal-Spring Listening and Ringing Key]

_Horizontal Spring Type._ Various Bell operating companies have
generally adhered to the horizontal spring type except in individual
and four-party-line keys. The construction of a Western Electric
Company horizontal spring key is shown in Fig. 266. In this particular
key, as illustrated, there are two cam levers operating upon three
sets of springs. The cam lever at the left operates the ordinary
ringing and listening set of springs according to whether it is pushed
one way or the other. In ringing on single-party lines the cam lever
at the left is the one to be used; while on two-party lines the lever
at the left serves to ring the first party and the ringing key at the
right the second party.

In order that the operator may have an indication as to which station
on a two-party line she has called, a small target _1_ carried on a
lever _2_ is provided. This target may display a black or a white
field, according to which of its positions it occupies. The lever _2_
is connected by the links _3_ and _4_ with the two key levers and the
target is thus moved into one position or the other, according to
which lever was last thrown into ringing position.

It will be noticed that the springs are mounted horizontally and on
edge. This on-edge feature has the advantage of permitting ready
inspection of the contacts and of avoiding the liability of dust
gathering between the contacts. As will be seen, at the lower end of
each switch lever there is a roller of insulating material which
serves as a wedge, when forced between the two long springs of any
set, to force them apart and into engagement with their respective
outer springs.

[Illustration: Fig. 267. Vertical-Spring Listening and Ringing Key]

_Vertical Spring Type._ The other type of ringing and listening key
employing vertical springs is almost universally used by the various
independent manufacturing companies. A good example of this is shown
in Fig. 267, which shows partly in elevation and partly in section a
double key of the Monarch Company. The operation of this is obvious
from its mode of construction. The right-hand set of springs of the
right-hand key in this cut are the springs of the listening key, while
the left-hand set of the right-hand key are those of the calling-plug
ringing key. The left-hand set of the left-hand key may be those of a
ring-back key on the answering plug, while the right-hand set of the
left-hand key may be for any special purpose. It is obvious that these
groups of springs may be grouped in different combinations or omitted
in part, as required. This same general form of key is also
manufactured by the Kellogg Company and the Dean Company, that of the
Kellogg Company being illustrated in perspective, Fig. 268. The keys
of this general type have the same advantages as those of the
horizontal on-edge arrangement with respect to the gathering of dust,
and while perhaps the contacts are not so readily get-at-able for
inspection, yet they have the advantage of being somewhat more simple,
and of taking up less horizontal space on the key shelf.

[Illustration: Fig. 268. Vertical Listening and Ringing Key]

[Illustration: Fig. 269. Four-Party Listening and Ringing Key]

_Party-Line Ringing Keys._ For party-line ringing the key matter
becomes somewhat more complicated. Usually the arrangement is such
that in connection with each calling plug there are a number of keys,
each arranged with respect to the circuits of the plug so as to send
out the proper combination and direction of current, if the polarity
system is used; or the proper frequency of current if the harmonic
system is used; or the proper number of impulses if the step-by-step
or broken-line system is used. The number of different kinds of
arrangements and combinations is legion, and we will here illustrate
only an example of a four-party line ringing key adapted for harmonic
ringing. A Kellogg party-line listening and ringing key is shown in
Fig. 269. In this, besides the regular listening key, are shown four
push-button keys, each adapted, when depressed, to break the
connection back of the key, and at the same time connect the proper
calling generator with the calling plug.

_Self-Indicating Keys._ A complication that has given a good deal of
trouble in the matter of party-line ringing is due to the fact that it
is sometimes necessary to ring a second or a third time on a
party-line connection, because the party called may not respond the
first time. The operator is not always able to remember which one of
the four keys associated with the plug connected with the desired
party she has pressed on the first occasion and, therefore, when it
becomes necessary to ring again, she may ring the wrong party. This is
provided for in a very ingenious way in the key shown in Fig. 269, by
making the arrangement such that after a given key has been depressed
to its full extent in ringing, and then released, it does not come
quite back to its normal position but remains slightly depressed. This
always serves as an indication to the operator, therefore, as to which
key she depressed last, and in the case of a re-ring, she merely
presses the key that is already down a little way. On the next call if
she is required to press another one of the four keys, the one which
remained down a slight distance on the last call will be released and
the one that is fully depressed will be the one that remains down as
an indication.

Such keys, where the key that was last used leaves an indication to
that effect, are called _indicating_ ringing keys. In other forms the
indication is given by causing the key lever to move a little target
which remains exposed until some other key in the same set is moved.
The key shown in Fig. 266 is an example of this type.

     NOTE. The matter of automatic ringing and other special forms of
     ringing will be referred to and discussed at their proper places
     in this work, but at this point they are not pertinent as they
     are not employed in simple switchboards.

Operator's Telephone Equipment. Little need be said concerning the
matter of the operator's talking apparatus, _i.e._, the operator's
transmitter and receiver, since as transmitters and receivers they are
practically the same as those in ordinary use for other purposes. The
watch-case receiver is nearly always employed for operators' purposes
on account of its lightness and compactness. It is used in connection
with a head band so as to be held continually at the operator's ear,
allowing both of her hands to be free.

The transmitter used by operators does not in itself differ from the
transmitters employed by subscribers, but the methods by which it is
supported differ, two general practices being followed. One of these
is to suspend the transmitter by flexible conducting cords so as to be
adjustable in a vertical direction. A good illustration of this is
given in Fig. 270. The other method, and one that is coming into more
and more favor, is to mount the transmitter on a light bracket
suspended by a flexible band from the neck of the operator, a breast
plate being furnished so that the transmitter will rest on her breast
and be at all times within proper position to receive her speech. To
facilitate this, a long curved mouthpiece is commonly employed, as
shown clearly in Fig. 47.

[Illustration: Fig. 270. Operator's Transmitter Suspension]

_Cut-in Jack._ It is common to terminate that portion of the apparatus
which is worn on the operator's person--that is, the receiver only if
the suspended type of transmitter is employed, and the receiver and
transmitter if the breast plate type of transmitter is employed--in a
plug, and a flexible cord connecting the plug terminates with the
apparatus. The portions of the operator's talking circuit that are
located permanently in the switchboard cabinet are in such cases
terminated in a jack, called an operator's _cut-in jack_. This is
usually mounted on the front rail of the switchboard cabinet just
below the key shelf. Such a cut-in jack is shown in Fig. 271 and it is
merely a specialized form of spring jack adapted to receive the short,
stout plug in which the operator's transmitter, or transmitter and
receiver, terminate. By this arrangement the operator is enabled
readily to connect or disconnect her talking apparatus, which is worn
on her person, whenever she comes to the board for work or leaves it
at the end of her work. A complete operator's telephone set, or that
portion that is carried on the person of the operator, together with
the cut-in plug, is shown in Fig. 272.

[Illustration: Fig. 271. Operator's Cut-in Jack]

[Illustration: Fig. 272. Operator's Talking Set]

Circuits of Complete Switchboard. We may now discuss the circuits of
a complete simple magneto switchboard. The one shown in Fig. 273 is
typical. Before going into the details of this, it is well to inform
the student that this general form of circuit representation is one
that is commonly employed in showing the complete circuits of any
switchboard. Ordinarily two subscribers' lines are shown, these
connecting their respective subscribers' stations with two different
line equipments at the central office. The jacks and signals of these
line equipments are turned around so as to face each other, in order
to clearly represent how the connection between them may be made by
means of the cord circuit. The elements of the cord circuit are also
spread out, so that the various parts occupy relative positions which
they do not assume at all in practice. In other words it must be
remembered that, in circuit diagrams, the relative positions of the
parts are sacrificed in order to make clear the circuit connections.
However, this does not mean that it is often not possible to so locate
the pieces of apparatus that they will in a certain way indicate
relative positions, as may be seen in the case of the drop and jack in
Fig. 273, the drop being shown immediately above the jack, which is
the position in which these parts are located in practice.

[Illustration: Fig. 273. Circuit of Simple Magneto Switchboard]

Little need be said concerning this circuit in view of what has
already been said in connection with Figs. 240 to 245. It will be seen
in the particular sub-station circuit here represented, that the
talking apparatus is arranged in the usual manner and that the ringer
and generator are so arranged that when the generator is operated the
ringer will be cut out of circuit, while the generator will be placed
across the circuit; while, when the generator is idle, the ringer is
bridged across the circuit and the generator is cut out.

The line terminates in each case in the tip and sleeve contacts of the
jack, and in the normal condition of the jack the line drop is bridged
across the line. The arrangement by which the drop is restored and at
the same time cut out of circuit when the operator plugs in the jack,
is obvious from the diagrammatic illustration. The cord circuit is the
same as that already discussed, with the exception that two ringing
keys are provided, one in connection with the calling plug, as is
universal practice, and the other in connection with the answering plug
as is sometimes practiced in order that the operator may, when occasion
requires, ring back the calling subscriber without the necessity of
changing the plug in the jack. The outer contacts of these two ringing
keys are connected to the terminals of the ringing generator and, when
either key is operated, the connection between the plug, on which the
ringing is to be done, and the rest of the cord circuit will be broken,
while the generator will be connected with the terminals of the plug.
The listening key and talking apparatus need no further explanation, it
being obvious that when the key is operated the subscriber's telephone
set will be bridged across the cord circuit and, therefore, connected
with either or both of the talking subscribers.

[Illustration: Fig. 274. Night-Alarm Circuit]

Night-Alarm Circuits. The circuit of Fig. 273, while referred to as
a complete circuit, is not quite that. The night-alarm circuit is not
shown. In order to clearly indicate how a single battery and bell, or
buzzer, may serve in connecting a number of line drops, reference is
made to Fig. 274 which shows the connection between three different
line drops and the night-alarm circuit. The night-alarm apparatus
consists in the battery _1_ and the buzzer, or bell, _2_. A switch _3_
adapted to be manually operated is connected in the circuit with the
battery and the buzzer so as to open this circuit when the night alarm
is not needed, thus making it inoperative. During the portions of the
day when the operator is needed constantly at the board it is
customary to leave this switch _3_ open, but during the night period
when she is not required constantly at the board this switch is closed
so that an audible signal will be given whenever a drop falls. The
night-alarm contact _4_ on each of the drops will be closed whenever a
shutter falls, and as the two members of this contact, in the case of
each drop, are connected respectively with the two sides of the
night-alarm circuit, any one shutter falling will complete the
necessary conditions for causing the buzzer to sound, assuming of
course that the switch _3_ is closed.

_Night Alarm with Relay._ A good deal of trouble has been caused in
the past by uncertainty in the closure of the night-alarm circuit at
the drop contact. Some of the companies have employed the form of
circuit shown in Fig. 275 to overcome this. Instead of the night-alarm
buzzer being placed directly in the circuit that is closed by the
drop, a relay _5_ and a high-voltage battery _6_ are placed in this
circuit. The buzzer and the battery for operating it are placed in a
local circuit controlled by this relay. It will be seen by reference
to Fig. 275 that when the shutter falls, it will, by closing the
contact _4_, complete the circuit from the battery _6_ through the
relay _5_--assuming switch _3_ to be closed--and thus cause the
operation of the relay. The relay, in turn, by pulling up its
armature, will close the circuit of the buzzer _2_ through the battery
_7_ and cause the buzzer to sound.

[Illustration: Fig. 275. Night-Alarm Circuit with Relay]

The advantage of this method over the direct method of operating the
buzzer is that any imperfection in the night-alarm contact at the drop
is much less likely to prevent the flow of current of the high-voltage
battery _6_ than of the low-voltage battery _1_, shown in connection
with Fig. 274. This is because the higher voltage is much more likely
to break down any very thin bit of insulation, such as might be caused
by a minute particle of dust or oxide between contacts that are
supposed to be closed by the falling of the shutter. It has been
common to employ for battery _6_ a dry-cell battery giving about 20
or 24 volts, and for the operation of the buzzer itself, a similar
battery of about two cells giving approximately 3 volts.

_Night-Alarm Contacts._ The night-alarm contact _4_ of the drop shown
diagrammatically in Figs. 274 and 275 would, if taken literally,
indicate that the shutter itself actually forms one terminal of the
circuit and the contact against which it falls, the other. This has
not been found to be a reliable way of closing the night-alarm
contacts and this method is indicated in these figures and in other
figures in this work merely as a convenient way of representing the
matter diagrammatically. As a matter of fact the night-alarm contacts
are ordinarily closed by having the shutter fall against one spring,
which is thereby pressed into engagement with another spring or
contact, as shown in Fig. 249. This method employs the shutter only as
a means for mechanically causing the one spring to press against the
other, the shutter itself forming no part of the circuit. The reason
why it is not a good plan to have the shutter itself act as one
terminal of the circuit is that this necessitates the circuit
connections being led to the shutter through the trunnions on which
the shutter is pivoted. This is bad because, obviously, the shutter
must be loosely supported on its trunnions in order to give it
sufficiently free movement, and, as is well known, loose connections
are not conducive to good electrical contacts.

Grounded-and Metallic-Circuit Lines. When grounded circuits were the
rule rather than the exception, many of the switchboards were
particularly adapted for their use and could not be used with
metallic-circuit lines. These grounded-circuit switchboards provided
but a single contact in the jack and a single contact on the plug, the
cords having but a single strand reaching from one plug to the other.
The ringing keys and listening keys were likewise single-contact keys
rather than double. The clearing-out drop and the operator's talking
circuit and the ringing generator were connected between the single
strand of the cord and the ground as was required.

The grounded-circuit switchboard has practically passed out of
existence, and while a few of them may be in use, they are not
manufactured at present. The reason for this is that while many
grounded circuits are still in use, there are very few places where
there are not some metallic-circuit lines, and while the
grounded-circuit switchboard will not serve for metallic-circuit
lines, the metallic-circuit switchboard will serve equally well for
either metallic-circuit or grounded lines, and will interconnect them
with equal facility. This fact will be made clear by a consideration
of Figs. 276, 277, and 278.

[Illustration: Fig. 276. Connection Between Metallic Lines]

[Illustration: Fig. 277. Connection Between Grounded Lines]

_Connection between Two Similar Lines._ In Fig. 276 a common magneto
cord circuit is shown connecting two metallic-circuit lines; in Fig.
277 the same cord circuit is shown connecting two grounded lines. In
this case the line wire _1_ of the left-hand line is, when the plugs
are inserted, continued to the tip of the answering plug, thence
through the tip strand of the cord circuit to the tip of the calling
plug, then to the tip spring of the right-hand jack and out to the
single conductor of that line. The entire sleeve portion of the cord
circuit becomes grounded as soon as the plugs are inserted in the
jacks of such a line. Hence, we see that the sleeve contacts of the
plug and the sleeve conductor of the cord are connected to ground
through the permanent ground connection of the sleeve conductors of
the jack as soon as the plug is inserted into the jack. Thus, when the
cord circuit of a metallic-circuit switchboard is used to connect two
grounded circuits together, the tip strand of the cord is the
connecting link between the two conductors, while the sleeve strand of
the cord merely serves to ground one side of the clearing-out drop
and one side each of the operator's telephone set and the ringing
generator when their respective keys are operated.

_Connection between Dissimilar Lines._ Fig. 278 shows how the same
cord circuit and the same arrangement of line equipment may be used
for connecting a grounded line to a metallic-circuit line. The
metallic circuit line is shown on the left and the grounded line on
the right. When the two plugs are inserted into the respective jacks
of this figure, the right-hand conductor of the metallic circuit shown
on the left will be continued through the tip strand of the cord
circuit to the line conductor of the grounded line shown on the right.
The left-hand conductor of the metallic-circuit line will be connected
to ground because it will be continued through the sleeve strand of
the cord circuit to the sleeve contact of the calling plug and thence
to the sleeve contact of the jack of the grounded line, which sleeve
contact is shown to be grounded. The talking circuit between the two
connected lines in this case may be traced as follows: From the
subscriber's station at the left through the right-hand limb of the
metallic-circuit line, through the tip contact and tip conductor of
the cord circuit, to the single limb of the grounded-circuit line,
thence to the sub-station of that line and through the talking
apparatus there to ground. The return path from the right-hand station
is by way of ground to the ground connection at the central office,
thence to the sleeve contact of the grounded line jack, through the
sleeve conductor of the cord circuit, to the sleeve contact of the
metallic-circuit line jack, and thence by the left-hand limb of the
metallic-circuit line to the subscriber's station.

[Illustration: Fig. 278. Connection Between Dissimilar Lines]

A better way of connecting a metallic-circuit line to a grounded line
is by the use of a special cord circuit involving a repeating coil,
such a connection being shown in Fig. 279. The cord circuit in this
case differs in no respect from those already shown except that a
repeating coil is associated with it in such a way as to conductively
divide the answering side from the calling side. Obviously, whatever
currents come over the line connected with the answering plug will
pass through the windings _1_ and _2_ of this coil and will induce
corresponding currents in the windings _3_ and _4_, which latter
currents will pass out over the circuit of the line connected with the
calling plug. When a grounded circuit is connected to a metallic
circuit in this manner, no ground is thrown onto the metallic circuit.
The balance of the metallic circuit is, therefore, maintained.

To ground one side of a metallic circuit frequently so unbalances it
as to cause it to become noisy, that is, to have currents flowing in
it, by induction or from other causes, other than the currents which
are supposed to be there for the purpose of conveying speech.

[Illustration: Fig. 279. Connection of Dissimilar Lines through
Repeating Coil]

_Convertible Cord Circuits._ The consideration of Fig. 279 brings us
to the subject of so-called convertible cord circuits. Some
switchboards, serving a mixture of metallic and grounded lines, are
provided with cord circuits which may be converted at will by the
operator from the ordinary type shown in Fig. 276 to the type shown in
Fig. 279. The advantage of this will be obvious from the following
consideration. When a call originates on any line, either grounded or
metallic, the operator does not know which kind of a line is to be
called for. She, therefore, plugs into this line with any one of her
answering plugs and completes the connection in the usual way. If the
call is for the same kind of a circuit as that over which the call
originated, she places the converting key in such a position as will
connect the conductors of the cord circuit straight through; while if
the connection is for a different kind of a line than that on which
the call originated she throws the converting key into such a
position as to include the repeating coil. A study of Fig. 280 will
show that when the converting key, which is commonly referred to as
the repeating-coil key, is in one position, the cord conductors will
be cut straight through, the repeating coil being left open in both
its windings; and when it is thrown to its other position, the
connection between the answering and calling sides of the cord circuit
will be severed and the repeating coil inserted so as to bring about
the same effects and circuit arrangements as are shown in Fig. 279.

[Illustration: Fig. 280. Convertible Cord Circuit]

Cord-Circuit Considerations. _Simple Bridging Drop Type._ The matter
of cord circuits in magneto switchboards is deserving of much
attention. So far as talking requirements are concerned, the ordinary
form of cord circuit with a clearing-out drop bridged across the two
strands is adequate for nearly all conditions except those where a
grounded-and a metallic-circuit line are connected together, in which
case the inclusion of a repeating coil has some advantages.

[Illustration: Fig. 281. Bridging Drop-Cord Circuit]

From the standpoint of signaling, however, this type of cord circuit
has some disadvantages under certain conditions. In order to simplify
the discussion of this and other cord-circuit matters, reference will
be made to some diagrams from which the ringing and listening keys and
talking apparatus have been entirely omitted. In Fig. 281 the regular
bridging type of clearing-out drop-cord circuit is shown, this being
the type already discussed as standard. For ordinary practice it is
all right. Certain difficulties are experienced with it, however,
where lines of various lengths and various types of sub-station
apparatus are connected. For instance, if a long bridging line be
connected with one end of this cord circuit and a short line having a
low-resistance series ringer be connected with the other end, then a
station on the long line may have some difficulty in throwing the
clearing-out drop, because of the low-resistance shunt that is placed
around it through the short line and the low-resistance ringer. In
other words, the clearing-out drop is shunted by a comparatively
low-resistance line and ringer and the feeble currents arriving from a
distant station over the long line are not sufficient to operate the
drop thus handicapped. The advent of the various forms of party-line
selective signaling and the use of such systems in connection with
magneto switchboards has brought in another difficulty that sometimes
manifests itself with this type of cord circuit. If two ordinary
magneto telephones are connected to the two ends of this cord circuit,
it is obvious that when one of the subscribers has hung up his
receiver and the other subscriber rings off, the bell of the other
subscriber will very likely be rung even though the clearing-out drop
operates properly; it would be better in any event not to have this
other subscriber's bell rung, for he may understand it to be a recall
to his telephone. When, however, a party line is connected through
such a cord circuit to an ordinary line having bridging instruments,
for instance, the difficulty due to ringing off becomes even greater.
When the subscriber on the magneto line operates his generator to give
the clearing-out signal, he is very likely to ring some of the bells
on the other line and this, of course, is an undesirable thing. This
may happen even in the case of harmonic bells on the party line, since
it is possible that the subscriber on the magneto line in turning his
generator will, at some phase of the operation, strike just the proper
frequency to ring some one of the bells on the harmonic party line. It
is obvious, therefore, that there is a real need for a cord circuit
that will prevent _through ringing._

One way of eliminating the through-ringing difficulty in the type of
cord circuit shown in Fig. 281 would be to use such a very low-wound
clearing-out drop that it would practically short-circuit the line
with respect to ringing currents and prevent them from passing on to
the other line. This, however, is not a good thing to do, since a
winding sufficiently low to shunt the effective ringing current would
also be too low for good telephone transmission.

[Illustration: Fig. 282. Series Drop-Cord Circuit]

_Series Drop Type._ Another type of cord circuit that was largely used
by the Stromberg-Carlson Telephone Manufacturing Company at one time is
shown in Fig. 282. In this the clearing-out drop was not bridged but
was placed in series in the tip side of the line and was shunted by a
condenser. The resistance of the clearing-out drop was 1,000 ohms and
the capacity of the condenser was 2 microfarads. It is obvious that
this way of connecting the clearing-out drop was subject to the
_ringing-through_ difficulty, since the circuit through which the
clearing-out current necessarily passed included the telephone
instrument of the line that was not sending the clearing-out signal.
This form was also objectionable because it was necessary for the
subscriber to ring through the combined resistance of two lines, and in
case the other line happened to be open, no clearing-out signal would
be received. While this circuit, therefore, was perhaps not quite so
likely as the other to tie up the subscriber, that is, to leave him
connected without the ability to send a clearing-out signal, yet it was
sure to ring through, for the clearing-out drop could not be thrown
without the current passing through the other subscriber's station.

[Illustration: Fig. 283. Dean Non-Ring-Through Cord Circuit]

_Non-Ring-Through Type._ An early attempt at a non-ring-through cord
is shown in Fig. 283, this having once been standard with the Dean
Electric Company. It made use of two condensers of 1 microfarad each,
one in each side of the cord circuit. The clearing-out drop was of 500
ohms resistance and was connected from the answering side of the tip
conductor to the calling side of the sleeve conductor. In this way
whatever clearing-out current reached the central office passed
through at least one of the condensers and the clearing-out drop. In
order for the clearing-out current to pass on beyond the central
office it was necessary for it to pass through the two condensers in
series. This arrangement had the advantage of giving a positive
ring-off, regardless of the condition of the connected line.
Obviously, even if the line was short-circuited, the ringing currents
from the other line would still be forced through the clearing-out
drop on account of the high effective resistance of the 1-microfarad
condenser connected in series with the short-circuited line. Also the
clearing-out signal would be properly received if the connected line
were open, since the clearing-out drop would still be directly across
the cord circuit. This arrangement also largely prevented through
ringing, since the currents would pass through the 1-microfarad
condenser and the 500-ohm drop more readily than through the two
condensers connected in series.

[Illustration: Fig. 284. Monarch Non-Ring-Through Cord Circuit]

In Fig. 284 is shown the non-ring-through arrangement of cord circuit
adopted by the Monarch Company. In this system the clearing-out drop
has two windings, either of which will operate the armature. The two
windings are bridged across the cord circuit, with a 1/2-microfarad
condenser in series in the tip strand between the two winding
connections. While the low-capacity condenser will allow the
high-frequency talking current to pass readily without affecting it to
any appreciable extent, it offers a high resistance to a low-frequency
ringing current, thus preventing it from passing out on a connected
line and forcing it through one of the windings of the coil. There is
a tendency to transformer action in this arrangement, one of the
windings serving as a primary and the other as a secondary, but this
has not prevented the device from being highly successful.

A modification of this arrangement is shown in Fig. 285, wherein a
double-wound clearing-out drop is used, and a 1/2-microfarad condenser
is placed in series in each side of the cord circuit between the
winding connections of the clearing-out drop. This circuit should give
a positive ring-off under all conditions and should prevent through
ringing except as it may be provided by the transformer action between
the two windings on the same core.

[Illustration: Fig. 285. Non-Ring-Through Cord Circuit]

Another rather ingenious method of securing a positive ring-off and
yet of preventing in a certain degree the undesirable ringing-through
feature is shown in the cord circuit, Fig. 286. In this two
non-inductive coils _1_ and _2_ are shown connected in series in the
tip and sleeve strands of the coils, respectively. Between the neutral
point of these two non-inductive windings is connected the
clearing-out drop circuit. Voice currents find ready path through
these non-inductive windings because of the fact that, being
non-inductive, they present only their straight ohmic resistance. The
impedance of the clearing-out drop prevents the windings being shunted
across the two sides of the cord circuit. With this circuit a positive
ring-off is assured even though the line connected with the one
sending the clearing-out signal is short-circuited or open. If it is
short-circuited, the shunt around the clearing-out drop will still
have the resistance of two of the non-inductive windings included in
it, and thus the drop will never be short-circuited by a very
low-resistance path. Obviously, an open circuit in the line will not
prevent the clearing-out signal being received. While this is an
ingenious scheme, it is not one to be highly recommended since the
non-inductive windings, in order to be effective so far as signaling
is concerned, must be of considerable resistance and this resistance
is in series in the talking circuit. Even non-inductive resistance is
to be avoided in the talking circuit when it is of considerable
magnitude and where there are other ways of solving the problem.

[Illustration: Fig. 286. Cord Circuit with Differential Windings]

_Double Clearing-out Type. _Some people prefer two clearing-out drops
in each cord circuit, so arranged that the one will be responsive to
currents sent from the line with which the answering plug is connected
and the other responsive only to currents sent from the line with
which the calling plug is connected. Such a scheme, shown in Fig. 287,
is sometimes employed by the Dean, the Monarch, and the Kellogg
companies. Two 500-ohm clearing-out drops of ordinary construction are
bridged across the cord circuit and in each side of the cord circuit
there is included between the drop connections a 1-microfarad
condenser. Ringing currents originating on the line with which the
answering plug is connected will pass through the clearing-out drop,
which is across that side of the cord circuit, without having to pass
through any condensers. In order to reach the other clearing-out drop
the ringing current must pass through the two 1-microfarad condensers
in series, this making in effect only 1/2-microfarad. As is well
known, a 1/2-microfarad condenser not only transmits voice currents
with ease but also offers a very high apparent resistance to ringing
currents. With the double clearing-out drop system the operator is
enabled to tell which subscriber is ringing off. If both shutters fall
she knows that both subscribers have sent clearing-out signals and
she, therefore, pulls down the connection without the usual precaution
of listening to see whether one of the subscribers may be waiting for
another connection. This double clearing-out system is analogous to
the complete double-lamp supervision that will be referred to more
fully in connection with common-battery circuits. There is not the
need for double supervision in magneto work, however, that there is in
common-battery work because of the fact that in magneto work the
subscribers frequently fail to remember to ring off, this act being
entirely voluntary on their part, while in common-battery work, the
clearing-out signal is given automatically by the subscriber when he
hangs up his receiver, thus accomplishing the desired end without the
necessity of thoughtfulness on his part.

[Illustration: Fig. 287. Double Clearing-Out Drops]

Another form of double clearing-out cord circuit is shown in Fig. 288.
In this the calling and the answering plugs are separated by repeating
coils, a condenser of 1-microfarad capacity being inserted between each
pair of windings on the two ends of the circuit. The clearing-out
drops are placed across the calling and answering cords in the usual
manner. The condenser in this case prevents the drop being
short-circuited with respect to ringing currents and yet permits the
voice currents to flow readily through it. The high impedance of the
drop forces the voice currents to take the path through the repeating
coil rather than through the drop. This circuit has the advantage of a
repeating-coil cord circuit in permitting the connection of metallic
and grounded lines without causing the unbalancing of the metallic
circuits by the connection to them of the grounded circuits.

[Illustration: Fig. 288. Double Clearing-Out Drops]

Recently there has been a growing tendency on the part of some
manufacturers to control their clearing-out signals by means of relays
associated with cord circuits, these signals sometimes being ordinary
clearing-out drops and sometimes incandescent lamps.

[Illustration: Fig. 289. Relay-Controlled Clearing-Out Drop]

In Fig. 289 is shown the cord circuit sometimes used by the L.M.
Ericsson Telephone Manufacturing Company. A high-wound relay is
normally placed across the cord and this, besides having a
high-resistance and impedance winding has a low-resistance locking
winding so arranged that when the relay pulls up its armature it will
close a local circuit including this locking winding and local battery.
When once pulled up the relay will, therefore, stay up due to the
energizing of this locking coil. Another contact operated by the relay
closes the circuit of a low-wound clearing-out drop placed across the
line, thus bridging it across the line. The condition of high impedance
is maintained across the cord circuit normally while the subscribers
are talking; but when either of them rings off, the high-wound relay
pulls up and locks, thus completing the circuit of the clearing-out
drop across the cords. The subsequent impulses sent from the
subscribers' generators operate this drop. The relay is restored or
unlocked and the clearing-out drop disconnected from the cord circuit
by means of a key which opens the locking circuit of the relay. This
key is really a part of the listening key and serves to open this
locking circuit whenever the listening key is operated. The
clearing-out drop is also automatically restored by the action of the
listening key, this connection being mechanical rather than electrical.

Recall Lamp:--The Monarch Company sometimes furnishes what it terms a
recall lamp in connection with the clearing-out drops on its magneto
switchboards. The circuit arrangement is shown in Fig. 290, wherein
the drop is the regular double-wound clearing-out drop like that of
Fig. 284. The armature carries a contact spring adapted to close the
local circuit of a lamp whenever it is attracted. The object of this
is to give the subscriber, whose line still remains connected by a
cord circuit, opportunity to recall the central office if the operator
has not restored the clearing-out drop.

[Illustration: Fig. 290. Cord Circuit with Recall Lamp]

_Lamp-Signal Type._ There has been a tendency on the part of some
manufacturing companies to advocate, instead of drop signals,
incandescent lamp signals for the cord circuits, and sometimes for the
line circuits on magneto boards. In most cases this may be looked upon
as a "frill." Where line lamps instead of drops have been used on
magneto switchboards, it has been the practice to employ, instead of a
drop, a locking relay associated with each lamp, which was so arranged
that when the relay was energized by the magneto current from the
subscriber's station, it would pull up and lock, thus closing the lamp
circuit.

The local circuit, or locking circuit, which included the lamp was
carried through a pair of contacts in the corresponding jacks so
arranged that when the plug was inserted in answer to the call, this
locking lamp circuit would be open, thereby extinguishing the lamp and
also unlocking the relay. There seems to be absolutely no good reason
why lamp signals should be substituted for mechanical drops in magneto
switchboards. There is no need for the economy in space which the lamp
signal affords, and the complications brought in by the locking
relays, and the requirements for maintaining a local battery suitable
for energizing the lamps are not warranted for ordinary cases.

[Illustration: Fig. 291. Cord Circuit with Double Lamp Signals]

In Fig. 291 is shown a cord circuit, adaptable to magneto
switchboards, provided with double lamp signals instead of
clearing-out drops. Two high-wound locking relays are bridged across
the line, the cord strands being divided by 1-microfarad condensers.
When the high-wound coil of either relay is energized by the magneto
current from the subscriber's station, the relay pulls up and closes a
locking circuit including a battery and a coil _2_, the contact _3_ of
the locking relay, and also the contact _4_ of a restoring key. This
circuit may be traced from the ground through battery, coil _2_,
contact _3_ controlled by the relay, and contact _4_ controlled by the
restoring key, and back to ground. In multiple with the locking coil
_2_ is the lamp, which is illuminated, therefore, whenever the locking
circuit is closed. Pressure on the restoring key breaks the locking
circuit of either of the lamps, thereby putting out the lamp and at
the same time restoring the locking relay to its normal position.

_Lamps vs. Drops in Cord Circuits._ So much has been said and written
about the advantages of incandescent lamps as signals in switchboards
and about the merits of the common-battery method of supplying current
to the subscribers, that there has been a tendency for people in
charge of the operation of small exchanges to substitute the lamp for
the drop in a magneto switchboard in order to give the general
appearance of common-battery operations. There has also been a
tendency to employ the common-battery system of operation in many
places where magneto service should have been used, a mistake which
has now been realized and corrected. In places where the simple
magneto switchboard is the thing to use, the simpler it is the better,
and the employment of locking relays and lamp signals and the
complications which they carry with them, is not warranted.

Switchboard Assembly. The assembly of all the parts of a simple
magneto switchboard into a complete whole deserves final
consideration. The structure in which the various parts are mounted,
referred to as the cabinet, is usually of wood.

_Functions of Cabinet._ The purpose of the cabinet is not only to form
a support for the various pieces of apparatus but also to protect them
from dust and mechanical injury, and to hold those parts that must be
manipulated by the operator in such relation that they may be most
convenient for use, and thus best adapted for carrying out their
various functions. Other points to be provided for in the design of
the cabinet and the arrangement of the various parts within are: that
all the apparatus that is in any way liable to get out of order may be
readily accessible for inspection and repairs; and that provision
shall be made whereby the wiring of these various pieces of apparatus
may be done in a systematic and simple way so as to minimize the
danger of crossed, grounded, or open circuits, and so as to provide
for ready repair in case any of these injuries do occur.


_Wall-Type Switchboards._ The simplest form of switchboard is that for
serving small communities in rural districts. Ordinarily the telephone
industry in such a community begins by a group of farmers along a
certain road building a line connecting the houses of several of them
and installing their own instruments. This line is liable to be
extended to some store at the village or settlement, thus affording
communication between these farmers and the center of their community.
Later on those residing on other roads do the same thing and connect
their lines to the same store or central point. Then it is that some
form of switchboard is established, and perhaps the storekeeper's
daughter or wife is paid a small fee for attendance.

[Illustration: Fig. 292. Wall Switchboard with Telephone]

A switchboard well-adapted for this class of service where the number
of lines is small, is shown in Fig. 292. In this the operator's
talking apparatus and her calling apparatus are embodied in an
ordinary magneto wall telephone. The switchboard proper is mounted
alongside of this, and the two line binding posts of the telephone are
connected by a pair of wires to terminals of the operator's plug,
which plug is shown hanging from the left-hand portion of the
switchboard. The various lines centering at this point terminate in
the combined drops and jacks on the switchboard, of which there are 20
shown in this illustration. Beside the operator's plug there are a
number of pairs of plugs shown hanging from the switchboard cabinet.
These are connected straight through in pairs, there being no
clearing-out drops or keys associated with them in the arrangement.
Each line shown is provided with an extra jack, the purpose of which
will be presently understood.

The method of operation is as follows: When a subscriber on a certain
line desires to get connection through the switchboard he turns his
generator and throws the drop. The operator in order to communicate
with him inserts the plug in which her telephone terminates into the
jack, and removes her receiver from its hook. Having learned that it
is for a certain subscriber on another line, she withdraws her plug
from the jack of the calling line and inserts it into the jack of the
called line, then, hanging up her receiver, she turns the generator
crank in accordance with the proper code to call that subscriber. When
that subscriber responds she connects the two lines by inserting the
two plugs of a pair into their respective jacks, and the subscribers
are thus placed in communication. The extra jack associated with each
line is merely an open jack having its terminals connected
respectively with the two sides of the line. Whenever an operator
desires to listen in on two connected lines she does so by inserting
the operator's plug into one of these extra jacks of the connected
lines, and she may thus find out whether the subscribers are through
talking or whether either one of them desires another connection. The
drops in such switchboards are commonly high wound and left
permanently bridged across the line so as to serve as clearing-out
drops. The usual night-alarm attachment is provided, the buzzer being
shown at the upper right-hand portion of the cabinet.

[Illustration: Fig. 293. Combined Telephone and Switchboard]

Another type of switchboard commonly employed for this kind of
service is shown in Fig. 293, in which the telephone and the
switchboard cabinet are combined. The operation of this board is
practically the same as that of Fig. 292, although it has
manually-restored drops instead of self-restoring drops; the
difference between these two types, however, is not material for this
class of service. For such work the operator has ample time to attend
to the restoring of the drop and the only possible advantage in the
combined drop-and-jack for this class of work is that it prevents the
operator from forgetting to restore the drops. However, she is not
likely to do this with the night-alarm circuit in operation, since the
buzzer or bell would continue to ring as long as the drop was down.

[Illustration: Fig. 294. Upright Magneto Switchboard]

[Illustration: Fig. 295. Upright Magneto Switchboard--Rear View]

_Upright Type Switchboard._ By far the most common type of magneto
switchboard is the so-called upright type, wherein the drops and
jacks are mounted on the face of upright panels rising from a
horizontal shelf, which shelf contains the plugs, the keys, and any
other apparatus which the operator must manipulate. Front and rear
views of such a switchboard, as manufactured by the Kellogg Company,
are shown in Figs. 294 and 295. This particular board is provided with
fifty combined drops and jacks and, therefore, equipped for fifty
subscribers' lines. The drops and jacks are mounted in strips of five,
and arranged in two panels. The clearing-out drops, of which there are
ten, are arranged at the bottom of the two panels in a single row and
may be seen immediately above the switchboard plugs. There are ten
pairs of cords and plugs with their associated ringing and listening
keys, the plugs being mounted on the rear portion of the shelf,
while the ringing and listening keys are mounted on the hinged portion
of the shelf in front of the plugs.

[Illustration: Fig. 296. Details of Drop, Jack, Plug, and Key
Arrangement]

[Illustration: Fig. 297. Cross-Section of Upright Switchboard]

A better idea of the arrangement of drops, jacks, plugs, and keys may
be had from an illustration of a Dean magneto switchboard shown in
Fig. 296. The clearing-out drops and the arrangement of the plugs and
keys are clearly shown. The portion of the switchboard on which the
plugs are mounted is always immovable, the plugs being provided with
seats through which holes are bored of sufficient size to permit the
switchboard cord to pass beneath the shelf. When one of these plugs is
raised, the cord is pulled up through this hole thus allowing the plug
to be placed in any of the jacks.

The key arrangement shown in this particular cut is instructive. It
will be noticed that the right-hand five pairs of plugs are provided
with ordinary ringing and listening keys, while the left-hand five are
provided with party-line ringing keys and listening keys. The
listening key in each case is the one in the rear and is alike for all
of the cord pairs. The right-hand five ringing keys are so arranged
that pressing the lever to the rear will ring on the answering cord,
while pressing it toward the front will cause ringing current to flow
on the calling plug. In the left-hand five pairs of cords shown in
this cut, the pressure of any one of the keys causes a ringing current
of a certain frequency to flow on the calling cord, this frequency
depending upon which one of the keys is pressed.

[Illustration: Fig. 298. Cord Weight]

An excellent idea of the grouping of the various pieces of apparatus
in a complete simple magneto switchboard may be had from Fig. 297.
While the arrangement here shown is applicable particularly to the
apparatus of the Dean Electric Company, the structure indicated is
none-the-less generally instructive, since it represents good practice
in this respect. In this drawing the stationary plug shelf with the
plug seat is clearly shown and also the hinged key shelf. The hinge of
the key shelf is an important feature and is universally found in all
switchboards of this general type. The key shelf may be raised and
thus expose all of the wiring leading to the keys, as well as the
various contacts of the keys themselves, to inspection.

[Illustration: Fig. 299. Magneto Switchboard, Target Signals]

As will be seen, the switchboard cords leading from the plugs extend
down to a point near the bottom of the cabinet where they pass through
pulley weights and then up to a stationary cord rack. On this cord
rack are provided terminals for the various conductors in the cord,
and it is at this point that the cord conductors join the other wires
leading to the other portions of the apparatus as required. A good
form of cord weight is shown in Fig. 298; and obviously the function
of these weights is to keep the cords taut at all times and to prevent
their tangling.

[Illustration: Fig. 300. Rear View of Target Signal, Magneto
Switchboard]

The drawing, Fig. 297, also gives a good idea of the method of
mounting the hand generator that is ordinarily employed with such
magneto switchboards. The shaft of the generator is merely continued
out to the front of the key shelf where the usual crank is provided,
by means of which the operator is able to generate the necessary
ringing current. Beside the hand generator at each operator's
position, it is quite common in magneto boards, of other than the
smallest sizes, to employ some form of ringing generator, either a
power-driven generator or a pole changer driven by battery current for
furnishing ringing current without effort on the part of the operator.

[Illustration: Fig. 301. Dean Two-Position Switchboard]

Switchboards as shown in Figs. 294 and 295, are called single-position
switchboards because they afford room for a single operator.
Ordinarily for this class of work a single operator may handle from
one to two hundred lines, although of course this depends on the
amount of traffic on the line, and this, in turn, depends on the
character of the subscribers served, and also on the average number of
stations on a line. Another single-position switchboard is shown in
Figs. 299 and 300, being a front and rear view of the simple magneto
switchboard of the Western Electric Company, which is provided with
the target signals of that company rather than the usual form of drop.

Where a switchboard must accommodate more lines than can be handled by
a single operator, the cabinet is made wider so as to afford room for
more than one operator to be seated before it. Sometimes this is
accomplished by building the cabinet wider, or by putting two such
switchboard sections as are shown in Figs. 294 or 299 side by side. A
two-position switchboard section is shown in front and rear views in
Figs. 301 and 302.

[Illustration: Fig. 302. Rear View of Dean Two-Position Switchboard]

_Sectional Switchboards._ The problem of providing for growth in a
switchboard is very much the same as that which confronts one in
buying a bookcase for his library. The Western Electric Company has
met this problem, for very small rural exchanges, in much the same way
that the sectional bookcase manufacturers have provided for the
possible increase in bookcase capacity. Like the sectional bookcase,
this sectional switchboard may start with the smallest of equipment--a
single sectional unit--and may be added to vertically as the
requirements increase, the original equipment being usable in its more
extended surroundings.

[Illustration: Fig. 303. Sectional Switchboard--Wall Type]

This line of switchboards is illustrated in Figs. 303 to 306. The
beginning may be made with either a wall type or an upright type of
switchboard, the former being mounted on brackets secured to the wall,
and the latter on a table. A good idea of the wall type is shown in
Fig. 303. Three different kinds of sectional units are involved in
this: first, the unit which includes the cords, plugs, clearing-out
drops, listening jacks, operator's telephone set and generator;
second, the unit containing the line equipment, including a strip of
ten magneto line signals and their corresponding jacks; third, the
finishing top, which includes no equipment except the support for the
operator's talking apparatus.

[Illustration: Fig. 301. Sectional Switchboard--Wall Type]

The first of the units in Fig. 303 forms the foundation on which the
others are built. Two of the line-equipment units are shown; these
provide for a total of twenty lines. The top rests on the upper
line-equipment unit, and when it becomes necessary to add one or more
line-equipment units as the switchboard grows, this top is merely
taken off, the other line-equipment units put in place on top of those
already existing, and the top replaced. The wall type of sectional
switchboard is so arranged that the entire structure may be swung out
from the wall, as indicated in Fig. 304, exposing all of the apparatus
and wiring for inspection. Each of the sectional units is provided
with a separate door, as indicated, so that the rear door equipment is
added to automatically as the sections are added. In the embodiment of
the sectional switchboard idea shown in these two figures just
referred to, no ringing and listening keys are provided, but the
operator's telephone and generator terminate in a special plug--the
left-hand one shown in Fig. 303--and when the operator desires to
converse with the connected subscribers, she does so by inserting the
operator's plug into one of the jacks immediately below the
clearing-out drop corresponding to the pair of plugs used in making
the connection. The arrangement in this case is exactly the same in
principle as that described in Fig. 292. The operator's generator is
so arranged in connection with this left-hand operator's plug that the
turning of the generator crank automatically switches the operator's
telephone set off and switches the generator on, just the same as a
switch hook may do in a subscriber's series telephone.

[Illustration: Fig. 305. Sectional Switchboard--Table Type]

[Illustration: Fig. 306. Sectional Switchboard--Table Type]

The upright type of sectional switchboard is shown in Figs. 305 and
306, which need no explanation in view of the foregoing, except to say
that, in the particular instrument illustrated, ringing and listening
keys are provided instead of the jack-and-plug arrangement of the wall
type. In this case also, the top section carries an arm for supporting
a swinging transmitter instead of the hook support for the combined
transmitter and receiver.




REVIEW QUESTIONS

[Blank Page]

REVIEW QUESTIONS

ON THE SUBJECT OF TELEPHONY

PAGES 11--62

       *       *       *       *       *

1. When was the telephone invented and by whom?

2. State the velocity of sound in air. Is it higher in air than in a
denser medium?

3. State and define the characteristics of sound.

4. Make sketch of Bell's original magneto telephone without permanent
magnets.

5. Describe and sketch Hughes' microphone.

6. Which is, at present, the best material for varying the resistance
in transmitters?

7. Give the fundamental differences between the magneto transmitter
and the carbon transmitter.

8. What is the function of the induction coil in the telephone
circuit?

9. Describe and sketch the different kinds of visible signals.

10. What should be the diameter of hard drawn copper wire in order to
allow economical spacing of poles?

11. State the four principal properties of a telephone line.

12. If in testing a line the capacity is changed what are the results
found on the receiver and transmitter end?

13. Why is paper used as an insulator of telephone cables?

14. How does a conductor behave in connection with direct current and
how with alternating current?

15. What influence has inductance on the telephone?

16. Define impedance and give the formula for it.

17. What is the usual specification for insulation of resistance in
telephone cables?

18. If 750 feet of cable have an insulation resistance of 9,135
megohms, how great is the insulation resistance for 7 miles and 1,744
feet of cable?

19. What is the practical limiting conversation distance for No. 10 B.
and S. wire?

20. Describe Professor Pupin's method of inserting inductance into the
telephone line.

21. What does _mho_ denote?

22. Why are Pupin's coils not so successful on open wires?

23. What is a repeater?

24. Define _reactive interference_.

25. State the frequencies of the pitches of the human voice.

26. What is the office of a diaphragm in a telephone apparatus?

27. What transmitter material has greatly increased the ranges of
speech?

28. Describe the different methods of measurements of telephone
circuits.

29. What are the two kinds of _electric calls_?

30. How many conductors has a telephone line?

31. Give formula for capacity reactance and the meaning of the
symbols.

32. Which American cities are joined by underground lines at present?

33. State the two practical ways of improving telephone transmission.




REVIEW QUESTIONS

ON THE SUBJECT OF TELEPHONY

PAGES 63--141

       *       *       *       *       *

1. On what general principle are most of the telephone transmitters of
today constructed?

2. Make sketch of the new Western Electric transmitter and describe
its working.

3. Make sketch and describe the Kellogg transmitter.

4. What troubles were encountered in the earlier forms of granular
carbon transmitters and how were they overcome?

5. What limits the current-carrying capacity of the transmitter? How
may this capacity be increased?

6. State in what kind of transmitters a maximum degree of
sensitiveness is desirable.

7. Show the conventional symbols for transmitters.

8. Describe a telephone receiver.

9. Sketch a Western Electric receiver and point out its deficiencies.

10. Make a diagram of the Kellogg receiver.

11. Describe the direct-current receiver of the Automatic Electric
Company.

12. Describe and sketch the Dean receiver.

13. Show the conventional symbols of a receiver.

14. Describe exactly how, in a cell composed of a tin and a silver
plate with dilute sulphuric acid as electrolyte, the current inside
and outside of the cell will flow.

15. Describe the phenomenon of polarization.

16. What is _local action_ of a cell? How may it be prevented?

17. Into how many classes may cells be divided? Which class is most
used in telephony?

18. Describe the LeClanché cell.

19. Sketch and describe an excellent form of dry cell.

20. Show the conventional symbols for batteries.

21. Sketch and describe the generator shunt switch and the generator
cut-in switch.

22. How may a pulsating current be derived from a magneto generator?

23. Show conventional symbols for magneto generators.

24. Sketch and describe the Western Electric polarized bell.

25. Give conventional ringer symbols.

26. What is the purpose of the hook switch?

27. Make sketch and give description of Kellogg's long lever hook
switch.

28. Describe and sketch the Western Electric short lever hook switch.

29. Point out the principal difference between the desk stand hook
switches of the Western Electric Company and of the Kellogg
Switchboard and Supply Company.

30. Give conventional symbols of hook switches.




REVIEW QUESTIONS

ON THE SUBJECT OF TELEPHONY

PAGES 143--225

       *       *       *       *       *

1. Describe an electromagnet and its function in telephony.

2. Sketch an iron-clad electromagnet.

3. What is a differential electromagnet? Sketch and describe one type.

4. State the desirable characteristics of good enamel insulation for
magnet wire.

5. If you have a coil of No. 23 double cotton B. and S. wire of 115
ohms resistance and you have to rewind it for 1,070 ohms resistance
with double cotton wire, what number of wire would you take? Show
calculation.

     NOTE. No. 23 d. c wire has res. 1.772 ohms per cubic inch; for
     the core, 115 ohms. There are required in the coil 1,070 ohms,
     that is, 9.3 times as much. 1.772 x 9.3 = 16.47 ohms, which must
     be the resistance per cu. in. This resistance gives, according to
     Table IV, No. 29 wire.

6. What is an impedance coil? State how it differs from an
electromagnet coil.

7. Describe the different kinds of impedance coils.

8. Give symbol of impedance coil.

9. What are the principal parts of an induction coil?

10. What is the function of an induction coil in telephony?

11. What is a repeating coil and how does it differ from an induction
coil?

12. Give conventional symbols of induction coils and repeating coils.

13. Enumerate the different types of non-inductive resistance devices
and give a short description of each.

14. Define condenser.

15. What is the meaning of the word _dielectrics_?

16. State what you understand by the specific inductive capacity of a
dielectric.

17. Upon what factors does the capacity of a condenser depend?

18. What is the usual capacity of condensers in telephone practice?

19. Give conventional condenser symbols.

20. By what two methods may the current be supplied to a telephone
transmitter?

21. Make sketch of local-battery stations with metallic circuit.

22. Sketch common-battery circuit in series with two lines.

23. State the objections against the preceding arrangement.

24. Make sketch of the standard arrangement of the Western Electric
Company in bridging the common battery with repeating coils.

25. Sketch the arrangement of bridging the battery with impedance
coils and state the purpose of the coils.

26. Make diagram of a common-source current supply for many lines with
repeating coils and point out the travel of the voice currents.

27. Name the different parts which comprise a telephone set.

28. What is a magneto telephone?

29. Make diagram of the circuit of a series magneto set with receiver
on the hook and explain how the different currents are flowing.

30. Show diagram of the Stromberg-Carlson magneto desk telephone
circuit and describe its working.

31. Give sketch of the Stromberg-Carlson common-battery wall set
circuit.

32. Describe briefly the microtelephone set.

33. Make sketch of the Monarch common-battery wall set.




REVIEW QUESTIONS

ON THE SUBJECT OF TELEPHONY

PAGES 227--286

       *       *       *       *       *

1. What is a party line?

2. What is usually understood by private lines?

3. What problem is there to overcome in connection with party lines?

4. State the two general classes of party-line systems.

5. Point out the defects of the series system.

6. Make sketch of a metallic bridging line and show the circuit for
the voice currents.

7. What is a signal code?

8. Give classification of selective party-line systems with short
definitions.

9. Describe the principle of selection by polarity and make sketch
illustrating this principle.

10. Make diagram of the circuit of a four-party station with relay.

11. Describe the process of tuning in the harmonic system.

12. What is the difference between the under-tune and in-tune systems?

13. Sketch circuit of Kellogg's harmonic system.

14. Illustrate the principle of a broken-line system by a sketch.

15. In what particulars does the party-line system in rural districts
differ from that within urban limits?

16. Describe and sketch Pool's lock-out system.

17. Make diagram of the K.B. lock-out system.

18. What is the object of the ratchet in this system?

19. Make diagram of simplified circuits of Roberts system.

20. Sketch and describe Roberts latching key and connections.

21. Sketch circuits of bridging station for non-selective party line.

22. How would you arrange the signal code for six stations on a
non-selective party line?

23. What is the limit of number of stations on a non-selective party
line under ordinary circumstances?

24. State the objections against the party polarity system as shown in
Fig. 172.

25. What are the advantages of the harmonic party-line system?

26. To how many frequencies is the harmonic system usually limited?

27. What can you say about the commercial success of the step-by-step
method?

28. State the principles of a lock-out party line.

29. For what purpose is a condenser placed in the receiver circuit of
each station in the K.B. lock-out system?

30. How are the selecting relays in Roberts line restored to their
normal position after a conversation is finished?

31. What are the objections against the Roberts system?




REVIEW QUESTIONS

ON THE SUBJECT OF TELEPHONY

PAGES 287--315

       *       *       *       *       *

1. What are electrical hazards?

2. When is the lightning hazard least?

3. What actions can electricity produce? Which involves the greater
hazard to the value of property?

4. When is a piece of apparatus called "self-protecting"?

5. Why must a protector for telephone apparatus work more quickly for
a large current than for a small one?

6. State the general problem which heating hazards present with
relation to telephone apparatus.

7. What is the most nearly universal electrical hazard?

8. Sketch and describe the saw-tooth lightning arrester.

9. Make diagram of the carbon-block arrester and state its advantages.

10. Describe a vacuum arrester.

11. Explain the reason for placing an impedance in connection with the
lightning arrester.

12. What is the purpose of the globule of low-melting alloy in the
Western Electric Company's arrester?

13. Why are not fuses good lightning arresters?

14. What is the proper function of a fuse?

15. Make sketch of a mica slip fuse.

16. Define _sneak currents_.

17. Make a diagram of a sneak-current arrester and describe its
principles and working.

18. Describe a heat coil.

19. Sketch a complete line protection.

20. Where is the proper position of the fuse?

21. Which wires are considered exposed and which unexposed?

22. Why is it not necessary to install sneak-current arresters in
central-battery subscribers' stations?

23. Sketch and describe the action of a combined sneak-current and
air-gap arrester, as widely used by Bell companies.

24. Describe the self-soldering heat-coil arrester.

25. What is the purpose of ribbon fuses?

26. What is a drainage coil?




REVIEW QUESTIONS

ON THE SUBJECT OF TELEPHONY

PAGES 317--386

       *       *       *       *       *

1. What is a central office?

2. What are (_a_) subscriber's lines? (_b_) Trunk lines? (_c_) Toll
lines?

3. For what purpose is the switchboard?

4. Give short descriptions of the different classes of switchboards.

5. How are manual switchboards subdivided? Describe briefly the
different types.

6. Define A and B boards.

7. What is a call circuit?

8. What kind of calls are handled on a toll switchboard?

9. Give drop symbol and describe its principles.

10. What is a jack?

11. Make a sketch of a plug inserted into a jack.

12. Give jack and plug symbols.

13. What are ringing and listening keys?

14. Show symbols for ringing and listening keys.

15. State the parts of which a cord equipment consists.

16. Show step by step the various operations of a telephone system
wherein the lines center in a magneto switchboard. Make all the
necessary diagrams and give brief descriptions to show that you
understand each operation.

17. On what principle does a drop with night-alarm contact operate?

18. What is the advantage of associating jacks and drops?

19. Describe the mechanical restoration as employed in the Miller
drop and jack.

20. Describe the electrical restoration of drop shutters as
manufactured by the Western Electric Company.

21. What complications arise in ringing of party lines and how are
they overcome?

22. Give diagram of the complete circuit of a simple magneto
switchboard.

23. Sketch night-alarm circuit with relay.

24. What is a convertible cord circuit?

25. State what disadvantages may be encountered under certain
conditions with a bridging drop-cord circuit.

26. Are lamps in cord circuits to be advocated on magneto
switchboards?

27. What is the function of the cabinet?

28. Give cross-section of upright switchboard as used in the magneto
system.

29. What is the purpose of a sectional switchboard?

30. Give a short description of the essential parts of a sectional
switchboard.




INDEX




INDEX

_The page numbers of this volume will be found at the bottom of the
pages; the numbers at the top refer only to the section._


A

Acousticon transmitter
Acoustics
    characteristics of sound
        loudness
        pitch
        timbre
    human ear
    human voice
    propagation of sound
Air-gap vs. fuse arresters
Amalgamated zincs
Arrester separators
Audible signals
    magneto bell
    telegraph sounder
    telephone receiver
    vibrating bell
Automatic Electric Company
    direct-current receiver
    transmitter
Automatic shunt


B

Bar electromagnet
Battery bell
Battery symbols
Blake single electrode
Brazed bell
Broken-back ringer
Broken-line method of selective signaling


C

Capacity reactance
Carbon
    adaptability
    limitations
    preparation of
    superiority
Carbon air-gap arrester
Carbon-block arrester
Carrying capacity of transmitter
Central-office protectors
Characteristics of sound
    loudness
    pitch
    timbre
Chloride of silver cell
Closed-circuit cells
Closed-circuit impedance coil
Common-battery telephone sets
Condensers
    capacity
    charge
    conventional symbols
    definition of
    dielectric
    dielectric materials
    functions
    means for assorting current
    sizes
    theory
Conductivity of conductors
Conductors, conductivity of
Conventional symbols
Cook
    air-gap arrester
    arrester
    arrester for magneto stations
Crowfoot cell
Current supply to transmitters
    common battery
        advantages
        bell substation arrangement
        bridging battery with impedance coils
        bridging battery with repeating coil
        current supply from distant point
        current supply over limbs of line in parallel
        Dean substation arrangement
        double battery with impedance coil
        Kellogg substation arrangement
        North Electric Company system
        series battery
        series substation arrangement
        Stromberg-Carlson system
        supply many lines from common source
            repeating coil
            retardation coil
  local battery


D

Dean
  drop and jack
  receiver
  wall telephone hook
Desk stand hooks
  Kellogg
  Western Electric
Dielectric
Dielectric materials
  dry paper
  mica
Differential electromagnet
Direct-current receiver
Drainage coils


E

Electric lamp signal
Electrical hazards
Electrical reproduction of speech
  carbon
  conversion from sound waves to vibration of diaphragm
  conversion from vibration to voice currents
  conversion from voice currents to vibration
  cycle of conversion
  detrimental effects of capacity
  early conceptions
  electrostatic telephone
  induction coil
  limitations of magneto transmitter
  loose contact principle
  magneto telephone
  measurements of telephone currents
  variation of electrical pressure
  variation of resistance
Electrical signals
  audible
    magneto-bell
    telegraph sounder
    telephone receiver
    vibrating bell
  visible
    electric lamp signal
    electromagnetic signal
Electrodes
  arrangement of
  carbon preparation
  multiple
  single
Electrolysis
Electromagnetic method of measuring telephone currents
Electromagnetic signal
Electromagnets and inductive coils
  conventional symbols
  differential electromagnet
  direction of armature motion
  direction of lines of force
  electromagnets
  low-resistance circuits
    horseshoe form
    iron-clad form
    special horseshoe form
  impedance coils
    kind of iron
    number of turns
    types
      closed-circuit
      open-circuit
      toroidal
  induction coil
    current and voltage ratios
    design
    functions
    use and advantage
  magnet wire
    enamel
    silk and cotton insulation
    space utilization
    wire gauges
  magnetic flux
  magnetization curves
  magnetizing force
  mechanical details
  permeability
  reluctance
  repeating coil
  winding methods
    winding calculations
    winding data
    winding terminals
Electrostatic capacity
  unit of
Electrostatic telephone
Enamel


F

Five-bar generator
Fuller cell


G

Galvani
Generator armature
Generator cut-in switch
Generator shunt switch
Generator symbols
Granular carbon
Gravity cell


H

Hand receivers
Harmonic method of selective signaling
  advantages
  circuits
  in-tune system
  limitations
  principles
  tuning
  under-tune system
Head receivers
Heat coil
Holtzer-Cabot arrester
Hook switch
  automatic operation
  contact material
  design
  desk stand hooks
    Kellogg
    Western Electric
  purpose
  symbols
  wall telephone hooks
    Dean
    Kellogg
    Western Electric
Horseshoe electromagnet
Human ear
Human voice


I

Impedance coils
  kind of iron
  number of turns
  symbols of
  types
    closed-circuit
    open-circuit
    toroidal
Inductance vs. capacity
Induction coil
  current and voltage ratios
  design
  functions
  use and advantage
Inductive neutrality
Inductive reactance
Insulation of conductors
Introduction to telephony
Iron-clad electromagnet
Iron wire ballast


K

Kellogg
  air-gap arrester
  desk stand hook
  drop and jack
  receiver
  ringer
  transmitter
  wall telephone hook

L

Lalande cell
Lamp filament
Le Clanché cell
Lenz law
Line signals
Lines of force, direction of
Loading coils
Lock-out party-line systems
  broken-line method
  operation
  Poole system
  step-by-step system
Loudness of sound
Low-reluctance circuits
  horseshoe form
  iron-clad form


M

Magnetic flux
Magnetization curves
Magnetizing force
Magneto bell
Magneto operator
Magneto signaling apparatus
  armature
  automatic shunt
  battery bell
  generator symbols
  magneto bell
  magneto generator
  method of signaling
  polarized ringer
  pulsating current
  ringer symbols
  theory
Magneto switchboard
  automatic restoration
    mechanical
      Dean type
      Kellogg type
      Monarch type
      Western Electric type
  circuits of complete switchboard
  code signaling
  commercial types of drops and jacks
    early drops
    jack mounting
    manual vs. automatic restoration
    methods of associating
    night alarm
    tubular drops
  component parts
    jacks and plugs
    keys
    line and cord equipments
    line signal
    operators' equipment
  cord-circuit considerations
    double clearing-out type
    lamp-signal type
    non-ring through type
    series drop type
    simple bridging drop type
  definitions
  electrical restoration
  grounded and metallic-circuit lines
  mode of operation
  night-alarm circuits
  operation in detail
    clearing out
    essentials of operation
    normal condition of line
    operator answering
    operator calling
    subscriber calling
    subscribers conversing
    operator's telephone equipment
    cut-in jack
  ringing and listening keys
    horizontal spring type
    party-line ringing keys
    self-indicating keys
    vertical spring type
  switchboard assembly
    functions of cabinet
    sectional switchboards
    upright type of switchboard
    wall type switchboard
  switchboard cords
    concentric conductors
    parallel tinsel conductors
    steel spiral conductors
  switchboard plugs
Magneto telephone
Magneto telephone sets
Mica card resistance
Mica slip fuse
Microtelephone set
Monarch drop and jack
Monarch receiver
Monarch transmitter
Multiple electrode
Mutual induction


N

Non-inductive resistance devices
  inductive neutrality
  provisions against heating
  temperature coefficient
  types
    differentially-wound unit
    iron wire ballast
    lamp filament
    mica card unit
Non-selective party-line systems
  bridging
  limitations
  series
  signal code


O

Open-circuit cells
Open-circuit impedance coil
Operator's receiver


P

Packing of transmitters
Permeability
Pitch
  Doppler's principle
  vibration of diaphragms
Polarity method of selective signaling
Polarization of cells
Polarized ringer
  brazed bell
  Kellogg
  Western Electric
Poole lock-out system
Primary cells
  conventional symbol
  series and multiple connections
  simple voltaic
  types of
    closed-circuit
      Fuller
      gravity
      Lalande
      prevention of creeping
      setting up
    open-circuit
      Le Clanché
    standard
      chloride of silver
Propagation of sound
Protective means
  against high potentials
    air-gap arrester
      advantages of carbon
      commercial types
      continuous arcs
      discharge across gaps
      dust between carbons
      introduction of impedance
      metallic electrodes
      vacuum arresters
    against sneak currents
      heat coil
      sneak-current arresters
    against strong currents
      fuses
        enclosed
        mica
        proper functions
    central-office protectors
        self-soldering heat coils
        sneak-current and air-gap arrester
    city exchange requirements
    complete line protection
    electrolysis
    subscribers' station protectors
      ribbon fuses
Pulsating-current commutator


R

Receivers
  Dean
  direct-current
  early
  Kellogg
  modern
  Monarch
  operator's
  single-pole
  symbols
  Western Electric
Reluctance
Repeating coil
Ribbon fuses
Ringer symbols
Ringing and listening key
Robert's latching relay
Robert's self-cleansing arrester
Rolled condenser


S

Saw-tooth arrester
Selective party-line systems
  broken-line method
  classification
    broken-line systems
    harmonic systems
    polarity systems
    step-by-step systems
  harmonic method
  polarity method
  step-by-step method
Self-induction
Signal code
Signaling, method of
Silk and cotton insulation
Single electrode
Single-pole receiver
Sneak-current arresters
Solid-back transmitter
Sound
  characteristics of
    loudness
    pitch
    timbre
Standard cell
Step-by-step lock-out system
Step-by-step method of selective signaling
Subscribers' station protectors
Switchboard cords
Switchboard plugs
Switchboard transmitter
Symbols
  battery
  condenser
  generator
  hook switch
  impedance coil
  induction coil
  receiver
  repeating coil
  ringer
  ringing and listening key
  transmitter


T

Table
  condenser data
  copper wire
  German silver wire--18 per cent
  German silver wire--30 per cent
  metals, behavior of, in different electrolysis
  signal code
  specific inductive capacities
  temperature coefficients
  transmission distances, limiting
  winding data for insulating wires
Tandem differential electromagnet
Telegraph sounder
Telephone currents, measurements of
  electromagnetic method
  thermal method
Telephone exchange, features of
  districts
  subscribers' lines
  switchboards
  toll lines
  trunk lines
Telephone lines
  conductivity of conductors
  electrostatic capacity
  inductance of circuit
  inductance vs. capacity
  insulation of conductors
  transmission
Telephone sets
  classification of
    common-battery telephone
    magneto telephone
    wall and desk telephones
  common-battery
    desk
    hotel
    wall
  magneto
    circuits of
      bridging
      series
    desk
    wall
Temperature coefficients
Thermal method of measuring telephone currents
Timbre
Toroidal impedance coil
Toroidal repeating coil
Transmission, ways of improving
Transmitters
  acousticon
  Automatic Electric Company
  carrying capacity
  conventional diagram
  electrode
    arrangement of
    multiple
    single
  granular carbon
  Kellogg
  materials
  Monarch
  packing
  sensitiveness
  switchboard
  symbols
  variable resistance
  Western Electric solid-back


U

Under-tuned ringer


V

Vacuum arrester
Variable resistance
Vibrating bell
Visible signals
  electric lamp
  electromagnetic
Volta
Voltaic cell
  amalgamated zincs
  difference of potential
  local action
  polarization
  theory


W

Wall telephone hooks
  Dean
  Kellogg
  Western Electric
Western Electric
  air-gap arrester
  desk stand hook
  drop and jack
  receiver
  ringer
  solid-back transmitter
  station arrester
  wall telephone hook
White transmitter
Wire gauges