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[Transcriber's Note: References to page numbers in table of contents
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[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.