The Project Gutenberg EBook of Cyclopedia of Telephony & Telegraphy Vol. 1
by Kempster Miller, George Patterson, Charles Thom, Robert Millikan,
Samuel McMeen
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Title: Cyclopedia of Telephony & Telegraphy Vol. 1
A General Reference Work on Telephony, etc. etc.
Author: Kempster Miller
George Patterson
Charles Thom
Robert Millikan
Samuel McMeen
Release Date: April 14, 2005 [EBook #15617]
Language: English
Character set encoding: ISO-8859-1
*** START OF THIS PROJECT GUTENBERG EBOOK CYCLOPEDIA OF TELEPHONY 1 ***
Produced by Ronald Holder and the Online Distributed
Proofreading Team at http://www.pgdp.net.
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
F O U R V O L U M E S
CHICAGO
AMERICAN SCHOOL OF CORRESPONDENCE
1919
ToC
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
The editors have freely consulted the standard technical literature of America and Europe in the preparation of these volumes. They desire to express their indebtedness particularly to the following eminent authorities, whose well-known works should be in the library of every telephone and telegraph engineer.
Grateful acknowledgment is here made also for the invaluable co-operation of the foremost engineering firms and manufacturers in making these volumes thoroughly representative of the very best and latest practice in the transmission of intelligence, also for the valuable drawings, data, suggestions, criticisms, and other courtesies.
ARTHUR E. KENNELY, D.Sc.
Professor of Electrical Engineering, Harvard University.
Joint Author of "The Electric Telephone."
"The Electric Telegraph," "Alternating
Currents," "Arc Lighting," "Electric Heating,"
"Electric Motors," "Electric Railways,"
"Incandescent Lighting," etc.
HENRY SMITH CARHART, A.M., LL.D.
Professor of Physics and Director of the Physical Laboratory, University of Michigan.
Author of "Primary Batteries," "Elements of Physics,"
"University Physics," "Electrical
Measurements," "High School Physics," etc.
FRANCIS B. CROCKER, M.E., Ph.D.
Head of Department of Electrical Engineering, Columbia University, New York; Past-President,
American Institute of Electrical Engineers.
Author of "Electric Lighting;" Joint Author of "Management of Electrical Machinery."
HORATIO A. FOSTER
Consulting Engineer; Member of American Institute of Electrical Engineers; Member
of American Society of Mechanical Engineers.
Author of "Electrical Engineer's Pocket-Book."
WILLIAM S. FRANKLIN, M.S., D.Sc.
Professor of Physics, Lehigh University.
Joint Author of "The Elements of Electrical Engineering," "The Elements of Alternating
Currents."
LAMAR LYNDON, B.E., M.E.
Consulting Electrical Engineer; Associate Member of American Institute of Electrical
Engineers; Member, American Electro-Chemical Society.
Author of "Storage Battery Engineering."
ROBERT ANDREWS MILLIKAN, Ph.D.
Professor of Physics, University of Chicago.
Joint Author of "A First Course in Physics," "Electricity, Sound and Light," etc.
KEMPSTER B. MILLER, M.E.
Consulting Engineer and Telephone Expert; of the Firm of McMeen and Miller,
Electrical Engineers and Patent Experts, Chicago.
Author of "American Telephone Practice."
WILLIAM H. PREECE
Chief of the British Postal Telegraph.
Joint Author of "Telegraphy," "A Manual of Telephony," etc.—
LOUIS BELL, Ph.D.
Consulting Electrical Engineer; Lecturer on Power Transmission, Massachusetts Institute of Technology.
Author of "Electric Power Transmission," "Power Distribution for Electric Railways,"
"The Art of Illumination," "Wireless Telephony," etc.
OLIVER HEAVISIDE, F.R.S.
Author of "Electro-Magnetic Theory," "Electrical Papers," etc.
SILVANUS P. THOMPSON, D.Sc, B.A., F.R.S., F.R.A.S.
Principal and Professor of Physics in the City and Guilds of London Technical College.
Author of "Electricity and Magnetism," "Dynamo-Electric Machinery,"
"Polyphase Electric Currents and Alternate-Current Motors," "The Electromagnet," etc.
ANDREW GRAY, M.A., F.R.S.E.
Author of "Absolute Measurements in Electricity and Magnetism."
ALBERT CUSHING CREHORE, A.B., Ph.D.
Electrical Engineer; Assistant Professor of Physics, Dartmouth College; Formerly instructor
in Physics, Cornell University.
Author of "Synchronous and Other Multiple Telegraphs;" Joint Author of "Alternating
Currents."
J. J. THOMSON, D.Sc, LL.D., Ph.D., F.R.S.
Fellow of Trinity College, Cambridge University; Cavendish Professor of Experimental
Physics, Cambridge University.
Author of "The Conduction of Electricity through Gases," "Electricity and Matter."
FREDERICK BEDELL, Ph. D.
Professor of Applied Electricity, Cornell University.
Author of "The Principles of the Transformer;" Joint Author of "Alternating Currents."
DUGALD C. JACKSON, C.E.
Head of Department of Electrical Engineering, Massachusetts Institute of Technology;
Member, American Institute of Electrical Engineers, etc.
Author of "A Textbook on Electromagnetism and the Construction of Dynamos;"
Joint Author of "Alternating Currents and Alternating-Current Machinery."
MICHAEL IDVORSKY PUPIN, A.B., Sc.D., Ph.D.
Professor of Electro-Mechanics, Columbia University, New York.
Author of "Propagation of Long Electric Waves," and "Wave-Transmission over
Non-Uniform Cables and Long-Distance Air Lines."
FRANK BALDWIN JEWETT, A.B., Ph.D.
Transmission and Protection Engineer, with American Telephone & Telegraph Co.
Author of "Modern Telephone Cable," "Effect of Pressure on Insulation Resistance."
ARTHUR CROTCH
Formerly Lecturer on Telegraphy and Telephony at the Municipal Technical Schools,
Norwich, Eng.
Author of "Telegraphy and Telephony."
JAMES ERSKINE-MURRAY, D.Sc.
Fellow of the Royal Society of Edinburgh; Member of the Institution of Electrical
Engineers.
Author of "A Handbook of Wireless Telegraphy."
A.H. MCMILLAN, A.B., LL.B.
Author of "Telephone Law, A Manual on the Organization and Operation of Telephone Companies."
WILLIAM ESTY, S.B., M.A.
Head of Department of Electrical Engineering, Lehigh University.
Joint Author of "The Elements of Electrical Engineering."
GEORGE W. WILDER, Ph.D.
Formerly Professor of Telephone Engineering, Armour Institute of Technology.
Author of "Telephone Principles and Practice," "Simultaneous Telegraphy and Telephony,"
etc.
WILLIAM L. HOOPER, Ph.D.
Head of Department of Electrical Engineering, Tufts College.
Joint Author of "Electrical Problems for Engineering Students."
DAVID S. HULFISH
Technical Editor, The Nickelodeon; Telephone and Motion-Picture Expert; Solicitor of
Patents.
Author of "How to Read Telephone Circuit Diagrams."
J.A. FLEMING, M.A., D.Sc. (Lond.), F.R.S.
Professor of Electrical Engineering in University College, London;
Late Fellow and Scholar of St. John's College, Cambridge; Fellow of University College, London.
Author of "The Alternate-Current Transformer," "Radiotelegraphy and Radiotelephony,"
"Principles of Electric Wave Telegraphy," "Cantor Lectures on Electrical
Oscillations and Electric Waves," "Hertzian Wave Wireless Telegraphy," etc.
F.A.C. PERRINE, A.M., D.Sc.
Consulting Engineer: Formerly President, Stanley Electric Manufacturing Company;
Formerly Professor of Electrical Engineering, Leland Stanford, Jr. University.
Author of "Conductors for Electrical Distribution."
A. FREDERICK COLLINS
Editor, Collins Wireless Bulletin.
Author of "Wireless Telegraphy, Its History, Theory and Practice," "Manual of Wireless
Telegraphy," "Design and Construction of Induction Coils," etc.
SCHUYLER S. WHEELER, D.Sc.
President, Crocker-Wheeler Co.; Past-President, American Institute of Electrical Engineers.
Joint Author of "Management of Electrical Machinery."
CHARLES PROTEUS STEINMETZ
Consulting Engineer, with the General Electric Co.; Professor of Electrical Engineering, Union College.
Author of "The Theory and Calculation of Alternating-Current Phenomena," "Theoretical
Elements of Electrical Engineering", etc.
GEORGE W. PATTERSON, S.B., Ph.D.
Head of Department of Electrical Engineering, University of Michigan.
Joint Author of "Electrical Measurements."
WILLIAM MAVER, JR.
Ex-Electrician Baltimore and Ohio Telegraph Company; Member of the American Institute of Electrical Engineers.
Author of "American Telegraphy and Encyclopedia of the Telegraph," "Wireless Telegraphy."
JOHN PRICE JACKSON, M.E.
Professor of Electrical Engineering, Pennsylvania State College.
Joint Author of "Alternating Currents and Alternating-Current Machinery."
AUGUSTUS TREADWELL, JR., E.E.
Associate Member, American Institute of Electrical Engineers.
Author of "The Storage Battery, A Practical Treatise on Secondary Batteries."
EDWIN J. HOUSTON, Ph.D.
Professor of Physics, Franklin Institute, Pennsylvania; Joint Inventor of Thomson-Houston
System of Arc Lighting; Electrical Expert and Consulting Engineer.
Joint Author of "The Electric Telephone," "The Electric Telegraph," "Alternating
Currents," "Arc Lighting," "Electric Heating,"
"Electric Motors," "Electric Railways,"
"Incandescent Lighting," etc.
WILLIAM J. HOPKINS
Professor of Physics in the Drexel Institute of Art, Science, and Industry, Philadelphia.
Author of "Telephone Lines and their Properties."
ToC
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
Page 9
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.
Fundamental Principles By K. B. Miller and S. G. McMeen [A] Page 11
CHAPTER I—Acoustics—Characteristics of Sound—Loudness—Pitch—Vibration of Diaphragms—Timbre—Human Voice—Human Ear
CHAPTER II—Speech—Magneto Telephones—Loose-Contact Principle—Induction Coils
CHAPTER III—Simple Telephone Circuit—Capacity—Telephone Currents—Audible and Visible Signals
CHAPTER IV—Telephone
Lines—Conductors—Inductance—Insulation
Substation Equipment By K. B. Miller and S. G. McMeen Page 63
CHAPTER V—Transmitters—Variable Resistance—Materials—Single and Multiple Electrodes—Solid-Back Transmitter—Types of Transmitters—Electrodes—Packing—Acousticon Transmitter—Switchboard Transmitter
CHAPTER VI—Receivers—Types of Receivers—Operator's Receiver
CHAPTER VII—Primary Cells—Series and Multiple Connections—Types of Primary Cells
CHAPTER VIII—Magneto Signaling Apparatus—Battery Bell—Magneto Bell—Magneto Generator—Armature—Automatic Shunt—Polarized Ringer
CHAPTER IX—Hook Switch
CHAPTER X—Electromagnets—Impedance, Induction, and Repeating Coils
CHAPTER XI—Non-Inductive Resistance Devices—Differentially-Wound Unit
CHAPTER XII—Condensers—Materials
CHAPTER XIII—Current Supply to Transmitters—Local Battery—Common Battery—Diagrams of Common-Battery Systems
CHAPTER XIV—Telephone Sets: Magneto,
Series and Bridging, Common-Battery
Party-Line Systems By K. B. Miller and S. G. McMeen Page 227
CHAPTER XV—Non-Selective Party-Line Systems—Series and Bridging—Signal Code
CHAPTER XVI—Selective Party-Line Systems: Polarity, Harmonic, Step-by-Step, and Broken-Line
CHAPTER XVII—Lock-Out Party-Line
Systems: Poole, Step-by-Step, and Broken-Line
Protection By K. B. Miller and S. G. McMeen Page 287
CHAPTER XVII—Electrical Hazards
CHAPTER XIX—High Potentials—Air-Gap Arrester—Discharge across Gaps—Types of Arrester—Vacuum Arrester—Strong Currents—Fuses—Sneak Currents—Line Protection—Central-Office and Subscribers' Station Protectors—City Exchange Requirements—Electrolysis
Manual Switchboards By K. B. Miller and S. G. McMeen Page 317
CHAPTER XX—The Telephone Exchange—Subscribers', Trunk, and Toll Lines—Districts—Switchboards
CHAPTER XXI—Simple Magneto Switchboard—Operation—Commercial Types of Drops and Jacks—Manual vs. Automatic Restoration—Switchboard Plugs and Cords—Ringing and Listening Keys—Operator's Telephone Equipment—Circuits of Complete Switchboard—Night-Alarm Circuits—Grounded and Metallic Circuit Line—Cord Circuit—Switchboard Assembly
Review Questions Page 387
Index Page 401
[A] For professional standing of authors, see list of Authors and Collaborators at front of volume.
A TYPICAL SMALL MAGNETO SWITCHBOARD INSTALLATION
A TYPICAL CENTRAL OFFICE
FOR RURAL EXCHANGE
Line Protectors on Wall at Left.
OLD BRANCH-TERMINAL MULTIPLE BOARD, PARIS, FRANCE
No. 10 SERIES MULTIPLE SWITCHBOARD
Monarch Telephone Mfg. Co.
OPERATOR'S EQUIPMENT
Clement Automanual System
INTERIOR OF WAREHOUSE FOR TELEPHONE CONSTRUCTION MATERIAL
COMPRESSED AIR WAGON FOR PNEUMATIC DRILLING AND CHIPPING IN MANHOLES
SOUTH OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO
THE OPERATING ROOM OF THE EXCHANGE AT WEBB CITY, MISSOURI
A TYPICAL MEDIUM-SIZED MULTIPLE SWITCHBOARD EQUIPMENT
MAIN OFFICE, KEYSTONE TELEPHONE COMPANY, PHILADELPHIA, PA.
MAIN OFFICE, KANSAS CITY HOME TELEPHONE CO., KANSAS CITY, MO.
VENTILATING PLANT FOR LARGE TELEPHONE OFFICE BUILDING
OPERATING ROOM AT TOKYO, JAPAN
VIEW OF A LARGE FOREIGN MULTIPLE SWITCHBOARD
OLD SWITCHBOARD OF BELL EXCHANGE SERVING CHINATOWN, SAN FRANCISCO, CALIFORNIA
A SPECIALLY FORMED CABLE FOR KEY SHELF OF MONARCH SWITCHBOARD
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 Page 12 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 Page 13 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 Page 14 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 Page 15 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 Page 16 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. Page 17 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 Page 18 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.
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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. Page 20
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 Page 21 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 Page 22 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. Page 23 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.
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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 Page 25 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.
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 Page 26 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.
Magneto Telephones. The device in Fig. 1 is a practical magneto receiver and transmitter. It is chosen as best picturing the idea to be proposed. Fig. 2 illustrates a pair of magneto telephones of the early Bell type; 1-1 are diaphragms; 2-2 are permanent magnets with a free end of each brought as near as possible, without touching, to the diaphragm. Each magnet bears on its end nearest the diaphragm a winding of fine wire, the two ends of each of these windings being joined by means of a two-wire line. All that has been said concerning Fig. 1 is true also of the electrical and magnetic actions of the devices of Fig. 2. In the latter, the flux which threads the fine wire winding is disturbed by motions of the transmitting diaphragm. This disturbance of the flux creates electromotive forces in those windings. Similarly, a variation of the electromotive forces in the windings varies the pull of the permanent magnet of the receiving instrument upon its diaphragm.
Fig. 3 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 Page 28 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:
"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 Page 29 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.
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.
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.
Page 30 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.
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 Page 31 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.
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 Page 32 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.
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.
Page 33 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.
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 Page 34 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.
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.
Page 35Let 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.
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 Page 36 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.
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 Page 37 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.
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 Page 38 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.
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Page 39
Electric calls or signals are of two kinds: audible and visible.
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.
Page 40 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.
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 Page 41 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.
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.
"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 Page 42 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.
If, now, such an electromagnet, a permanent magnet, and a pivoted armature be related to a pair of gongs as shown in Fig. 21, a polarized ringer results. It should be noted that a permanent magnet has both its poles presented (though one of the poles is not actually attached) to two parts of the iron of the electro-magnet. The result is that the ends of the armature are of south polarity and those of the core are of north polarity. All the markings of Fig. 21 relate to the polarity produced by the permanent magnet. If, now, a current flow in the ringer winding from plus to minus, obviously the right-hand pole will be additively magnetized, the current tending to produce north magnetism there; also the left-hand pole will be subtractively magnetized, the current tending to produce south magnetism there. If the current be of a certain strength, relative to the certain ringer under study, magnetism in the left pole will be neutralized and that in the right pole doubled. Hence the armature will be attracted more by the right pole than by the left and will strike the right-hand gong. A reversal of current produces an opposite action, the left-hand gong being struck. The current ceasing, the armature remains where last thrown.
It is important to note that the strength of action depends upon the strength of the current up to a certain point only. That depends Page 43 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.
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 Page 44 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.
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 Page 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.
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, Page 46 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.
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.
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Page 47
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 Page 48 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.
Page 49Lines 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 Page 50 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 ÷ RC 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 Page 51 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.
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.
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 Page 52 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.
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.
Page 53 The electrostatic capacity of two open wires .165 inch diameter, 1 ft. apart, and 30 ft. above the earth, is of the order of .009 microfarads per mile. This quantity would be higher if the wires were closer together; or nearer the earth; or if they were surrounded by a gas other than the air or hydrogen; or if the wires were insulated not by a gas but by any solid covering. As another example, a line composed of two wires of a diameter of .036 inch, if wrapped with paper and twisted into a pair as a part of a telephone-cable, has a mutual electrostatic capacity of approximately .08 microfarads per mile, this quantity being greater if the cable be more tightly compressed.
The use of paper as an insulator for wires in telephone cables is due to its low specific inductive capacity. This is because the insulation of the wires is so largely dry air. Rubber and similar insulating materials give capacities as great as twice that of dry paper.
The condenser or other capacity acts as an effective barrier to the steady flow of direct currents. Applying a fixed potential causes a mere rush of current to charge its surface to a definite degree, dependent upon the particular conditions. The condenser does not act as such a barrier to alternating currents, for it is possible to talk through a condenser by means of the alternating voice currents of telephony, or to pass through it alternating currents of much lower frequency. A condenser is used in series with a polarized ringer for the purpose of letting through alternating current for ringing the bell, and of preventing the flow of direct current.
The degree to which the condenser allows alternating currents to pass while stopping direct currents, depends on the capacity of the condenser and on the frequencies of alternating current. The larger the condenser capacity or the higher the frequency of the alternations, the greater will be the current passing through the circuit. The degree to which the current is opposed by the capacity is the reactance of that capacity for that frequency. The formula is
Capacity reactance = 1 ÷ C ω
wherein C is the capacity in farads and ω is 2 π n, or twice 3.1416 times the frequency.
All the foregoing leads to the generalization that the higher the frequency, the less the opposition of a capacity to an alternating Page 54 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.
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. Page 55 The listening observer will notice that the sounds reaching his ear steadily grow faint as the inductance in the line increases and the speaking observer will notice the same thing through the receiver in series with the line.
Both observations in this test show that the amount of current entering and emerging from the line decreased as the inductance increased. Compare this with the test with bridged capacity and the loading of lines described later herein, observing the curious beneficial result when both hurtful properties are present in a line. The test is illustrated in Fig. 34.
The degree in which any current is opposed by inductance is termed the reactance of that inductance. Its formula is
Inductive reactance = L ω
wherein L is the inductance in henrys and ω is 2 π n, or twice 3.1416 times the frequency. To distinguish the two kinds of reactance, that due to the capacity is called capacity reactance and that due to inductance is called inductive reactance.
All the foregoing leads to the generalization that the higher the frequency, the greater the opposition of an inductance to an alternating current. If the frequency be zero, the reactance is zero, i.e., the circuit conducts direct current as mere resistance. If the frequency be infinite, the reactance is infinite, i.e., the circuit is "open" to the alternating current and that current cannot pass through it. Compare this with the correlative generalization following the preceding thought upon capacity.
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 Page 56 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,
)](images/p56eq1.gif "Equation: Z = [sqrt](R^2 + (1/C^2[omega]^2))"){kind=small}
and
](images/p56eq2.gif "Equation: Z = [sqrt](R^2 + L^2[omega]^2)"){kind=small}
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 Page 57 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 Page 58 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 Page 59 this case from the shaken to the fixed point is small, but if the string be loaded by attaching bullets to it, uniformly throughout its length, it now may transmit much more energy to the fixed end.
The addition of inductance to a telephone line is analogous to the addition of bullets to the string, so that a telephone line is said to be loaded when inductances are inserted in it, and the inductances themselves are known as loading coils.
Fig. 35 shows the general relation of Pupin loading coils to the capacity of the line. The condensers of the figure are merely conventionals to represent the condenser which the line itself forms. The inductances of the figure are the actual loading coils.
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. Page 60 That is, the larger a is, the more the voice current is reduced in passing over the line. The equation is
The quantities are
R = Resistance in ohmsThe 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 Page 61 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.
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. Page 62 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.
Enough has been accomplished by practical tests of the Shreeve device and others like it to show that the search for a method of relaying telephone voice currents is not looking for a pot of gold at the end of the rainbow. The most remarkable truth established by the success of repeaters of the Shreeve type is that a device embodying so large inertia of moving parts can succeed at all. If this mean anything, it is that a device in which inertia is absolutely eliminated might do very much better. Many of the methods already proposed by inventors attack the problem in this way and one of the most recent and most promising ways is that of Mr. J.B. Taylor, the circuit of whose telephone-relay patent is shown in Fig. 37. In it, 1 is an electromagnet energized by voice currents; its varying field varies an arc between the electrodes 2-2 and 3 in a vacuum tube. These fluctuations are transformed into line currents by the coil 4.
ToC
Page 63
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 Page 64 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.
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 Page 65 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 Page 66 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.
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. Page 67 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.
Page 68 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.
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 Page 69 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.
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 Page 70 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 Page 71 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.
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 Page 72 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 Page 73 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.
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 Page 74 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.
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 Page 75 which has a base of the same size as the electrode. The carbon-chamber wall consists of a brass ring to which are fastened the mica disks of the front and the back electrodes by means of brass collars clamped over the edge of the mica and around the rim of the brass ring forming the chamber.
Electrodes. The electrode plates of nearly all modern transmitters are of specially treated carbon. These are first copper-plated and soldered to their brass supporting disks. After this they are turned and ground so as to be truly circular in form and to present absolutely flat faces toward each other. These faces are then highly polished and the utmost effort is made to keep them absolutely clean. Great pains are taken to remove from the pores of the carbon, as well as from the surface, all of the acids or other chemicals that may have entered them during the process of electroplating them or of soldering them to the brass supporting disk. That the two electrodes, when mounted in a transmitter, should be parallel with each other, is an item of great importance as will be pointed out later.
In a few cases, as previously stated, gold or platinum has been substituted for the carbon electrodes in transmitters. These are capable of giving good results when used in connection with the proper form of granular carbon, but, on the whole, the tendency has been to abandon all forms of electrode material except carbon, and its use is now well nigh universal.
Preparation of Carbon. The granular carbon is prepared from carefully selected anthracite coal, which is specially treated by roasting or "re-carbonizing" and is then crushed to approximately the proper fineness. The crushed carbon is then screened with extreme care to eliminate all dust and to retain only granules of uniform size.
Packing. In the earlier forms of granular-carbon transmitters a great deal of trouble was experienced due to the so-called packing of the instrument. This, as the term indicates, was a trouble due to the tendency of the carbon granules to settle into a compact mass and thus not respond to the variable pressure. This was sometimes due to the presence of moisture in the electrode chamber; sometimes to the employment of granules of varying sizes, so that they would finally arrange themselves under the vibration of the diaphragm into a fairly compact mass; or sometimes, and more frequently, to the granules Page 76 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 Page 77 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.
Page 78 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.
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 Page 79 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.
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.
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.
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The telephone receiver is the device which translates the energy of the voice currents into the energy of corresponding sound waves. All telephone receivers today are of the electromagnetic type, the voice currents causing a varying magnetic pull on an armature or diaphragm, which in turn produces the sound waves corresponding to the undulations of the voice currents.
Early Receivers. The early forms of telephone receivers were of the single-pole type; that is, the type wherein but one pole of the electromagnet was presented to the diaphragm. The single-pole receiver that formed the companion piece to the old Blake transmitter and that was the standard of the Bell companies for many years, is shown in Fig. 49. While this has almost completely passed out of use, it may be profitably studied in order that a comparison may be made between certain features of its construction and those of the later forms of receivers.
The coil of this receiver was wound on a round iron core 2, flattened at one end to afford means for attaching the permanent magnet. The permanent magnet was of laminated construction, consisting of four hard steel bars 1, extending nearly the entire length of the receiver shell. These steel bars were all magnetized separately and placed with like poles together so as to form a single bar magnet. They were laid together in pairs so as to include between the pairs the flattened end of the pole piece 2 at one end and the flattened portion of the tail piece 3 at the other end. This whole magnet structure, including the core, the tail piece, and the permanently magnetized steel bars, was clamped together by screws as shown. The containing shell was of hard rubber consisting of three pieces, the barrel 4, the ear-piece 5, and the tail cap 6. The barrel and the ear piece engaged each other by means of a screw thread and served to clamp the diaphragm between them. The compound bar magnet Page 81 was held in place within the shell by means of a screw 7 passing through the hard rubber tail cap 6 and into the tail block 3 of the magnet. External binding posts mounted on the tail cap, as shown, were connected by heavy leading-in wires to the terminals of the electromagnet.
A casual consideration of the magnetic circuit of this instrument will show that it was inefficient, since the return path for the lines of force set up by the bar magnet was necessarily through a very long air path. Notwithstanding this, these receivers were capable of giving excellent articulation and were of marvelous delicacy of action. A very grave fault was that the magnet was supported in the shell at the end farthest removed from the diaphragm. As a result it was difficult to maintain a permanent adjustment between the pole piece and the diaphragm. One reason for this was that hard rubber and steel contract and expand under changes of temperature at very different rates, and therefore the distance between the pole piece and the diaphragm changed with changes of temperature. Another grave defect, brought about by this tying together of the permanent magnet and the shell which supported the diaphragm at the end farthest from the diaphragm, was that any mechanical shocks were thus given a good chance to alter the adjustment.
Modern Receivers. Receivers of today differ from this old single-pole receiver in two radical respects. In the first place, the modern receiver is of the bi-polar type, consisting essentially of a horseshoe magnet presenting both of its poles to the diaphragm. In the second place, the modern practice is to either support all of the working parts of the receiver, i.e., the magnet, the coils, and the diaphragm, by an inner metallic frame Page 82 entirely independent of the shell; or, if the shell is used as a part of the structure, to rigidly fasten the several parts close to the diaphragm rather than at the end farthest removed from the diaphragm.
Western Electric Receiver. The standard bi-polar receiver of the Western Electric Company, in use by practically all of the Bell operating companies throughout this country and in large use abroad, is shown in Fig. 50. In this the shell is of three pieces, consisting of the barrel 1, the ear cap 2, and the tail cap 3. The tail cap and the barrel are permanently fastened together to form substantially a single piece. Two permanently magnetized bar magnets 4-4 are employed, these being clamped together at their upper ends, as shown, so as to include the soft iron block 5 between them. The north pole of one of these magnets is clamped to the south pole of the other, so that in reality a horseshoe magnet is formed. At their lower ends, these two permanent magnets are clamped against the soft iron pole pieces 6-6, a threaded block 7 also being clamped rigidly between these pole pieces at this point. On the ends of the pole pieces the bobbins are wound. The whole magnet structure is secured within the shell 1 by means of a screw thread on the block 7 which engages a corresponding internal screw thread in the shell 1. As a result of this construction the whole magnet structure is bound rigidly to the shell structure at a point close to the diaphragm, comparatively speaking, and as a result of this close coupling, the relation between the diaphragm and the pole piece is very much more rigid and substantial than in the case where the magnet structure and the shell were secured together at the end farthest removed from the diaphragm.
Although this receiver shown in Fig. 50 is the standard in use Page 83 by the Bell companies throughout this country, its numbers running well into the millions, it cannot be said to be a strictly modern receiver, because of at least one rather antiquated feature. The binding posts, by which the circuit conductors are led to the coils of this instrument, are mounted on the outside of the receiver shell, as indicated, and are thus subject to danger of mechanical injury and they are also exposed to the touch of the user, so that he may, in case of the wires being charged to an abnormal potential, receive a shock. Probably a more serious feature than either one of these is that the terminals of the flexible cords which attach to these binding posts are attached outside of the receiver shell, and are therefore exposed to the wear and tear of use, rather than being protected as they should be within the shell. Notwithstanding this undesirable feature, this receiver is a very efficient one and is excellently constructed.
Kellogg Receiver. In Fig. 51 is shown a bi-polar receiver with internal or concealed binding posts. This particular receiver is typical of a large number of similar kinds and is manufactured by the Kellogg Switchboard and Supply Company. Two straight permanently magnetized bar magnets 1-1 are clamped together at their opposite ends so as to form a horseshoe magnet. At the end opposite the diaphragm these bars clamp between them a cylindrical piece of iron 2, so as to complete the magnetic circuit at the end. At the end nearest the diaphragm they clamp between them the ends of the soft iron pole pieces 3-3, and also a block of composite metal 4 having a large circular flange 4' which serves as a means for supporting the magnet structure within the shell. The screws by means of which the disk 4' is clamped to the shouldered seat in the shell do not enter the shell directly, but rather enter screw-threaded brass blocks which are moulded into the structure of the shell. It is seen from this construction that the diaphragm and the pole pieces and the magnet Page 84 structure itself are all rigidly secured together through the medium of the shell at a point as close as possible to the diaphragm.
Between the magnets 1-1 there is clamped an insulating block 5, to which are fastened the terminal plates 6, one on each side of the receiver. These terminal plates are thoroughly insulated from the magnets themselves and from all other metallic parts by means of sheets of fiber, as indicated by the heavy black lines. On these plates 6 are carried the binding posts for the receiver cord terminals. A long tongue extends from each of the plates 6 through a hole in the disk 4', into the coil chamber of the receiver, at which point the terminal of the magnet winding is secured to it. This tongue is insulated from the disk 4', where it passes through it, by means of insulating bushing, as shown. The other terminal of the magnet coils is brought out to the other plate 6 by means of a similar tongue on the other side.
In order that the receiver terminals proper may not be subjected to any strain in case the receiver is dropped and its weight caught on the receiver cord, a strain loop is formed as a continuation of the braided covering of the receiver cord, and this is tied to the permanent magnet structure, as shown. By making this strain loop short, it is obvious that whatever pull the cord receives will not be taken by the cord conductors leading to the binding posts or by the binding posts or the cord terminals themselves.
A number of other manufacturers have gone even a step further than this in securing permanency of adjustment between the receiver diaphragm and pole pieces. They have done this by not depending at all on the hard rubber shell as a part of the structure, but by enclosing the magnet coil in a cup of metal upon which the diaphragm is mounted, so that the permanency of relation between the diaphragm and the pole pieces is dependent only upon the metallic structure and not at all upon the less durable shell.
Direct-Current Receiver. Until about the middle of the year 1909, it was the universal practice to employ permanent magnets for giving the initial polarization to the magnet cores of telephone receivers. This is still done, and necessarily so, in receivers employed in connection with magneto telephones. In common-battery systems, however, where the direct transmitter current is fed from the central office to the local stations, it has been found that this current which Page 85 must flow at any rate through the line may be made to serve the additional purpose of energizing the receiver magnets so as to give them the necessary initial polarity. A type of receiver has come into wide use as a result, which is commonly called the direct-current receiver, deriving its name from the fact that it employs the direct current that is flowing in the common-battery line to magnetize the receiver cores. The Automatic Electric Company, of Chicago, was probably the first company to adopt this form of receiver as its standard type. Their receiver is shown in cross-section in Fig. 52, and a photograph of the same instrument partially disassembled is given in Fig. 53. The most noticeable thing about the construction of this receiver is the absence of permanent magnets. The entire working parts are contained within the brass cup 1, which serves not only as a container for the magnet, but also as a seat for the diaphragm. This receiver is therefore illustrative of the type mentioned above, wherein the relation between the diaphragm and the pole pieces is not dependent upon any connection through the shell.
The coil of this instrument consists of a single cylindrical spool 2, mounted on a cylindrical core. This bobbin lies within a soft iron-punching 3, the form of which is most clearly shown in Fig. 53, Page 86 and this punching affords a return path to the diaphragm for the lines of force set up in the magnet core. Obviously a magnetizing current passing through the winding of the coil will cause the end of the core toward the diaphragm to be polarized, say positively, while the end of the enclosing shell will be polarized in the other polarity, negatively. Both poles of the magnet are therefore presented to the diaphragm and the only air gap in the magnetic circuit is that between the diaphragm and these poles. The magnetic circuit is therefore one of great efficiency, since it consists almost entirely of iron, the only air gap being that across which the attraction of the diaphragm is to take place.
The action of this receiver will be understood when it is stated that in common-battery practice, as will be shown in later chapters, a steady current flows over the line for energizing the transmitter. On this current is superposed the incoming voice currents from a distant station. The steady current flowing in the line will, in the case of this receiver, pass through the magnet winding and establish a normal magnetic field in the same way as if a permanent magnet were employed. The superposed incoming voice currents will then be able to vary this magnetic field in exactly the same way as in the ordinary receiver.
An astonishing feature of this recent development of the so-called direct-current receiver is that it did not come into use until after about twenty years of common-battery practice. There is nothing new in the principles involved, as all of them were already understood and some of them were employed by Bell in his original telephone; in fact, the idea had been advanced time and again, and thrown aside as not being worth consideration. This is an illustration of a frequent occurrence in the development of almost any rapidly growing art. Ideas that are discarded as worthless in the early stages of the art are finally picked up and made use of. The reason for this is that in some cases the ideas come in advance of the art, or they are proposed before the art is ready to use them. In other cases the idea as originally proposed lacked some small but essential detail, or, as is more often the case, the experimenter in the early days did not have sufficient skill or knowledge to make it fit the requirements as he saw them.
Monarch Receiver. The receiver of the Automatic Electric Page 87 Company just discussed employs but a single electromagnet by which the initial magnetization of the cores and also the variable magnetization necessary for speech reproduction is secured. The problem of the direct-current receiver has been attacked in another way by Ernest E. Yaxley, of the Monarch Telephone Manufacturing Company, with the result shown in Fig. 54. The construction in this case is not unlike that of an ordinary permanent-magnet receiver, except that in the place of the permanent magnets two soft iron cores 1-1 are employed. On these are wound two long bobbins of insulated wire so that the direct current flowing over the telephone line will pass through these and magnetize the cores to the same degree and for the same purpose as in the case of permanent magnets. These soft iron magnet cores 1-1 continue to a point near the coil chamber, where they join the two soft iron pole pieces 2-2, upon which the ordinary voice-current coils are wound. The two long coils 4-4, which may be termed the direct-current coils, are of somewhat lower resistance than the two voice-current coils 3-3. They are, however, by virtue of their greater number of turns and the greater amount of iron that is included in their cores, of much higher impedance than the voice-current coils 3-3. These two sets of coils 4-4 and 3-3 are connected in multiple. As a result of their lower ohmic resistance the coils 4-4 will take a greater amount of the steady current which comes over the line, and therefore the greater proportion of the steady current will be employed in magnetizing the bar magnets. On account of their higher impedance to alternating currents, however, nearly all of the voice currents which are superposed on the steady currents, flowing in the line will pass through the voice-current coils 3-3, and, being near the diaphragm, these currents will so vary the steady magnetism in the cores 2-2 as to produce the necessary vibration of the diaphragm.
Page 88 This receiver, like the one of the Automatic Electric Company, does not rely on the shell in any respect to maintain the permanency of relation between the pole pieces and the diaphragm. The cup 5, which is of pressed brass, contains the voice-current coils and also acts as a seat for the diaphragm. The entire working parts of this receiver may be removed by merely unscrewing the ear piece from the hard rubber shell, thus permitting the whole works to be withdrawn in an obvious manner.
Dean Receiver. Of such decided novelty as to be almost revolutionary in character is the receiver recently put on the market by the Dean Electric Company and shown in Fig. 55. This receiver is of the direct-current type and employs but a single cylindrical bobbin of wire. The core of this bobbin and the return path for the magnetic lines of force set up in it are composed of soft iron punchings of substantially E shape. These punchings are laid together so as to form a laminated soft-iron field, the limbs of which are about square in cross-section. The coil is wound on the center portion of this E as a core, the core being, as stated, approximately square in cross-section. The general form of magnetic circuit in this instrument is therefore similar to that of the Automatic Electric Company's receiver, shown in Figs. 52 and 53, but the core is laminated instead of being solid as in that instrument.
The most unusual feature of this Dean receiver is that the use of hard rubber or composition does not enter into the formation of the shell, but instead a shell composed entirely of steel stampings has been substituted therefor. The main portion of this shell is the barrel 1. Great skill has evidently been exercised in the forming of this by the cold-drawn process, it presenting neither seams nor welds. The ear piece 2 is also formed of steel of about the same gauge as the barrel 1. Instead of screw-threading the steel parts, Page so that they would directly engage each other, the ingenious device has been employed of swaging a brass ring 3 in the barrel portion and a similar brass ring 4 in the ear cap portion, these two being slotted and keyed, as shown at 8, so as to prevent their turning in their respective seats. The ring 3 is provided with an external screw thread and the ring 4 with an internal screw thread, so that the receiver cap is screwed on to the barrel in the same way as in the ordinary rubber shell. By the employment of these brass screw-threaded rings, the rusting together of the parts so that they could not be separated when required—a difficulty heretofore encountered in steel construction of similar parts—has been remedied.
The entire working parts of this receiver are contained within the cup 5, the edge of which is flanged outwardly to afford a seat for the diaphragm. The diaphragm is locked in place on the shell by a screw-threaded ring 6, as is clearly indicated. A ring 7 of insulating material is seated within the enlarged portion of the barrel 1, and against this the flange of the cup 5 rests and is held in place by the cap 2 when it is screwed home. The working parts of this receiver partially disassembled are shown in Fig. 56, which gives a clear idea of some of the features not clearly illustrated in Fig. 55.
It cannot be denied that one of the principal items of maintenance of subscribers' station equipment has been due to the breakage of receiver shells. The users frequently allow their receiver to fall and strike heavily against the wall or floor, thus not only subjecting the cords to great strain, but sometimes cracking or entirely breaking the receiver shell. The innovation thus proposed by the Dean Company Page 90 of making the entire receiver shell of steel is of great interest. The shell, as will be seen, is entirely insulated from the circuit of the receiver so that no contact exists by which a user could receive a shock. The shell is enameled inside and out with a heavy black insulating enamel baked on, and said to be of great durability. How this enamel will wear remains to be seen. The insulation of the interior portions of the receiver is further guarded by providing a lining of fiber within the shell at all points where it seems possible that a cross could occur between some of the working parts and the metal of the shell. This type of receiver has not been on the market long enough to draw definite conclusions, based on experience in use, as to what its permanent performance will be.
Thus far in this chapter only those receivers which are commonly called hand receivers have been discussed. These are the receivers that are ordinarily employed by the general public.
Operator's Receiver. At the central office in telephone exchanges the operators are provided with receivers in order that they may communicate with the subscribers or with other operators. In order that they may have both of their hands free to set up and take down the connections and to perform all of the switching operations required, a special form of receiver is employed for this purpose, which is worn as a part of a head-gear and is commonly termed a head receiver. These are necessarily of very light construction, in Page 91 order not to be burdensome to the operators, and obviously they must be efficient. They are ordinarily held in place at the ear by a metallic head band fitting over the head of the operator.
Such a receiver is shown in cross-section in Fig. 57, and completely assembled with its head band in Fig. 58. Referring to Fig. 57 the shell 1 of the receiver is of aluminum and the magnets are formed of steel rings 2, cross-magnetized so as to present a north pole on one side of the ring and a south pole on the other. The two L-shaped pole pieces 3 are secured by screws to the poles of these ring magnets, and these pole pieces carry the magnet coils, as is clearly indicated. These poles are presented to a soft iron diaphragm in exactly the same way as in the larger hand receivers, the diaphragm being clamped in place by a hard rubber ear piece, as shown. The head bands are frequently of steel covered with leather. They have assumed numerous forms, but the general form shown in Fig. 58 is the one commonly adopted.
Conventional Symbols. The usual diagrammatic symbols for hand and head receivers are shown in Fig. 59. They are self-explanatory. The symbol at the left in this figure, showing the general outline of the receiver, is the one most commonly used where any sort of a receiver is to be indicated in a circuit diagram, but where it becomes desirable to indicate in the diagram the actual connections with the coil or coils of the receiver, the symbol shown at the right is to be preferred, and obviously it may be modified as to number of windings and form of core as desired.
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Galvani, an Italian physician, discovered, in 1786, that a current of electricity could be produced by chemical action. In 1800, Volta, a physicist, also an Italian, threw further light on Galvani's discovery and produced what we know as the voltaic, or galvanic, cell. In honor of these two discoverers we have the words volt, galvanic, and the various words and terms derived therefrom.
Simple Voltaic Cell. A very simple voltaic cell may be made by placing two plates, one of copper and one of zinc, in a glass vessel partly filled with dilute sulphuric acid, as shown in Fig. 60. When the two plates are not connected by a wire or other conductor, experiment shows that the copper plate bears a positive charge with respect to the zinc plate, and the zinc plate bears a negative charge with respect to the copper. When the two plates are connected by a wire, a current flows from the copper to the zinc plate through the metallic path of the wire, just as is to be expected when any conductor of relatively high electrical potential is joined to one of relatively low electrical potential. Ordinarily, when one charged body is connected to another of different potential, the resulting current is of but momentary duration, due to the redistribution of the charges and consequent equalization of potential. In the case of the simple cell, however, the current is continuous, showing that some action is maintaining the charges on the two plates and therefore maintaining the difference of potential between them. The energy of this current is derived from the chemical action of the acid on the zinc. The cell is in reality a sort of a zinc-burning furnace.
In the action of the cell, when the two plates are joined by a wire, it may be noticed that the zinc plate is consumed and that bubbles of hydrogen gas are formed on the surface of the copper plate.
Page 93 Theory. Just why or how chemical action in a voltaic cell results in the production of a negative charge on the consumed plate is not known. Modern theory has it that when an acid is diluted in water the molecules of the acid are split up or dissociated into two oppositely charged atoms, or groups of atoms, one bearing a positive charge and the other a negative charge of electricity. Such charged atoms or groups of atoms are called ions. This separation of the molecules of a chemical compound into positively and negatively charged ions is called dissociation.
Thus, in the simple cell under consideration the sulphuric acid, by dissociation, splits up into hydrogen ions bearing positive charges, and SO4 ions bearing negative charges. The solution as a whole is neutral in potential, having an equal number of equal and opposite charges.
It is known that when a metal is being dissolved by an acid, each atom of the metal which is torn off by the solution leaves the metal as a positively charged ion. The carrying away of positive charges from a hitherto neutral body leaves that body with a negative charge. Hence the zinc, or consumed plate, becomes negatively charged.
In the chemical attack of the sulphuric acid on the zinc, the positive hydrogen ions are liberated, due to the affinity of the negative SO4 ions for the positive zinc ions, this resulting in the formation of zinc sulphate in the solution. Now the solution itself becomes positively charged, due to the positive charges leaving the zinc plate with the zinc ions, and the free positively charged hydrogen ions liberated in the solution as just described are repelled to the copper plate, carrying their positive charges thereto. Hence the copper plate, or the unconsumed plate, becomes positively charged and also coated with hydrogen bubbles.
The plates or electrodes of a voltaic cell need not consist of zinc Page 94 and copper, nor need the fluid, called the electrolyte, be of sulphuric acid; any two dissimilar elements immersed in an electrolyte that attacks one of them more readily than the other will form a voltaic cell. In every such cell it will be found that one of the plates is consumed, and that on the other plate some element is deposited, this element being sometimes a gas and sometimes a solid. The plate which is consumed is always the negative plate, and the one on which the element is deposited is always the positive, the current through the connecting wire always being, therefore, from the unconsumed to the consumed plate. Thus, in the simple copper-zinc cell just considered, the zinc is consumed, the element hydrogen is deposited on the copper, and the current flow through the external circuit is from the copper to the zinc.
The positive charges, leaving the zinc, or consumed, plate, and passing through the electrolyte to the copper, or unconsumed, plate, constitute in effect a current of electricity flowing within the electrolyte. The current within the cell passes, therefore, from the zinc plate to the copper plate. The zinc is, therefore, said to be positive with respect to the copper.
Difference of Potential. The amount of electromotive force, that is generated between two dissimilar elements immersed in an electrolyte is different for different pairs of elements and for different electrolytes. For a given electrolyte each element bears a certain relation to another; i.e., they are either electro-positive or electro-negative relative to each other. In the following list a group of elements are arranged with respect to the potentials which they assume with respect to each other with dilute sulphuric acid as the electrolyte. The most electro-positive elements are at the top and the most electro-negative at the bottom.
| + | Sodium | Lead | Copper | |
| Magnesium | Iron | Silver | ||
| Zinc | Nickel | Gold | ||
| Cadmium | Bismuth | Platinum | ||
| Tin | Antimony | - | Graphite (Carbon) |
Any two elements selected from this list and immersed in dilute sulphuric acid will form a voltaic cell, the amount of difference of potential, or electromotive force, depending on the distance apart in this series of the two elements chosen. The current within the cell Page 95 will always flow from the one nearest the top of the list to the one nearest the bottom, i.e., from the most electro-positive to the most electro-negative; and, therefore, the current in the wire joining the two plates will flow from the one lowest down in the list to the one highest up.
From this series it is easy to see why zinc and copper, and also zinc and carbon, are often chosen as elements of voltaic cells. They are widely separated in the series and comparatively cheap.
This series may not be taken as correct for all electrolytes, for different electrolytes alter somewhat the order of the elements in the series. Thus, if two plates, one of iron and the other of copper, are immersed in dilute sulphuric acid, a current is set up which proceeds through the liquid from the iron to the copper; but, if the plates after being carefully washed are placed in a solution of potassium sulphide, a current is produced in the opposite direction. The copper is now the positive element.
Table II shows the electrical deportment of the principal metals in three different liquids. It is arranged like the preceding one, each metal being electro-positive to any one lower in the list.
TABLE II
Behavior of Metals in Different Electrolytes
| Caustic Potash | Hydrochloric Acid | Potassium Sulphide |
| + Zinc | + Zinc | + Zinc |
| Tin | Cadmium | Copper |
| Cadmium | Tin | Cadmium |
| Antimony | Lead | Tin |
| Lead | Iron | Silver |
| Bismuth | Copper | Antimony |
| Iron | Bismuth | Lead |
| Copper | Nickel | Bismuth |
| Nickel | Silver | Nickel |
| - Silver | - Antimony | - Iron |
It is important to remember that in all cells, no matter what elements or what electrolyte are used, the electrode which is consumed is the one that becomes negatively charged and its terminal, therefore, becomes the negative terminal or pole, while the electrode which is not consumed is the one that becomes positively charged, and its terminal is, therefore, the positive terminal or pole of the cell. However, Page 96 because the current in the electrolyte flows from the consumed plate to the unconsumed plate, the consumed plate is called the positive plate and the unconsumed, the negative. This is likely to become confusing, but if one remembers that the active plate is the positive plate, because it sends forth positive ions in the electrolyte, and, therefore, itself becomes negatively charged, one will have the proper basis always to determine the direction of the current flow, which is the important thing.
Polarization. If the simple cell already described have its terminals connected by a wire for some time, it will be found that the current rapidly weakens until it ceases to be manifest. This weakening results from two causes: first, the hydrogen gas which is liberated in the action of the cell is deposited in a layer on the copper plate, thereby covering the plate and reducing the area of contact with the liquid. This increases the internal resistance of the cell, since hydrogen is a non-conductor. Second, the plate so covered becomes in effect a hydrogen electrode, and hydrogen stands high as an electro-positive element. There is, therefore, actual reduction in the electromotive force of the cell, as well as an increase in internal resistance. This phenomenon is known as polarization, and in commercial cells means must be taken to prevent such action as far as possible.
The means by which polarization of cells is prevented or reduced in practice may be divided into three general classes:
First—mechanical means. If the hydrogen bubbles be simply brushed away from the surface of the electrode the resistance and the counter polarity which they cause will be diminished. The same result may be secured if air be blown into the solution through a tube, or if the liquid be kept agitated. If the surface of the electrode be roughened or covered with points, the bubbles collect more freely at the points and are more quickly carried away to the surface of the liquid. These means are, however, hardly practical except in cells for laboratory use.
Second—chemical means. If a highly oxidizing substance be added to the electrolyte, it will destroy the hydrogen bubbles by combining with them while they are in a nascent state, and this will prevent the increase in internal resistance and the opposing electromotive force. Such substances are bichromate of potash, nitric acid, and chlorine, and are largely used.
Third—electro-chemical means. Double cells, arranged to separate the elements and liquids by means of porous partitions or by gravity, may be so arranged that solid copper is liberated instead of hydrogen at a point where the current leaves the liquid, thereby entirely obviating polarization. This method also is largely used.
Page 97Local Action. When a simple cell stands idle, i.e., with its circuit open, small hydrogen bubbles may be noticed rising from the zinc electrode instead of from copper, as is the case where the circuit is closed. This is due to impurities in the zinc plate, such as particles of iron, tin, arsenic, carbon, etc. Each of these particles acts with the surrounding zinc just as might be expected of any pair of dissimilar elements opposed to each other in an electrolyte; in other words, they constitute small voltaic cells. Local currents, therefore, are generated, circulating between the two adjacent metals, and, as a result, the zinc plate and the electrolyte are needlessly wasted and the general condition of the cell is impaired. This is called local action.
Amalgamated Zincs. Local action might be prevented by the use of chemically pure zinc, but this, on account of its expense, cannot be employed commercially. Local action, however, may be overcome to a great extent by amalgamating the zinc, i.e., coating it with mercury. The iron particles or other impurities do not dissolve in the mercury, as does the zinc, but they float to the surface, whence the hydrogen bubbles which may form speedily carry them off, and, in other cases, the impurities fall to the bottom of the cell. As the zinc in the pasty amalgam dissolves in the acid, the film of mercury unites with fresh zinc, and so always presents a clear, bright, homogeneous surface to the action of the electrolyte.
The process of amalgamating the zinc may be performed by dipping it in a solution composed of
Nitric Acid 1 lb.
Muriatic Acid 2 lbs.
Mercury 8 oz.
The acids should be first mixed and then the mercury slowly added until dissolved. Clean the zinc with lye and then dip it in the solution for a second or two. Rinse in clean water and rub with a brush.
Another method of amalgamating zincs is to clean them by dipping them in dilute sulphuric acid and then in mercury, allowing the surplus to drain off.
Commercial zincs, for use in voltaic cells as now manufactured, Page 98 usually have about 4 per cent of mercury added to the molten zinc before casting into the form of plates or rods.
Series and Multiple Connections. When a number of voltaic cells are joined in series, the positive pole of one being connected to the negative pole of the next one, and so on throughout the series, the electromotive forces of all the cells are added, and the electromotive force of the group, therefore, becomes the sum of the electromotive forces of the component cells. The currents through all the cells in this case will be equal to that of one cell.
If the cells be joined in multiple, the positive poles all being connected by one wire and the negative poles by another, then the currents of all the cells will be added while the electromotive force of the combination remains the same as that of a single cell, assuming all the cells to be alike in electromotive force.
Obviously combinations of these two arrangements may be made, as by forming strings of cells connected in series, and connecting the strings in multiple or parallel.
The term battery is frequently applied to a single voltaic cell, but this term is more properly used to designate a plurality of cells joined together in series, or in multiple, or in series multiple so as to combine their actions in causing current to flow through an external circuit. We may therefore refer to a battery of so many cells. It has, however, become common, though technically improper, to refer to a single cell as a battery, so that the term battery, as indicating necessarily more than one cell, has largely lost its significance.
Cells may be of two types, primary and secondary.
Primary cells are those consisting of electrodes of dissimilar elements which, when placed in an electrolyte, become immediately ready for action.
Secondary cells, commonly called storage cells and accumulators, consist always of two inert plates of metal, or metallic oxide, immersed in an electrolyte which is incapable of acting on either of them until a current has first been passed through the electrolyte from one plate to the other. On the passage of a current in this way, the decomposition of the electrolyte is effected and the composition of the plates is so changed that one of them becomes electro-positive and the other electro-negative. The cell is then, when the charging current ceases, capable of acting as a voltaic cell.
Page 99 This chapter is devoted to the primary cell or battery alone.
Types of Primary Cells. Primary cells may be divided into two general classes: first, those adapted to furnish constant current; and second, those adapted to furnish only intermittent currents. The difference between cells in this respect rests largely in the means employed for preventing or lessening polarization. Obviously in a cell in which polarization is entirely prevented the current may be allowed to flow constantly until the cell is completely exhausted; that is, until the zinc is all eaten up or until the hydrogen is exhausted from the electrolyte or both. On the other hand some cells are so constituted that polarization takes place faster than the means intended to prevent it can act. In other words, the polarization gradually gains on the preventive means and so gradually reduces the current by increasing the resistance of the cell and lowering its electromotive force. In cells of this kind, however, the arrangement is such that if the cell is allowed to rest, that is, if the external circuit is opened, the depolarizing agency will gradually act to remove the hydrogen from the unattacked electrode and thus place the cell in good condition for use again.
Of these two types of primary cells the intermittent-current cell is of far greater use in telephony than the constant-current cell. This is because the use of primary batteries in telephony is, in the great majority of cases, intermittent, and for that reason a cell which will give a strong current for a few minutes and which after such use will regain practically all of its initial strength and be ready for use again, is more desirable than one which will give a weaker current continuously throughout a long period of time.
Since the cells which are adapted to give constant current are commonly used in connection with circuits that are continuously closed, they are called closed-circuit cells. The other cells, which are better adapted for intermittent current, are commonly used on circuits which stand open most of the time and are closed only occasionally when their current is desired. For this reason these are termed open-circuit cells.
Open-Circuit Cells. LeClanché Cell:—By far the most important primary cell for telephone work is the so-called LeClanché cell. This assumes a large variety of forms, but always employs zinc as the negatively charged element, carbon as the positively charged element, Page 100 and a solution of sal ammoniac as the electrolyte. This cell employs a chemical method of taking care of polarization, the depolarizing agent being peroxide of manganese, which is closely associated with the carbon element.
The original form of the LeClanché cell, a form in which it was very largely used up to within a short time ago, is shown in Fig. 61. In this the carbon element is placed within a cylindrical jar of porous clay, the walls of this jar being of such consistency as to allow moisture slowly to permeate through it. Within this porous cup, as it is called, a plate or disk of carbon is placed, and around this the depolarizing agent, consisting of black oxide of manganese. This is usually mixed with, broken carbon, so as to increase the effective area of the carbon element in contact with the depolarizing agent, and also to reduce the total internal resistance of the cell. The zinc electrode usually consisted merely in a rod of zinc, as shown, with a suitable terminal at its upper end.
The chemical action taking place within the LeClanché cell is, briefly, as follows: Sal ammoniac is chemically known as chloride of ammonium and is a combination of chlorine and ammonia. In the action which is assumed to accompany the passage of current in this cell, the sal ammoniac is decomposed, the chlorine leaving the ammonia to unite with an atom of the zinc plate, forming chloride of zinc and setting free ammonia and hydrogen. The ammonia is immediately dissolved in the water of the cell, and the hydrogen enters the porous cup and would speedily polarize the cell by adhering to the carbon plate but for the fact that it encounters the peroxide of manganese. This material is exceedingly rich in oxygen and it therefore readily gives up a part of its oxygen, which forms water by combination with the already liberated hydrogen and leaves what is termed a sesquioxide of manganese. This absorption or combination of the hydrogen prevents immediate polarization, but hydrogen is evolved during the operation of the cell more rapidly Page 101 than it can combine with the oxygen of the manganese, thereby leading to polarization more rapidly than the depolarizer can prevent it when the cell is heavily worked. When, however, the cell is left with its external circuit open for a time, depolarization ensues by the gradual combination of the hydrogen with the oxygen of the peroxide of manganese, and as a result the cell recuperates and in a short time attains its normal electromotive force.
The electromotive force of this cell when new is about 1.47 volts. The internal resistance of the cell of the type shown in Fig. 61 is approximately 1 ohm, ordinarily less rather than more.
A more recent form of LeClanché cell is shown in cross-section in Fig. 62. This uses practically the same materials and has the same chemical action as the old disk LeClanché cell shown in Fig. 61. It dispenses, however, with the porous cup and instead employs a carbon electrode, which in itself forms a cup for the depolarizing agent.
The carbon electrode is in the form of a corrugated hollow cylinder which engages by means of an internal screw thread a corresponding screw thread on the outer side of the carbon cover. Within this cylinder is contained a mixture of broken carbon and peroxide of manganese. The zinc electrode is in the form of a hollow cylinder almost surrounding the carbon electrode and separated therefrom by means of heavy rubber bands stretched around the carbon. The rod, forming the terminal of the zinc, passes through a porcelain bushing on the cover plate to obviate short circuits. This type of cell has an electromotive force of about 1.55 volts and recuperates very quickly after severe use. It also has considerably lower internal resistance than the type of LeClanché cell employing a porous cup, and, therefore, is capable of generating a considerably larger current.
Page 102 Cells of this general type have assumed a variety of forms. In some the carbon electrode, together with the broken carbon and peroxide of manganese, were packed into a canvas bag which was suspended in the electrolyte and usually surrounded by the zinc electrode. In other forms the carbon electrode has moulded with it the manganese depolarizer.
In order to prevent the salts within the cell from creeping over the edge of the containing glass jar and also over the upper portion of the carbon electrode, it is common practice to immerse the upper end of the carbon element and also the upper edge of the glass jar in hot paraffin.
In setting up the LeClanché cell, place not more than four ounces of white sal ammoniac in the jar, fill the jar one-third full of water, and stir until the sal ammoniac is all dissolved. Then put the carbon and zinc elements in place. A little water poured in the vent hole of the porous jar or carbon cylinder will tend to hasten the action.
An excess of sal ammoniac should not be used, as a saturated solution tends to deposit crystals on the zinc; on the other hand, the solution should not be allowed to become too weak, as in that case the chloride of zinc will form on the zinc. Both of these causes materially increase the resistance of the cell.
A great advantage of the LeClanché cell is that when not in use there is but little material waste. It contains no highly corrosive chemicals. Such cells require little attention, and the addition of water now and then to replace the loss due to evaporation is about all that is required until the elements become exhausted. They give a relatively high electromotive force and have a moderately low internal resistance, so that they are capable of giving rather large currents for short intervals of time. If properly made, they recuperate quickly after polarization due to heavy use.
Dry Cell. All the forms of cells so far considered may be quite properly termed wet cells because of the fact that a free liquid electrolyte is used. This term is employed in contradistinction to the later developed cell, commonly termed the dry cell. This term "dry cell" is in some respects a misnomer, since it is not dry and if it were dry it would not work. It is essential to the operation of these cells that they shall be moist within, and when such moisture is dissipated the cell is no longer usable, as there is no further useful chemical action.
Page 103 The dry cells are all of the LeClanché type, the liquid electrolyte of that type being replaced by a semi-solid substance that is capable of retaining moisture for a considerable period.
As in the ordinary wet LeClanché cell, the electrodes are of carbon and zinc, the zinc element being in the form of a cylindrical cup and forming the retaining vessel of the cell, while the carbon element is in the form of a rod or plate and occupies a central position with regard to the zinc, being held out of contact with the zinc, however, at all points.
A cross-section of an excellent form of dry cell is shown in Fig. 63. The outer casing is of zinc, formed in the shape of a cylindrical cup, and serves not only as the retaining vessel, but as the negatively charged electrode. The outer surface of the zinc is completely covered on its sides and bottom with heavy pasteboard so as to insulate it from bodies with which it may come in contact, and particularly from the zinc cups of other cells used in the same battery. The positively charged electrode is a carbon rod corrugated longitudinally, as shown, in order to obtain greater surface. This rod is held in the center of the zinc cup out of contact therewith, and the intervening space is filled with a mixture of peroxide of manganese, powdered carbon, and sal ammoniac. Several thicknesses of blotting paper constitute a lining for the inner portion of the zinc electrode and serve to prevent the manganese mixture from coming directly into contact therewith. The cell is sealed with pitch, which is placed on a layer of sand and sawdust mixed in about equal parts.
The electrolyte in such cells varies largely as to quantities and proportions of the materials employed in various types of cells, and Page 104 also varies in the method in which the elements are introduced into the container.
The following list and approximate proportions of material will serve as a fair example of the filling mixture in well-known types of cells.
| Manganese dioxide | 45 per cent |
| Carbon or graphite, or both | 45 per cent |
| Sal ammoniac | 7 per cent |
| Zinc chloride | 3 per cent |
Water is added to the above and a sufficient amount of mixture is taken for each cell to fill the zinc cup about seven-eighths full when the carbon is in place. The most suitable quantity of water depends upon the original dryness and fineness of material and upon the quality of the paper lining.
In some forms of dry batteries, starch or other paste is added to improve the contact of the electrolyte with the zinc and promote a more even distribution of action throughout the electrolyte. Mercury, too, is often added to effect amalgamation of the zinc.
As in the ordinary wet type of LeClanché cell, the purpose of the manganese is to act as a depolarizer; the carbon or graphite being added to give conductivity to the manganese and to form a large electrode surface. It is important that the sal ammoniac, which is the active agent of the cell, should be free from lumps in order to mix properly with the manganese and carbon.
A small local action takes place in the dry cell, caused by the dissimilar metals necessarily employed in soldering up the zinc cup and in soldering the terminal rod of zinc to the zinc cup proper. This action, however, is slight in the better grades of cells. As a result of this, and also of the gradual drying out of the moisture within the cell, these cells gradually deteriorate even when not in use—this is commonly called shelf-wear. Shelf-wear is much more serious in the very small sizes of dry cells than in the larger ones.
Dry cells are made in a large number of shapes and sizes. The most useful form, however, is the ordinary cylindrical type. These are made in sizes varying from one and one-half inches high and three-quarters inch in diameter to eight inches high and three and three-quarters inches in diameter. The most used and standard size of dry cell is of cylindrical form six inches high and two and three-quarters inches in diameter. The dry cell when new and in good Page 105 condition has an open-circuit voltage of from 1.5 to 1.6 volts. Perhaps 1.55 represents the usual average.
A cell of the two and three-quarters by six-inch size will give throughout its useful life probably thirty ampere hours as a maximum, but this varies greatly with the condition of use and the make of cell. Its effective voltage during its useful life averages about one volt, and if during this life it gives a total discharge of thirty ampere hours, the fair energy rating of the cell will be thirty watt-hours. This may not be taken as an accurate figure, however, as the watt-hour capacity of a cell depends very largely, not only on the make of the cell, but on the rate of its discharge.
An examination of Fig. 63 shows that the dry cell has all of the essential elements of the LeClanché cell. The materials of which the electrodes are made are the same and the porous cup of the disk LeClanché cell is represented in the dry cell by the blotting-paper cylinder, which separates the zinc from the carbon electrode. The positively charged electrode must not be considered as merely the carbon plate or rod alone, but rather the carbon rod with its surrounding mixture of peroxide of manganese and broken carbon. Such being the case, it is obvious that the separation between the electrodes is very small, while the surface presented by both electrodes is very large. As a result, the internal resistance of the cell is small and the current which it will give on a short circuit is correspondingly large. A good cell of the two and three-quarters by six-inch size will give eighteen or twenty amperes on short-circuit, when new.
As the action of the cell proceeds, zinc chloride and ammonia are formed, and there being insufficient water to dissolve the ammonia, there results the formation of double chlorides of zinc and ammonium. These double chlorides are less soluble than the chlorides and finally occupy the pores of the paper lining between the electrolyte and the zinc and greatly increase the internal resistance of the cell. This increase of resistance is further contributed to by the gradual drying out of the cell as its age increases.
Within the last few years dry batteries have been so perfected mechanically, chemically, and electrically that they have far greater outputs and better recuperative power than any of the other types of LeClanché batteries, while in point of convenience and economy, resulting from their small size and non-breakable, Page 106 non-spillable features and low cost, they leave no room for comparison.
Closed-Circuit Cells. Gravity-Cell:—Coming now to the consideration of closed-circuit or constant-current cells, the most important is the well-known gravity, or blue-stone, cell, devised by Daniell. It is largely used in telegraphy, and often in telephony in such cases as require a constantly flowing current of small quantity. Such a cell is shown in Fig. 64.
The elements of the gravity cell are electrodes of copper and zinc. The solution in which the copper plate is immersed is primarily a solution of copper sulphate, commonly known as blue-stone, in water. The zinc plate after the cell is in action is immersed in a solution of sulphate of zinc which is formed around it.
The glass jar is usually cylindrical, the standard sizes being 5 inches diameter and 7 inches deep; and also 6 inches diameter and 8 inches deep. The copper electrode is of sheet copper of the form shown, and it is partly covered with crystals of blue-stone or copper sulphate. Frequently, in later forms of cells, the copper electrode consists merely of a straight, thick, rectangular bar of copper laid horizontally, directly on top of the blue-stone crystals. In all cases a rubber-insulated wire is attached by riveting to the copper electrode, and passes up through the electrolyte to form the positive terminal.
The zinc is, as a rule, of crowfoot form, as shown, whence this cell derives the commonly applied name of crowfoot cell. This is essentially a two-fluid cell, for in its action zinc sulphate is formed, and this being lighter than copper sulphate rises to the top of the jar and surrounds the zinc. Gravity, therefore, serves to keep the two fluids separate.
In the action of the cell, when the external circuit is closed, sulphuric acid is formed which attacks the zinc to form sulphate of zinc Page 107 and to liberate hydrogen, which follows its tendency to attach itself to the copper plate. But in so doing the hydrogen necessarily passes through the solution of sulphate of copper surrounding the copper plate. The hydrogen immediately combines with the SO4 radical, forming therewith sulphuric acid, and liberating metallic copper. This sulphuric acid, being lighter than the copper sulphate, rises to the surface of the zinc and attacks the zinc, thus forming more sulphate of zinc. The metallic copper so formed is deposited on the copper plate, thereby keeping the surface bright and clean. Since hydrogen is thus diverted from the copper plate, polarization does not ensue.
The zinc sulphate being colorless, while the copper sulphate is of a dark blue color, the separating line of the two liquids is easily distinguishable. This line is called the blue line and care should be taken that it does not reach the zinc and cause a deposit of copper to be placed thereon.
As has been stated, these two liquids do not mix readily, but they will eventually mingle unless the action of the cell is sufficient to use up the copper sulphate as speedily as it is dissolved. Thus it will be seen that while the cell is free from polarization and local action, there is, nevertheless, a deteriorating effect if the cell is allowed to remain long on open circuit. Therefore, it should be used when a constant current is required.
Prevention of Creeping:—Much trouble has been experienced in gravity cells due to the creeping of the salts over the edge of the jar. Frequently the upper edges of the jars are coated by dipping in hot paraffin wax in the hope of preventing this. Sometimes oil is poured on top of the fluid in the jar to prevent the creeping of the salts and the evaporation of the electrolyte. The following account of experiments performed by Mr. William Reid, of Chicago, throws light on the relative advantages of these and other methods of preventing creeping.
The experiment was made with gravity cells having 5-inch by 7-inch glass jars. Four cells were made up and operated in a rather dry, warm place, although perhaps under no more severe local conditions than would be found in most telephone exchanges. Cell No. 1 was a plain cell as ordinarily used. Cell No. 2 had the top of the rim of the jar treated with paraffin wax by dipping the rim to about one inch in depth in melted paraffin wax. Cell No. 3 had melted paraffin wax poured over the surface of the liquid forming a seal Page 108 about 3/16 inch in thickness. After cooling, a few small holes were bored through the seal to let gases escape. Cell No. 4 had a layer of heavy paraffin oil nearly 1/2 inch in thickness (about 6 oz. being used) on top of the solutions.
These cells were all run on a load of .22 to .29 amperes for 15-1/2 hours per day for thirty days, after which the following results were noted:
(a) The plain cell, or cell No. 1, had to have 26 ounces of water added to it to replace that which had evaporated. The creeping of zinc sulphate salts was very bad.
(b) The waxed rim cell, or cell No. 2, evaporated 26 ounces of water and the creeping of zinc sulphate salts was not prevented by the waxed rim. The wax proved of no value.
(c) The wax sealed cell, or cell No. 3, showed practically no evaporation and only very slight creeping of zinc sulphate salts. The creeping of salts that took place was only around spots where the edges of the seal were loose from the jar.
(d) The paraffin oil sealed cell, or cell No. 4, showed no evaporation and no creeping of salts.
It was concluded by Mr. Reid from the above experiments that the wax applied to the rim of the jar is totally ineffective and has no merits. The wax seal loosens around the edges and does not totally prevent creeping of the zinc sulphate salts, although nearly so. The wax-sealed jar must have holes drilled in it to allow the gases to escape. The method is hardly commercial, as it is difficult to make a neat appearing cell, besides making it almost impossible to manipulate its contents. A coat of paraffin oil approximately 1/2 inch in thickness (about 6 ounces) gives perfect protection against evaporation and creeping of the zinc sulphate salts. The cell, having the paraffin-oil seal, had a very neat, clean appearance as compared with cells No. 1 and No. 2. It was found that the zinc could be drawn out through the oil, cleaned, and replaced with no appreciable effect on voltage or current.
Setting Up:—In setting up the battery the copper electrode is first unfolded to form a cross and placed in the bottom of the jar. Enough copper sulphate, or blue-stone crystals, is then dropped into the jar to almost cover the copper. The zinc crowfoot is then hung in place, occupying a position about 4 inches above the top of the copper. Clear water is then poured in sufficient to fill the jar within about an inch of the top.
If it is not required to use the cell at once, it may be placed on short circuit for a time and allowed to form its own zinc sulphate. The cell may, however, be made immediately available for use by Page 109 drawing about one-half pint of a solution of zinc sulphate from a cell already in use and pouring it into the jar, or, when this is not convenient, by putting into the liquid four or five ounces of pulverized sulphate of zinc, or by adding about ten drops of sulphuric acid. When the cell is in proper working condition, one-half inch in thickness of heavy paraffin oil of good quality may be added.
If the blue line gets too low, and if there is in the bottom of the cell a sufficient quantity of sulphate of copper, it may be raised by drawing off a portion of the zinc sulphate with a battery syringe and replacing this with water. If the blue line gets too high, it may be lowered by short-circuiting the cell for a time, or by the addition of more sulphate of zinc solution from another battery. If the copper sulphate becomes exhausted, it should be replenished by dropping in more crystals.
Care should be taken in cold weather to maintain the temperature of the battery above 65° or 70° Fahrenheit. If below this temperature, the internal resistance of a cell increases very rapidly, so much so that even at 50° Fahrenheit the action becomes very much impaired. This follows from the facts that the resistance of a liquid decreases as its temperature rises, and that chemical action is much slower at lower temperatures.
The gravity cell has a practically constant voltage of 1.08 volts. Its internal resistance is comparatively high, seldom falling below 1 ohm and often rising to 6 ohms. At best, therefore, it is only capable of producing about 1 ampere. The gravity cell is perhaps the most common type of cell wherein depolarization is affected by electro-chemical means.
Fuller Cell:—A form of cell that is adapted to very heavy open-circuit work and also closed-circuit work where heavier currents are required than can be supplied by the gravity battery is the Fuller. In this the electrodes are of zinc and carbon, respectively, the zinc usually being in the form of a heavy cone and placed within a porous cup. The electrolyte of the Fuller cell is known as electropoion fluid, and consists of a mixture of sodium or potassium bichromate, sulphuric acid, and water.
The various parts of the standard Fuller cell, as once largely employed by the various Bell operating companies, are shown in Fig. 65. In this the jar was made of flint glass, cylindrical in form, Page 110 six inches in diameter and eight inches deep. It is important that a good grade of glass be used for the jar in this cell, because, on account of the nature of the electrolyte, breakage is disastrous in the effects it may produce on adjacent property. The carbon plate is rectangular in form, about four inches wide, eight and three-quarters inches long, and one-quarter inch thick. The metal terminal at the top of the carbon block is of bronze, both it and the lock nuts and bolts being nickel-plated to minimize corrosion. The upper end of the carbon block is soaked in paraffin so hot as to drive all of the moisture out of the paraffin and out of the pores of the block itself.
The zinc, as is noted from the cut, is in the form of a truncated cone. It is about two and one-eighth inches in diameter at the base and two and one-half inches high. Cast into the zinc is a soft copper wire about No. 12 B. & S. gauge. This wire extends above the top of the jar so as to form a convenient terminal for the cell.
The porous cup is cylindrical in form, about three inches in diameter and seven inches deep. The wooden cover is of kiln-dried white wood thoroughly coated with two coats of asphalt paint. It is provided with a slot for the carbon and a hole for the copper wire extending to the zinc.
The electrolyte for this cell is made as follows:
Sodium bichromate 6 oz.
Sulphuric acid 17 oz.
Soft water 56 oz.
This solution is mixed by dissolving the bichromate of sodium in the water and then adding slowly the sulphuric acid. Potassium bichromate may be substituted for the sodium bichromate.
In setting up this cell, the amalgamated zinc is placed within the porous cup, in the bottom of which are about two teaspoonfuls of mercury, the latter serving to keep the zinc well amalgamated. The porous cup is then placed in the glass jar and a sufficient quantity of the electrolyte is placed in the outer jar to come within about one and one-half inches of the top of the porous cup. About two teaspoonfuls of salt are then placed in the porous cup and sufficient soft water added to bring the level of the liquid within the porous cup even with the level of the electrolyte in the jar surrounding the cup. The carbon is then placed through the slot in the cover, and Page 111 the wire from the zinc is passed through the hole in the cover provided for it, and the cover is allowed to fall in place. The cell is now ready for immediate use.
The action of this cell is as follows: The sulphuric acid attacks the zinc and forms zinc sulphate, liberating hydrogen. The hydrogen attempts to pass to the carbon plate as usual, but in so doing it meets with the oxygen of the chromic acid and forms water therewith. The remainder of the chromic acid combines with the sulphuric acid to form chromium sulphate.
The mercury placed in the bottom of the porous cup with the zinc keeps the zinc in a state of perpetual amalgamation. This it does by capillary action, as the mercury spreads over the entire surface of the zinc. The initial amalgamation, while not absolutely essential, helps in a measure this capillary action.
In another well-known type of the Fuller battery the carbon is a hollow cylinder, surrounding the porous cup. In this type the zinc usually took the form of a long bar having a cross-shaped section, the length of this bar being sufficient to extend the entire depth of the porous cup. This type of cell has the advantage of a somewhat lower internal resistance than the standard form just described.
Should the electrolyte become supersaturated by virtue of the battery being neglected or too heavily overworked, a set of secondary reactions will occur in the cell, resulting in the formation of the Page 112 yellow crystals upon the carbon. This seriously affects the e.m.f. of the cell and also its internal resistance. Should this occur, some of the solution should be withdrawn and dilute sulphuric acid inserted in its place and the crystals which have formed on the carbon should be carefully washed off. Should the solution lose its orange tint and turn blue, it indicates that more bichromate of potash or bichromate of sodium is needed. This cell gives an electromotive force of 2.1 volts and a very large current when it is in good condition, since its internal resistance is low.
The Fuller cell was once largely used for supplying current to telephone transmitters at subscribers' stations, where very heavy service was demanded, but the advent of the so-called common-battery systems, in some cases, and of the high-resistance transmitter, in other cases, has caused a great lessening in its use. This is fortunate as the cell is a "dirty" one to handle and is expensive to maintain.
The Fuller cell still warrants attention, however, as an available source of current, which may be found useful in certain cases of emergency work, and in supplying special but temporary needs for heavier current than the LeClanché or gravity cell can furnish.
Lalande Cell:—A type of cell, specially adapted to constant-current work, and sometimes used as a central source of current in very small common-battery exchanges is the so-called copper oxide, or Lalande cell, of which the Edison and the Gordon are types. In all of these the negatively charged element is of zinc, the positively charged element a mass of copper oxide, and the electrolyte a solution of caustic potash in water. In the Edison cell the copper oxide is in the form of a compressed slab which with its connecting copper support forms the electrode. In the Gordon and other cells of this type the copper oxide is contained loosely in a perforated cylinder of sheet copper. The copper oxide serves not only as an electrode, but also as a depolarizing agent, the liberated hydrogen in the electrolyte uniting with the oxygen of the copper oxide to form water, and leaving free metallic copper.
On open circuit the elements are not attacked, therefore there is no waste of material while the cell is not in use. This important feature, and the fact that the internal resistance is low, make this cell well adapted for all forms of heavy open-circuit work. The Page 113 fact that there is no polarizing action within the cell makes it further adaptable to heavy closed-circuit service.
These cells are intended to be so proportioned that all of their parts become exhausted at once so that when the cell fails, complete renewals are necessary. Therefore, there is never a question as to which of the elements should be renewed.
After the elements and solution are in place about one-fourth of an inch of heavy paraffin oil is poured upon the surface of the solution in order to prevent evaporation. This cell requires little attention and will maintain a constant e.m.f. of about two-thirds of a volt until completely exhausted. It is non-freezable at all ordinary temperatures. Its low voltage is its principal disadvantage.
Standard Cell. Chloride of Silver Cell:—The chloride of silver cell is largely used as a standard for testing purposes. Its compactness and portability and its freedom from local action make it particularly adaptable to use in portable testing outfits where constant electromotive force and very small currents are required.
A cross-section of one form of the cell is shown in Fig. 66. Its elements are a rod of chemically-pure zinc and a rod of chloride of silver immersed in a water solution of sal ammoniac. As ordinarily constructed, the glass jar or tube is usually about 2-1/2 inches long by 1 inch in diameter. After the solution is poured in and the elements are in place the glass tube is hermetically sealed with a plug of paraffin wax.
The e.m.f. of a cell of this type is 1.03 volts and the external resistance varies with the age of the cell, being about 4 ohms at first. Care should be taken not to short-circuit these cells, or use them in any but high-resistance circuits, as they have but little energy and become quickly exhausted if compelled to work in low-resistance circuits.
Page 114 Conventional Symbol. The conventional symbol for a cell, either of the primary or the secondary type, consists of a long thin line and a short heavy line side by side and parallel. A battery is represented by a number of pairs of such lines, as in Fig. 67. The two lines of each pair are supposed to represent the two electrodes of a cell. Where any significance is to be placed on the polarity of the cell or battery the long thin line is supposed to represent the positively charged plate and the short thick line the negatively charged plate. The number of pairs may indicate the number of cells in the battery. Frequently, however, a few pairs of such lines are employed merely for the purpose of indicating a battery without regard to its polarity or its number of cells.
In Fig. 67 the representation at A is that of a battery of a number of cells connected in parallel; that at B of a battery with the cells connected in series; and that at C of a battery with one of its poles grounded.
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Method of Signaling. The ordinary apparatus, by which speech is received telephonically, is not capable of making sufficiently loud sounds to attract the attention of people at a distance from the instrument. For this reason it is necessary to employ auxiliary apparatus for the purpose of signaling between stations. In central offices where an attendant is always on hand, the sense of sight is usually appealed to by the use of signals which give a visual indication, but in the case of telephone instruments for use by the public, the sense of hearing is appealed to by employing an audible rather than a visual signal.
Battery Bell. The ordinary vibrating or battery bell, such as is employed for door bells, is sometimes, though not often, employed in telephony. It derives its current from primary batteries or from any direct-current source. The reason why they are not employed to a greater extent in telephony is that telephone signals usually have to be sent over lines of considerable length and the voltage that would be required to furnish current to operate such bells over such lengths of line is higher than would ordinarily be found in the batteries commonly employed in telephone work. Besides this the make-and-break contacts on which the, ordinary battery bell depends for its operation are an objectionable feature from the standpoint of maintenance.
Magneto Bell. Fortunately, however, there has been developed a simpler type of electric bell, which operates on smaller currents, and which requires no make-and-break contacts whatever. This simpler form of bell is commonly known as the polarized, or magneto, bell or ringer. It requires for its operation, in its ordinary form, an alternating current, though in its modified forms it may be used with pulsating currents, that is, with periodically recurring impulses of current always in the same direction.
Page 116Magneto Generator. In the early days of telephony there was nearly always associated with each polarized bell a magneto generator for furnishing the proper kind of current to ring such bells. Each telephone was therefore equipped, in addition to the transmitter and receiver, with a signal-receiving device in the form of a polarized bell, and with a current generator by which the user was enabled to develop his own currents of suitable kind and voltage for ringing the bells of other stations.
Considering the signaling apparatus of the telephones alone, therefore, each telephone was equipped with a power plant for generating currents used by that station in signaling other stations, the prime mover being the muscles of the user applied to the turning of a crank on the side of the instrument; and also with a current-consuming device in the form of a polarized electromagnetic bell adapted to receive the currents generated at other stations and to convert a portion of their energy into audible signals.
The magneto generator is about the simplest type of dynamo-electric machine, and it depends upon the same principles of operation as the much larger generators, employed in electric-lighting and street-railway power plants, for instance. Instead of developing the necessary magnetic field by means of electromagnets, as in the case of the ordinary dynamo, the field of the magneto generator is developed by permanent magnets, usually of the horseshoe form. Hence the name magneto.
In order to concentrate the magnetic field within the space in which the armature revolves, pole pieces of iron are so arranged in connection with the poles of the permanent magnet as to afford a substantially cylindrical space in which the armature conductors may revolve and through which practically all the magnetic lines of force Page 117 set up by the permanent magnets will pass. In Fig. 68 there is shown, diagrammatically, a horseshoe magnet with such a pair of pole pieces, between which a loop of wire is adapted to rotate. The magnet 1 is of hardened steel and permanently magnetized. The pole pieces are shown at 2 and 3, each being of soft iron adapted to make good magnetic contact on its flat side with the inner flat surface of the bar magnet, and being bored out so as to form a cylindrical recess between them as indicated. The direction of the magnetic lines of force set up by the bar magnet through the interpolar space is indicated by the long horizontal arrows, this flow being from the north pole (N) to the south pole (S) of the magnet. At 4 there is shown a loop of wire supposed to revolve in the magnetic field of force on the axis 5-5.
Theory. In order to understand how currents will be generated in this loop of wire 4, it is only necessary to remember that if a conductor is so moved as to cut across magnetic lines of force, an electromotive force will be set up in the conductor which will tend to make the current flow through it. The magnitude of the electromotive force will depend on the rate at which the conductor cuts through the lines of force, or, in other words, on the number of lines of force that are cut through by the conductor in a given unit of time. Again, the direction of the electromotive force depends on the direction of the cutting, so that if the conductor be moved in one direction across the lines of force, the electromotive force and the current will be in one direction; while if it moves in the opposite direction across the lines of force, the electromotive force and the current will be in the reverse direction.
It is, evident that as the loop of wire 4 revolves in the field of force about the axis 5-5, the portions of the conductor parallel to the axis will cut through the lines of force, first in one direction and then in the other, thus producing electromotive forces therein, first in one direction and then in the other.
Referring now to Fig. 68, and supposing that the loop 4 is revolving in the direction of the curved arrow shown between the upper edges of the pole pieces, it will be evident that just as the loop stands in the vertical position, its horizontal members will be moving in a horizontal direction, parallel with the lines of force and, therefore, not cutting them at all. The electromotive force and the current will, therefore, be zero at this time.
Page 118As the loop advances toward the position shown in dotted lines, the upper portion of the loop that is parallel with the axis will begin to cut downwardly through the lines of force, and likewise the lower portion of the loop that is parallel with the axis will begin to cut upwardly through the lines of force. This will cause electromotive forces in opposite directions to be generated in these portions of the loop, and these will tend to aid each other in causing a current to circulate in the loop in the direction shown by the arrows associated with the dotted representation of the loop. It is evident that as the motion of the loop progresses, the rate of cutting the lines of force will increase and will be a maximum when the loop reaches a horizontal position, or at that time the two portions of the loop that are parallel with the axis will be traveling at right angles to the lines of force. At this point, therefore, the electromotive force and the current will be a maximum.
From this point until the loop again assumes a vertical position, the cutting of the lines of force will still be in the same direction, but at a constantly decreasing rate, until, finally, when the loop is vertical the movement of the parts of the loop that are parallel with the axis will be in the direction of the lines of force and, therefore, no cutting will take place. At this point, therefore, the electromotive force and the current in the loop again will be zero. We have seen, therefore, that in this half revolution of the loop from the time when it was in a vertical position to a time when it was again in a vertical position but upside down, the electromotive force varied from zero to a maximum and back to zero, and the current did the same.
It is easy to see that, as the loop moves through the next half revolution, an exactly similar rise and fall of electromotive force and current will take place; but this will be in the opposite direction, since that portion of the loop which was going down through the lines of force is now going up, and the portion which was previously going up is now going down.
The law concerning the generation of electromotive force and current in a conductor that is cutting through lines of magnetic force, may be stated in another way, when the conductor is bent into the form of a loop, as in the case under consideration: Thus, if the number of lines of force which pass through a conducting loop be varied, electromotive forces will be generated in the loop. This will be true Page 119 whether the number of lines passing through the loop be varied by moving the loop within the field of force or by varying the field of force itself. In any case, if the number of lines of force be increased, the current will flow in one way, and if it be diminished the current will flow in the other way. The amount of the current will depend, other things being equal, on the rate at which the lines of force through the loop are being varied, regardless of the method by which the variation is made to take place. One revolution of the loop, therefore, results in a complete cycle of alternating current consisting of one positive followed by one negative impulse.
The diagram of Fig. 68 is merely intended to illustrate the principle involved. In the practical construction of magneto generators more than one bar magnet is used, and, in addition, the conductors in the armature are so arranged as to include a great many loops of wire. Furthermore, the conductors in the armature are wound around an iron core so that the path through the armature loops or turns, may present such low reluctance to the passage of lines of force as to greatly increase the number of such lines and also to cause practically all of them to go through the loops in the armature conductor.
Armature. The iron upon which the armature conductors are wound is called the core. The core of an ordinary armature is shown in Fig. 69. This is usually made of soft gray cast iron, turned so as to form bearing surfaces at 1 and 2, upon which the entire armature may rotate, and also turned so that the surfaces 3 will be truly cylindrical with respect to the axis through the center of the shaft. The armature conductors are put on by winding the space between the two parallel faces 4 as full of insulated wire as space will admit. One end of the armature winding is soldered to the pin 5 and, therefore, makes contact with the frame of the generator, while the other end of the winding is soldered to the pin 6, which engages the stud 7, carried in an insulating bushing in a longitudinal hole in the end of the armature shaft. It is thus seen that the frame of the machine will form one terminal of the armature winding, while the insulated stud 7 will form the other terminal.
Page 120Another form of armature largely employed in recent magneto generators is illustrated in Fig. 70. In this the shaft on which the armature revolves does not form an integral part of the armature core but consists of two cylindrical studs 2 and 3 projecting from the centers of disks 4 and 5, which are screwed to the ends of the core 1. This H type of armature core, as it is called, while containing somewhat more parts than the simpler type shown in Fig. 69, possesses distinct advantages in the matter of winding. By virtue of its simpler form of winding space, it is easier to insulate and easier to wind, and furthermore, since the shaft does not run through the winding space, it is capable of holding a considerably greater number of turns of wire. The ends of the armature winding are connected, one directly to the frame and the other to an insulated pin, as is shown in the illustration.
The method commonly employed of associating the pole pieces with each other and with the permanent magnets is shown in Fig. 71. It is very important that the space in which the armature revolves shall be truly cylindrical, and that the bearings for the armature shall be so aligned as to make the axis of rotation of the armature coincide with the axis of the cylindrical surface of the Page 121 pole pieces. A rigid structure is, therefore, required and this is frequently secured, as shown in Fig. 71, by joining the two pole pieces 1 and 2 together by means of heavy brass rods 3 and 4, the rods being shouldered and their reduced ends passed through holes in flanges extending from the pole pieces, and riveted. The bearing plates in which the armature is journaled are then secured to the ends of these pole pieces, as will be shown in subsequent illustrations. This assures proper rigidity between the pole pieces and also between the pole pieces and the armature bearings.
The reason why this degree of rigidity is required is that it is necessary to work with very small air gaps between the armature core and its pole pieces and unless these generators are mechanically well made they are likely to alter their adjustment and thus allow the armature faces to scrape or rub against the pole pieces. In Fig. 71 one of the permanent horseshoe magnets is shown, its ends resting in grooves on the outer faces of the pole pieces and usually clamped thereto by means of heavy iron machine screws.
With this structure in mind, the theory of the magneto generator developed in connection with Fig. 68 may be carried a little further. When the armature lies in the position shown at the left of Fig. 71, so that the center position of the core is horizontal, a good path is afforded for the lines of force passing from one pole to the other. Practically all of these lines will pass through the iron of the core rather than through the air, and, therefore, practically all of them will pass through the convolutions of the armature winding.
When the armature has advanced, say 45 degrees, in its rotation in the direction of the curved arrow, the lower right-hand portion of the armature flange will still lie opposite the lower face of the right-hand pole piece and the upper left-hand portion of the armature flange will still lie opposite the upper face of the left-hand pole piece. As a result there will still be a good path for the lines of force through the iron of the core and comparatively little change in the number of lines passing through the armature winding. As the corners of the armature flange pass away from the corners of the pole pieces, however, there is a sudden change in condition which may be best understood by reference to the right-hand portion of Fig. 71. The lines of force now no longer find path through the center portion of the armature core—that lying at right angles to their direction of flow. Page 122 Two other paths are at this time provided through the now horizontal armature flanges which serve almost to connect the two pole pieces. The lines of force are thus shunted out of the path through the armature coils and there is a sudden decrease from a large number of lines through the turns of the winding to almost none. As the armature continues in its rotation the two paths through the flanges are broken, and the path through the center of the armature core and, therefore, through the coils themselves, is reëstablished.
As a result of this consideration it will be seen that in actual practice the change in the number of lines passing through the armature winding is not of the gradual nature that would be indicated by a consideration of Fig. 68 alone, but rather, is abrupt, as the corners of the armature flanges leave the corners of the pole pieces. This abrupt change produces a sudden rise in electromotive force just at these points in the rotation, and, therefore, the electromotive force and the current curves of these magneto generators is not usually of the smooth sine-wave type but rather of a form resembling the sine wave with distinct humps added to each half cycle.
As is to be expected from any two-pole alternating generator, there is one cycle of current for each revolution of the armature. Under ordinary conditions a person is able to turn the generator handle at the rate of about two hundred revolutions a minute, and as the ratio of gearing is about five to one, this results in about one thousand revolutions per minute of the generator, and, therefore, in a Page 123 current of about one thousand cycles per minute, this varying widely according to the person who is doing the turning.
The end plates which support the bearings for the armature are usually extended upwardly, as shown in Fig. 72, so as to afford bearings for the crank shaft. The crank shaft carries a large spur gear which meshes with a pinion in the end of the armature shaft, so that the user may cause the armature to revolve rapidly. The construction shown in Fig. 72 is typical of that of a modern magneto generator, it being understood that the permanent magnets are removed for clearness of illustration.
Fig. 73 is a view of a completely assembled generator such as is used for service requiring a comparatively heavy output. Other types of generators having two, three, or four permanent magnets instead of five, as shown in this figure, are also standard.
Referring again to Fig. 69, it will be remembered that one end of the armature winding shown diagrammatically in that figure, is terminated in the pin 5, while the other terminates in the pin 7. When the armature is assembled in the frame of the generator it is evident that the frame itself is in metallic connection with one end of the armature winding, since the pin 5 is in metallic contact with the armature casting and this is in contact with the frame of the generator through the bearings. The frame of the machine is, therefore, one terminal of the generator. When the generator is assembled a spring of one form or another always rests against the terminal pin 7 of the armature so as to form a terminal for the armature winding of such a Page 124 nature as to permit the armature to rotate freely. Such spring, therefore, forms the other terminal of the generator.
Automatic Shunt. Under nearly all conditions of practice it is desirable to have the generator automatically perform some switching function when it is operated. As an example, when the generator is connected so that its armature is in series in a telephone line, it is quite obvious that the presence of the resistance and the impedance of the armature winding would be objectionable if left in the circuit through which the voice currents had to pass. For this reason, what is termed an automatic shunt is employed on generators designed for series work; this shunt is so arranged that it will automatically shunt or short-circuit the armature winding when it is at rest and also break this shunt when the generator is operated, so as to allow the current to pass to line.
A simple and much-used arrangement for this purpose is shown in Fig. 74, where 1 is the armature; 2 is a wire leading from the frame of the generator and forming one terminal of the generator circuit; and 3 is a wire forming the other terminal of the generator circuit, this wire being attached to the spring 4, which rests against the center pin of the armature so as to make contact with the opposite end of the armature winding to that which is connected with the frame. The circuit through the armature may be traced from the terminal wire 2 through the frame; thence through the bearings to the armature 1 and through the pin to the right-hand side of the armature winding. Continuing the circuit through the winding itself, it passes to the center pin projecting from the left-hand end of the armature shaft; thence to the spring 4 which rests against this pin; and thence to the terminal wire 3.
Normally, this path is shunted by what is practically a short circuit, which may be traced from the terminal 2 through the frame Page 125 of the generator to the crank shaft 5; thence to the upper end of the spring 4 and out by the terminal wire 3. This is the condition which ordinarily exists and which results in the removal of the resistance and the impedance on the armature winding from any circuit in which the generator is placed, as long as the generator is not operated.
An arrangement is provided, however, whereby the crank shaft 5 will be withdrawn automatically from engaging with the upper end of the spring 4, thus breaking the shunt around the armature circuit, whenever the generator crank is turned. In order to accomplish this the crank shaft 5 is capable of partial rotation and of slight longitudinal movement within the hub of the large gear wheel. A spring 7 usually presses the crank shaft toward the left and into engagement with the spring 4. A pin 8 carried by the crank shaft, rests in a V-shaped notch in the end of the hub 6 and as a result, when the crank is turned the pin rides on the surface of this notch before the large gear wheel starts to turn, and thus moves the crank shaft 5 to the right and breaks the contact between it and the spring 4. Thus, as long as the generator is being operated, its armature is connected in the circuit of the line, but as soon as it becomes idle the armature is automatically short-circuited. Such devices as this are termed automatic shunts.
In still other cases it is desirable to have the generator circuit normally open so that it will not affect in any way the electrical characteristics of the line while the line is being used for talking. In this case the arrangement is made so that the generator will automatically be placed in proper circuit relation with the line when it is operated.
A common arrangement for doing this is shown in Fig. 75, wherein the spring 1 normally rests against the contact pin of the armature and forms one terminal of the armature circuit. The spring 2 is adapted to form the other terminal of the armature circuit but it is normally insulated from everything. The circuit of the generator is, therefore, open between the spring 2 and the shaft 3, Page 126 but as soon as the generator is operated the crank shaft is bodily moved to the left by means of the V-shaped notch in the driving collar 4 and is thus made to engage the spring 2. The circuit of the generator is then completed from the spring 1 through the armature pin to the armature winding; thence to the frame of the machine and through shaft 3 to the spring 2. Such devices as this are largely used in connection with so-called "bridging" telephones in which the generators and bells are adapted to be connected in multiple across the line.
A better arrangement for accomplishing the automatic switching on the part of the generator is to make no use of the crank shaft as a part of the conducting path as is the case in both Figs. 74 and 75, but to make the crank shaft, by its longitudinal movement, impart the necessary motion to a switch spring which, in turn, is made to engage or disengage a corresponding contact spring. An arrangement of this kind that is in common use is shown in Fig. 76. This needs no further explanation than to say that the crank shaft is provided on its end with an insulating stud 1, against which a switching spring 2 bears. This spring normally rests against another switch spring 3, but when the generator crank shaft moves to the right upon the turning of the crank, the spring 2 disengages spring 3 and engages spring 4, thus completing the circuit of the generator armature. It is seen that this operation accomplishes the breaking of one circuit and the making of another, a function that will be referred to later on in this work.
Pulsating Current. Sometimes it is desirable to have a generator capable of developing a pulsating current instead of an alternating current; that is, a current which will consist of impulses all in one direction rather than of impulses alternating in direction. It is obvious that this may be accomplished if the circuit of the generator be broken during each half revolution so that its circuit is completed only when current is being generated in one direction.
Page 127Such an arrangement is indicated diagrammatically in Fig. 77. Instead of having one terminal of the armature winding brought out through the frame of the generator as is ordinarily done, both terminals are brought out to a commuting device carried on the end of the armature shaft. Thus, one end of the loop representing the armature winding is shown connected directly to the armature pin 1, against which bears a spring 2, in the usual manner. The other end of the armature winding is carried directly to a disk 3, mounted on but insulated from the shaft and revolving therewith. One-half of the circumferential surface of this disk is of insulating material 4 and a spring 5 rests against this disk and bears alternately upon the conducting portion 3 or the insulating portion 4, according to the position of the armature in its revolution. It is obvious that when the generator armature is in the position shown the circuit through it is from the spring 2 to the pin 1; thence to one terminal of the armature loop; thence through the loop and back to the disk 3 and out by the spring 5. If, however, the armature were turned slightly, the spring 5 would rest on the insulating portion 4 and the circuit would be broken.
It is obvious that if the brush 5 is so disposed as to make contact with the disk 3 only during that portion of the revolution while positive current is being generated, the generator will produce positive pulsations of current, all the negative ones being cut out. If, on the other hand, the spring 5 may be made to bear on the opposite side of the disk, then it is evident that the positive impulses would all be cut out and the generator would develop only negative impulses. Such a generator is termed a "direct-current" generator or a "pulsating-current" generator.
The symbols for magneto or hand generators usually embody a simplified side view, showing the crank and the gears on one side Page 128 and the shunting or other switching device on the other. Thus in Fig. 78 are shown three such symbols, differing from each other only in the details of the switching device. The one at the left shows the simple shunt, adapted to short-circuit the generator at all times save when it is in operation. The one in the center shows the cut-in, of which another form is described in connection with Fig. 75; while the symbol at the right of Fig. 78 is of the make-and-break device, discussed in connection with Fig. 76. In such diagrammatic representations of generators it is usual to somewhat exaggerate the size of the switching springs, in order to make clear their action in respect to the circuit connections in which the generator is used.
Polarized Ringer. The polarized bell or ringer is, as has been stated, the device which is adapted to respond to the currents sent out by the magneto generator. In order that the alternately opposite currents may cause the armature to move alternately in opposite directions, these bells are polarized, i.e., given a definite magnetic set, so to speak; so the effect of the currents in the coils is not to create magnetism in normally neutral iron, but rather to alter the magnetism in iron already magnetized.
Western Electric Ringer. A typical form of polarized bell is shown in Fig. 79, this being the standard bell or ringer of the Western Electric Company. The two electromagnets are mounted side by side, as shown, by attaching their cores to a yoke piece 1 of soft iron. This yoke piece also carries the standards 2 upon which the gongs are mounted. The method of mounting is such that the standards may be adjusted slightly so as to bring the gongs closer to or farther from, the tapper.
The soft iron yoke piece 1 also carries two brass posts 3 which, in turn, carry another yoke 4 of brass. In this yoke 4 is pivoted, by means of trunnion screws, the armature 5, this extending on each side of the pivot so that its ends lie opposite the free poles of the electromagnets. From the center of the armature projects the tapper rod carrying the ball or striker which plays between the two gongs.
In order that the armature and cores may be normally polarized, a permanent magnet 6 is secured to the center of the yoke piece 1. This bends around back of the electromagnets and comes into close proximity to the armature 5. By this means one end of each of the electromagnet cores is given one polarity—say north—while the armature Page 129 is given the other polarity—say south. The two coils of the electromagnet are connected together in series in such a way that current in a given direction will act to produce a north pole in one of the free poles and a south pole in the other. If it be assumed that the permanent magnet maintains the armature normally of south polarity and that the current through the coils is of such direction as to make the left-hand core north and the right-hand core south, then it is evident that the left-hand end of the armature will be attracted and the right-hand end repelled. This will throw the tapper rod to the right and sound the right-hand bell. A reversal in current will obviously produce the opposite effect and cause the tapper to strike the left-hand bell.
An important feature in polarized bells is the adjustment between the armature and the pole pieces. This is secured in the Western Electric bell by means of the nuts 7, by which the yoke 4 is secured to the standards 3. By moving these nuts up or down on the standards the armature may be brought closer to or farther from the poles, and the device affords ready means for clamping the parts into any position to which they may have been adjusted.
Kellogg Ringer. Another typical ringer is that of the Kellogg Switchboard and Supply Company, shown in Fig. 80. This differs from that of the Western Electric Company mainly in the details by which the armature adjustment is obtained. The armature supporting yoke 1 is attached directly to the cores of the magnets, no supporting side rods being employed. Instead of providing means whereby the armature may be adjusted toward or from the poles, the Page 130 reverse practice is employed, that is, of making the poles themselves extensible. This is done by means of the iron screws 2 which form extensions of the cores and which may be made to approach or recede from the armature by turning them in such direction as to screw them in or out of the core ends.
Biased Bell. The pulsating-current generator has already been discussed and its principle of operation pointed out in connection with Fig. 77. The companion piece to this generator is the so-called biased ringer. This is really nothing but a common alternating-current polarized ringer with a light spring so arranged as to hold the armature normally in one of its extreme positions so that the tapper will rest against one of the gongs. Such a ringer is shown in Fig. 81 and needs no further explanation. It is obvious that if a current flows in the coils of such a ringer in a direction tending to move the tapper toward the left, then no sound will result because the tapper is already moved as far as it can be in that direction. If, however, currents in the opposite direction are caused to flow through the windings, then the electromagnetic attraction on the armature will overcome the pull of the spring and the tapper will move over and strike the right-hand gong. A cessation of the current will allow the spring to exert itself and throw the tapper back into engagement with the left-hand gong. A series of such pulsations in the proper direction will, therefore, cause the tapper to play between the two gongs and ring the bell as usual. A series of currents in a wrong direction will, however, produce no effect.
Page 131Conventional Symbols. In Fig. 82 are shown six conventional symbols of polarized bells. The three at the top, consisting merely of two circles representing the magnets in plan view, are perhaps to be preferred as they are well standardized, easy to draw, and rather suggestive. The three at the bottom, showing the ringer as a whole in side elevation, are somewhat more specific, but are objectionable in that they take more space and are not so easily drawn.
Symbols A or B may be used for designating any ordinary polarized ringer. Symbols C and D are interchangeably used to indicate a biased ringer. If the bell is designed to operate only on positive impulses, then the plus sign is placed opposite the symbol, while a minus sign so placed indicates that the bell is to be operated only by negative impulses.
Some specific types of ringers are designed to operate only on a given frequency of current. That is, they are so designed as to be responsive to currents having a frequency of sixty cycles per second, for instance, and to be unresponsive to currents of any other frequency. Either symbols E or F may be used to designate such ringers, and if it is desired to indicate the particular frequency of the ringer this is done by adding the proper numeral followed by a short reversed curve sign indicating frequency. Thus 50~ would indicate a frequency of fifty cycles per second.
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Purpose. In complete telephone instruments, comprising both talking and signaling apparatus, it is obviously desirable that the two sets of apparatus, for talking and signaling respectively, shall not be connected with the line at the same time. A certain switching device is, therefore, necessary in order that the signaling apparatus alone may be left operatively connected with the line while the instrument is not being used in the transmission of speech, and in order that the signaling apparatus may be cut out when the talking apparatus is brought into play.
In instruments employing batteries for the supply of transmitter current, another switching function is the closing of the battery circuit through the transmitter and the induction coil when the instrument is in use for talking, since to leave the battery circuit closed all the time would be an obvious waste of battery energy.
In the early forms of telephones these switching operations were performed by a manually operated switch, the position of which the user was obliged to change before and after each use of the telephone. The objection to this was not so much in the manual labor imposed on the user as in the tax on his memory. It was found to be practically a necessity to make this switching function automatic, principally because of the liability of the user to forget to move the switch to the proper position after using the telephone, resulting not only in the rapid waste of the battery elements but also in the inoperative condition of the signal-receiving bell. The solution of this problem, a vexing one at first, was found in the so-called automatic hook switch or switch hook, by which the circuits of the instrument were made automatically to assume their proper conditions by the mere act, on the part of the user, of removing the receiver from, or placing it upon, a conveniently arranged hook or fork projecting from the side of the telephone casing.
Page 133Automatic Operation. It may be taken as a fundamental principle in the design of any piece of telephone apparatus that is to be generally used by the public, that the necessary acts which a person must perform in order to use the device must, as far as possible, follow as a natural result from some other act which it is perfectly obvious to the user that he must perform. So in the case of the switch hook, the user of a telephone knows that he must take the receiver from its normal support and hold it to his ear; and likewise, when he is through with it, that he must dispose of it by hanging it upon a support obviously provided for that purpose.
In its usual form a forked hook is provided for supporting the receiver in a convenient place. This hook is at the free end of a pivoted lever, which is normally pressed upward by a spring when the receiver is not supported on it. When, however, the receiver is supported on it, the lever is depressed by its weight. The motion of the lever is mechanically imparted to the members of the switch proper, the contacts of which are usually enclosed so as to be out of reach of the user. This switch is so arranged that when the hook is depressed the circuits are held in such condition that the talking apparatus will be cut out, the battery circuit opened, and the signaling apparatus connected with the line. On the other hand, when the hook is in its raised position, the signaling apparatus is cut out, the talking apparatus switched into proper working relation with the line, and the battery circuit closed through the transmitter.
In the so-called common-battery telephones, where no magneto generator or local battery is included in the equipment at the subscriber's station, the mere raising of the hook serves another important function. It acts, not only to complete the circuit through the substation talking apparatus, but, by virtue of the closure of the line circuit, permits a current to flow over the line from the central-office battery which energizes a signal associated with the line at the central office. This use of the hook switch in the case of the common-battery telephone is a good illustration of the principle just laid down as to making all the functions which the subscriber has to perform depend, as far as possible, on acts which his common sense alone tells him he must do. Thus, in the common-battery telephone the subscriber has only to place the receiver at his ear and ask for what he wants. This operation automatically displays a signal at the central Page 134 office and he does nothing further until the operator inquires for the number that he wants. He has then nothing to do but wait until the called-for party responds, and after the conversation his own personal convenience demands that he shall dispose of the receiver in some way, so he hangs it up on the most convenient object, the hook switch, and thereby not only places the apparatus at his telephone in proper condition to receive another call, but also conveys to the central office the signal for disconnection.
Likewise in the case of telephones operating in connection with automatic exchanges, the hook switch performs a number of functions automatically, of which the subscriber has no conception; and while, in automatic telephones, there are more acts required of the user than in the manual, yet a study of these acts will show that they all follow in a way naturally suggested to the user, so that he need have but the barest fundamental knowledge in order to properly make use of the instrument. In all cases, in properly designed apparatus, the arrangement is such that the failure of the subscriber to do a certain required act will do no damage to the apparatus or to the system, and, therefore, will inconvenience only himself.
Design. The hook switch is in reality a two-position switch, and while at present it is a simple affair, yet its development to its high state of perfection has been slow, and its imperfections in the past have been the cause of much annoyance.
Several important points must be borne in mind in the design of the hook switch. The spring provided to lift the hook must be sufficiently strong to accomplish this purpose and yet must not be strong enough to prevent the weight of the receiver from moving the switch to its other position. The movement of this spring must be somewhat limited in order that it will not break when used a great many times, and also it must be of such material and shape that it will not lose its elasticity with use. The shape and material of the restoring spring are, of course, determined to a considerable extent by the length of the lever arm which acts on the spring, and on the space which is available for the spring.
The various contacts by which the circuit changes are brought about upon the movement of the hook-switch lever usually take the form of springs of German silver or phosphor-bronze, hard rolled so as to have the necessary resiliency, and these are usually tipped with Page 135 platinum at the points of contact so as to assure the necessary character of surface at the points where the electric circuits are made or broken. A slight sliding movement between each pair of contacts as they are brought together is considered desirable, in that it tends to rub off any dirt that may have accumulated, yet this sliding movement should not be great, as the surfaces will then cut each other and, therefore, reduce the life of the switch.
Contact Material. On account of the high cost of platinum, much experimental work has been done to find a substitute metal suitable for the contact points in hook switches and similar uses in the manufacture of telephone apparatus. Platinum is unquestionably the best known material, on account of its non-corrosive and heat-resisting qualities. Hard silver is the next best and is found in some first-class apparatus. The various cheap alloys intended as substitutes for platinum or silver in contact points may be dismissed as worthless, so far as the writers' somewhat extensive investigations have shown.
In the more recent forms of hook switches, the switch lever itself does not form a part of the electrical circuit, but serves merely as the means by which the springs that are concerned in the switching functions are moved into their alternate cooperative relations. One advantage in thus insulating the switch lever from the current-carrying portions of the apparatus and circuits is that, since it necessarily projects from the box or cabinet, it is thus liable to come in contact with the person of the user. By insulating it, all liability of the user receiving shocks by contact with it is eliminated.
Wall Telephone Hooks. Kellogg. A typical form of hook switch, as employed in the ordinary wall telephone sets, is shown in Fig. 83, this being the standard hook of the Kellogg Switchboard and Supply Company. In this the lever 1 is pivoted at the point 3 in a bracket 5 that forms the base of all the working parts and the means of securing the entire hook switch to the box or framework of the telephone. This switch lever is normally pressed upward by a spring 2, mounted on the bracket 5, and engaging the under side of the hook lever at the point 4. Attached to the lever arm 1 is an insulated pin 6. The contact springs by which the various electrical circuits are made and broken are shown at 7, 8, 9, 10, and 11, these being mounted in one group with insulated bushings between them; the entire group is secured by machine screws to a lug projecting horizontally Page 136 from the bracket 5. The center spring 9 is provided with a forked extension which embraces the pin 6 on the hook lever. It is obvious that an up-and-down motion of the hook lever will move the long spring 9 in such manner as to cause electrical contact either between it and the two upper springs 7 and 8, or between it and the two lower springs 10 and 11. The hook is shown in its raised position, which is the position required for talking. When lowered the two springs 7 and 8 are disengaged from the long spring 9 and from each other, and the three springs 9, 10, and 11 are brought into electrical engagement, thus establishing the necessary signaling conditions.
The right-hand ends of the contact springs are shown projecting beyond the insulating supports. This is for the purpose of facilitating making electrical joints between these springs and the various wires which lead from them. These projecting ends are commonly referred to as ears, and are usually provided with holes or notches into which the connecting wire is fastened by soldering.
Western Electric. Fig. 84 shows the type of hook switch quite extensively employed by the Western Electric Company in wall telephone sets where the space is somewhat limited and a compact arrangement is desired. It will readily be seen that the principle on which this hook switch operates is similar to that employed in Fig. 83, although the mechanical arrangement of the parts differs radically. The hook lever 1 is pivoted at 3 on a bracket 2, which serves to support all the other parts of the switch. The contact springs are shown at 4, 5, and 6, and this latter spring 6 is so designed as to make it serve as an actuating spring for the hook. This is accomplished by having the curved end of this spring press against the lug 7 of the hook and thus tend to raise the hook when it is relieved of the weight of the receiver. The two shorter springs 8 and 9 have no electrical function but merely serve as supports against which the springs 4 and 5 may rest, when the receiver is on the Page 137 hook, these springs 4 and 5 being given a light normal tension toward the stop springs 8 and 9. It is obvious that in the particular arrangement of the springs in this switch no contacts are closed when the receiver is on the hook.
Concerning this latter feature, it will be noted that the particular form of Kellogg hook switch, shown in Fig. 83, makes two contacts and breaks two when it is raised. Similarly the Western Electric Company's makes two contacts but does not break any when raised. From such considerations it is customary to speak of a hook such as that shown in Fig. 83 as having two make and two break contacts, and such a hook as that shown in Fig. 84 as having two make contacts.
It will be seen from either of these switches that the modification of the spring arrangement, so as to make them include a varying number of make-and-break contacts, is a simple matter, and switches of almost any type are readily modified in this respect.
Dean. In Fig. 85 is shown a decidedly unique hook switch for wall telephone sets which forms the standard equipment of the Dean Electric Company. The hook lever 1 is pivoted at 2, an auxiliary Page 138 lever 3 also being pivoted at the same point. The auxiliary lever 3 carries at its rear end a slotted lug 4, which engages the long contact spring 5, and serves to move it up and down so as to engage and disengage the spring 6, these two springs being mounted on a base lug extending from the base plate 7, upon which the entire hook-switch mechanism is mounted. The curved spring 8, also mounted on this same base, engages the auxiliary lever 3 at the point 9 and normally serves to press this up so as to maintain the contact springs 5 in engagement with contact spring 6. The switch springs are moved entirely by the auxiliary lever 3, but in order that this lever 3 may be moved as required by the hook lever 1, this lever is provided with a notched lug 10 on its lower side, which notch is engaged by a forwardly projecting lug 11 that is integral with the auxiliary lever 3. The switch lever may be bodily removed from the remaining parts of the hook switch by depressing the lug 11 with the finger, so that it disengages the notch in lug 10, and then drawing the hook lever out of engagement with the pivot stud 2, as shown in the lower portion of the figure. It will be noted that the pivotal end of the hook lever is made with a slot instead of a hole as is the customary practice.
The advantage of being able to remove the hook switch bodily from the other portions arises mainly in connection with the shipment or transportation of instruments. The projecting hooks cause the instruments to take up more room and thus make larger packing boxes necessary than would otherwise be used. Moreover, in handling the telephones in store houses or transporting them to the places where they are to be used, the projecting hook switch is particularly liable to become damaged. It is for convenience under such conditions that the Dean hook switch is made so that the switch lever may be removed bodily and placed, for instance, inside the telephone box for transportation.
Desk-Stand Hooks. The problem of hook-switch design for portable desk telephones, while presenting the same general characteristics, differs in the details of construction on account of the necessarily restricted space available for the switch contacts in the desk telephone.
Western Electric. In Fig. 86 is shown an excellent example of hook-switch design as applied to the requirements of the ordinary portable desk set. This figure is a cross-sectional view of the Page 139 base and standard of a familiar type of desk telephone. The base itself is of stamped metal construction, as indicated, and the standard which supports the transmitter and the switch hook for the receiver is composed of a black enameled or nickel-plated brass tube 1, attached to the base by a screw-threaded joint, as shown. The switch lever 2 is pivoted at 3 in a brass plug 4, closing the upper end of the tube forming the standard. This brass plug supports also the transmitter, which is not shown in this figure. Attached to the plug 4 by the screw 5 is a heavy strip 6, which reaches down through the tube to the base plate of the standard and is held therein by a screw 7. The plug 4, carrying with it the switch-hook lever 2 and the brass strip 6, may be lifted bodily out of the standard 1 by taking out the screw 7 which holds the strip 6 in place, as is clearly indicated. On the strip 6 there is mounted the group of switch springs by which the circuit changes of the instrument are brought about when the hook is raised or lowered. The spring 8 is longer than the others, and projects upwardly far enough to engage the lug on the switch-hook lever 2. This spring, which is so bent as to close the contacts at the right when not prevented by the switch lever, also serves as an actuating spring to raise the lever 2 when the receiver is removed from it. This spring, when the receiver is removed from the hook, engages the two springs at the right, as shown, or when the receiver is placed on the hook, breaks contact with the two right-hand springs and makes contact respectively with the left-hand spring and also with the contact 9 which forms the transmitter terminal.
It is seen from an inspection of this switch hook that it has two make and two break contacts. The various contact springs are connected with the several binding posts shown, these forming the Page 140 connectors for the flexible cord conductors leading into the base and up through the standard of the desk stand. By means of the conductors in this cord the circuits are led to the other parts of the instrument, such as the induction coil, call bell, and generator, if there is one, which, in the case of the Western Electric Company's desk set, are all mounted separately from the portable desk stand proper.
This hook switch is accessible in an easy manner and yet not subject to the tampering of idle or mischievous persons. By taking out the screw 7 the entire hook switch may be lifted out of the tube forming the standard, the cords leading to the various binding posts being slid along through the tube. By this means the connections to the hook switch, as well as the contact of the switch itself, are readily inspected or repaired by those whose duty it is to perform such operations.
Kellogg. In Fig. 87 is shown a sectional view of the desk-stand hook switch of the Kellogg Switchboard and Supply Company. In this it will be seen that instead of placing the switch-hook springs within the standard or tube, as in the case of the Western Electric Company, they are mounted in the base where they are readily accessible by merely taking off the base plate from the bottom of the stand. The hook lever operates on the long spring of the group of switch springs by means of a toggle joint in an obvious manner. This switch spring itself serves by its own strength to raise the hook lever when released from the weight of the receiver.
In this switch, the hook lever, and in fact the entire exposed metal portions of the instrument, are insulated from all of the contact springs and, therefore, there is little liability of shocks on the part of the person using the instrument.
Page 141Conventional Symbols. The hook switch plays a very important part in the operation of telephone circuits; for this reason readily understood conventional symbols, by which they may be conveniently represented in drawings of circuits, are desirable. In Fig. 88 are shown several symbols such as would apply to almost any circuit, regardless of the actual mechanical details of the particular hook switch which happened to be employed. Thus diagram A in Fig. 88 shows a hook switch having a single make contact and this diagram might be used to refer to the hook switch of the Dean Electric Company shown in Fig. 85, in which only a single contact is made when the receiver is removed, and none is made when it is on the hook. Similarly, diagram B might be used to represent the hook switch of the Kellogg Company, shown in Fig. 83, the arrangement being for two make and two break contacts. Likewise diagram C might be used to represent the hook switch of the Western Electric Company, shown in Fig. 84, which, as before stated, has two make contacts only. Diagram D shows another modification in which contacts made by the hook switch, when the receiver is removed, control two separate circuits. Assuming that the solid black portion represents insulation, it is obvious that the contacts are divided into two groups, one insulated from the other.
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Page 143
Electromagnet. The physical thing which we call an electromagnet, consisting of a coil or helix of wire, the turns of which are insulated from each other, and within which is usually included an iron core, is by far the most useful of all the so-called translating devices employed in telephony. In performing the ordinary functions of an electromagnet it translates the energy of an electrical current into the energy of mechanical motion. An almost equally important function is the converse of this, that is, the translation of the energy of mechanical motion into that of an electrical current. In addition to these primary functions which underlie the art of telephony, the electromagnetic coil or helix serves a wide field of usefulness in cases where no mechanical motion is involved. As impedance coils, they serve to exert important influences on the flow of currents in circuits, and as induction coils, they serve to translate the energy of a current flowing in one circuit into the energy of a current flowing in another circuit, the translation usually, but not always, being accompanied by a change in voltage.
When a current flows through the convolutions of an ordinary helix, the helix will exhibit the properties of a magnet even though the substance forming the core of the helix is of non-magnetic material, such as air, or wood, or brass. If, however, a mass of iron, such as a rod or a bundle of soft iron wires, for instance, is substituted as a core, the magnetic properties will be enormously increased. The reason for this is, that a given magnetizing force will set up in iron a vastly greater number of lines of magnetic force than in air or in any other non-magnetic material.
Magnetizing Force. The magnetizing force of a given helix is that force which tends to drive magnetic lines of force through the magnetic circuit interlinked with the helix. It is called magnetomotive force and is analogous to electromotive force, that is, the force which tends to drive an electric current through a circuit.
Page 144The magnetizing force of a given helix depends on the product of the current strength and the number of turns of wire in the helix. Thus, when the current strength is measured in amperes, this magnetizing force is expressed as ampere-turns, being the product of the number of amperes flowing by the number of turns. The magnetizing force exerted by a given current, therefore, is independent of anything except the number of turns, and the material within the core or the shape of the core has no effect upon it.
Magnetic Flux. The total magnetization resulting from a magnetizing force is called the magnetic flux, and is analogous to current. The intensity of a magnetic flux is expressed by the number of magnetic lines of force in a square centimeter or square inch.
While the magnetomotive force or magnetizing force of a given helix is independent of the material of the core, the flux which it sets up is largely dependent on the material and shape of the core—not only upon this but on the material that lies in the return path for the flux outside of the core. We may say, therefore, that the amount of flux set up by a given current in a given coil or helix is dependent on the material in the magnetic path or magnetic circuit, and on the shape and length of that circuit. If the magnetic circuit be of air or brass or wood or any other non-magnetic material, the amount of flux set up by a given magnetizing force will be relatively small, while it will be very much greater if the magnetic circuit be composed in part or wholly of iron or steel, which are highly magnetic substances.
Permeability. The quality of material, which permits of a given magnetizing force setting up a greater or less number of lines of force within it, is called its permeability. More accurately, the permeability is the ratio existing between the amount of magnetization and the magnetizing force which produces such magnetization.
The permeability of a substance is usually represented by the Greek letter µ (pronounced mu). The intensity of the magnetizing force is commonly symbolized by H, and since the permeability of air is always taken as unity, we may express the intensity of magnetizing force by the number of lines of force per square centimeter which it sets up in air.
Now, if the space on which the given magnetizing force H were acting were filled with iron instead of air, then, owing to the greater permeability of iron, there would be set up a very much greater number Page 145 of lines of force per square centimeter, and this number of lines of force per square centimeter in the iron is the measure of the magnetization produced and is commonly expressed by the letter B.
From this we have
µ = B ÷ H
Thus, when we say that the permeability of a given specimen of wrought iron under given conditions is 2,000, we mean that 2,000 times as many lines of force would be induced in a unit cross-section of this sample as would be induced by the same magnetizing force in a corresponding unit cross-section of air. Evidently for air B = H, hence µ becomes unity.
The permeability of air is always a constant. This means that whether the magnetic density of the lines of force through the air be great or small the number of lines will always be proportional to the magnetizing force. Unfortunately for easy calculations in electromagnetic work, however, this is not true of the permeability of iron. For small magnetic densities the permeability is very great, but for large densities, that is, under conditions where the number of lines of force existing in the iron is great, the permeability becomes smaller, and an increase in the magnetizing force does not produce a corresponding increase in the total flux through the iron.
Magnetization Curves. This quality of iron is best shown by the curves of Fig. 89, which illustrate the degree of magnetization set up in various kinds of iron by different magnetizing forces. In these curves the ordinates represent the total magnetization B, while the abscissas represent the magnetizing force H. It is seen from an inspection of these curves that as the magnetizing force H increases, the intensity of flux also increases, but at a gradually lessening rate, indicating a reduction in permeability at the higher densities. These curves are also instructive as showing the great differences that exist between the permeability of the different kinds of iron; and also as showing how, when the magnetizing force becomes very great, the iron approaches what is called saturation, that is, a point at which the further increase in magnetizing force will result in no further magnetization of the core.
Page 146From the data of the curves of Fig. 89, which are commonly called magnetization curves, it is easy to determine other data from which so-called permeability curves may be plotted. In permeability curves the total magnetization of the given pieces of iron are plotted as abscissas, while the corresponding permeabilities are plotted as ordinates.
Direction of Lines of Force. The lines of force set up within the core of a helix always have a certain direction. This direction always depends upon the direction of the flow of current around the core. An easy way to remember the direction is to consider the helix as grasped in the right hand with the fingers partially encircling it and the thumb pointing along its axis. Then, if the current through the convolutions of the helix be in the direction in which the fingers of the hand are pointed around the helix, the magnetic lines of force will proceed through the core of the helix along the direction in which the thumb is pointed.
In the case of a simple bar electromagnet, such as is shown in Fig. 90, the lines of force emerging from one end of the bar must pass back through the air to the other end of the bar, as indicated by dotted Page 147 lines and arrows. The path followed by the magnetic lines of force is called the magnetic circuit, and, therefore, the magnetic circuit of the magnet shown in Fig. 90 is composed partly of iron and partly of air. From what has been said concerning the relative permeability of air and of iron, it will be obvious that the presence of such a long air path in the magnetic circuit will greatly reduce the number of lines of force that a given magnetizing force can set up. The presence of an air gap in a magnetic circuit has much the same effect on the total flow of lines of force as the presence of a piece of bad conductor in a circuit composed otherwise of good conductor, in the case of the flow of electric current.
Reluctance. As the property which opposes the flow of electric current in an electrical circuit is called resistance, so the property which opposes the flow of magnetic lines of force in a magnetic circuit is called reluctance. In the case of the electric circuit, the resistance is the reciprocal of the conductivity; in the case of the magnetic circuit, the reluctance is the reciprocal of the permeability. As in the case of an electrical circuit, the amount of flow of current is equal to the electromotive force divided by the resistance; so in a magnetic circuit, the magnetic flux is equal to the magnetizing force or magnetomotive force divided by the reluctance.
Types of Low-Reluctance Circuits. As the pull of an electromagnet upon its armature depends on the total number of lines of force passing from the core to the armature—that is, on the total flux—and as the total flux depends for a given magnetizing force on the reluctance of the magnetic circuit, it is obvious that the design of the electromagnetic circuit is of great importance in influencing the action of the magnet. Obviously, anything that will reduce the amount of air or other non-magnetic material that is in the magnetic circuit will tend to reduce the reluctance, and, therefore, to increase the total magnetization resulting from a given magnetizing force.
Horseshoe Form. One of the easiest and most common ways of reducing reluctance in a circuit is to bend the ordinary bar electromagnet Page 148 into horseshoe form. In order to make clear the direction of current flow, attention is called to Fig. 91. This is intended to represent a simple bar of iron with a winding of one direction throughout its length. The gap in the middle of the bar, which divides the winding into two parts, is intended merely to mark the fact that the winding need not cover the whole length of the bar and still will be able to magnetize the bar when the current passes through it. In Fig. 92 a similar bar is shown with similar winding upon it, but bent into U-form, exactly as if it had been grasped in the hand and bent without further change. The magnetic polarity of the two ends of the bar remain the same as before for the same direction of current, and it is obvious that the portion of the magnetic circuit which extends through air has been very greatly shortened by the bending. As a result, the magnetic reluctance of the circuit has been greatly decreased and the strength of the magnet correspondingly increased.
If the armature of the electromagnet shown in Fig. 92 is long enough to extend entirely across the air gap from the south to the north pole, then the air gap in the magnetic circuit is still further shortened, and is now represented only by the small gap between the ends of the armature and the ends of the core. Such a magnet, with an armature closely approaching the poles, is called a closed-circuit magnet, since Page 149 the only gap in the iron of the magnetic circuit is that across which the magnet pulls in attracting its armature.
In Fig. 93 is shown the electrical and magnetic counterpart of Fig. 92. The fact that the magnetic circuit is not a single iron bar but is made up of two cores and one backpiece rigidly secured together, has no bearing upon the principle, but only shows that a modification of construction is possible. In the construction of Fig. 93 the armature 1 is shown as being pulled directly against the two cores 2 and 3, these two cores being joined by a yoke 4, which, like the armature and the core, is of magnetic material. The path of the lines of force is indicated by dotted lines. This is a very important form of electromagnet and is largely used in telephony.
Iron-Clad Form. Another way of forming a closed-circuit magnet that is widely used in telephony is to enclose the helix or winding in a shell of magnetic material which joins the core at one end. This construction results in what is known as the tubular or iron-clad electromagnet, which is shown in section and in end view in Fig. 94. In this the core 1 is a straight bar of iron and it lies centrally within a cylindrical shell 2, also of iron. The bar is usually held in place within the shell by a screw, as shown. The lines of force set up in the core by the current flowing through the coil, pass to the center of the bottom of the iron shell and thence return through the metal of the shell, through the air gap between the edges of the shell and the armature, and then concentrate at the center of the armature and pass back to the end of the core. This is a highly efficient form of closed-circuit magnet, since the magnetic circuit is of low reluctance.
Such forms of magnets are frequently used where it is necessary to mount a large number of them closely together and where it is desired that the current flowing in one magnet shall produce no inductive effect in the coils of the adjacent magnets. The reason why mutual induction between adjacent magnets is obviated in the case Page 150 of the iron-clad or tubular magnet is that practically all stray field is eliminated, since the return path for the magnetic lines is so completely provided for by the presence of the iron shell.
Special Horseshoe Form. In Fig. 95 is shown a type of relay commonly employed in telephone circuits. The purpose of illustrating it in this chapter is not to discuss relays, but rather to show an adaptation of an electromagnet wherein low reluctance of the magnetic circuit is secured by providing a return leg for the magnetic lines developed in the core, thus forming in effect a horseshoe magnet with a winding on one of its limbs only. To the end of the core 1 there is secured an L-shaped piece of soft iron 2. This extends upwardly and then forwardly throughout the entire length of the magnet core. An L-shaped armature 3 rests on the front edge of the piece 2 so that a slight rocking motion will be permitted on the "knife-edge" bearing thus afforded. It is seen from the dotted lines that the magnetic circuit is almost a closed one. The only gap is that between the lower end of the armature 3 and the front end of the core. When the coil is energized, this gap is closed by the attraction of the armature. As a result, the rearwardly projecting end of the armature 3 is raised and this raises the spring 4 and causes it to break the normally existing contact with the spring 5 and to establish another contact with the spring 6. Thus the energy developed within the coil of the magnet is made to move certain parts which in turn operate the switching devices to produce changes in electrical circuits. These relays and other adaptations of the electromagnet will be discussed more fully later on.
There are almost numberless forms of electromagnets, but we have illustrated here examples of the principal types employed in telephony, and the modifications of these types will be readily understood in view of the general principles laid down.
Page 151Direction of Armature Motion. It may be said in general that the armature of an electromagnet always moves or tends to move, when the coil is energized, in such a way as to reduce the reluctance of the magnetic circuit through the coil. Thus, in all of the forms of electromagnets discussed, the armature, when attracted, moves in such a direction as to shorten the air gap and to introduce the iron of the armature as much as possible into the path of the magnetic lines, thus reducing the reluctance. In the case of a solenoid type of electromagnet, or the coil and plunger type, which is a better name than solenoid, the coil, when energized, acts in effect to suck the iron core or plunger within itself so as to include more and more of the iron within the most densely occupied portion of the magnetic circuit.
Differential Electromagnet. Frequently in telephony, the electromagnets are provided with more than one winding. One purpose of the double-wound electromagnet is to produce the so-called differential action between the two windings, i.e., making one of the windings develop magnetization in the opposite direction from that of the other, so that the two will neutralize each other, or at least exert different and opposite influences. The principle of the differential electromagnet may be illustrated in connection with Fig. 96. Here two wires 1 and 2 are shown wrapped in the same direction about an iron core, the ends of the wire being joined together at 3. Obviously, if one of these windings only is employed and a current sent through it, as by connecting the terminals of a battery with the points 4 and 3, for instance, the core will be magnetized as in an ordinary magnet. Likewise, the core will be energized if a current be sent from 5 to 3. Assuming that the two windings are of equal resistance and number of turns, the effects so produced, when either the coil 1 or the coil 2 is energized, will be equal. If the battery be connected between the terminals 4 and 5 with the positive pole, say, at 5, then the current will proceed through the winding 2 and tend to generate magnetism Page 152 in the core in the direction of the arrow. After traversing the winding 2, however, it will then begin to traverse the other winding 1 and will pass around the core in the opposite direction throughout the length of that winding. This will tend to set up magnetism in the core in the opposite direction to that indicated by the arrow. Since the two currents are equal and also the number of turns in each winding, it is obvious that the two magnetizing influences will be exactly equal and opposite and no magnetic effect will be produced. Such a winding, as is shown in Fig. 96, where the two wires are laid on side by side, is called a parallel differential winding.
Another way of winding magnets differentially is to put one winding on one end of the core and the other winding on the other end of the core and connect these so as to cause the currents through them to flow around the core in opposite directions. Such a construction is shown in Fig. 97 and is called a tandem differential winding. The tandem arrangement, while often good enough for practical purposes, cannot result in the complete neutralization of magnetic effect. This is true because of the leakage of some of the lines of force from intermediate points in the length of the core through the air, resulting in some of the lines passing through more of the turns of one coil than of the other. Complete neutralization can only be attained by first twisting the two wires together with a uniform lay and then winding them simultaneously on the core.
Mechanical Details. We will now consider the actual mechanical construction of the electromagnet. This is a very important feature of telephone work, because, not only must the proper electrical and magnetic effects be produced, but also the whole structure of the magnet must be such that it will not easily get out of order and not be affected by moisture, heat, careless handling, or other adverse conditions.
Page 153The most usual form of magnet construction employed in telephony is shown in Fig. 98. On the core, which is of soft Norway iron, usually cylindrical in form, are forced two washers of either fiber or hard rubber. Fiber is ordinarily to be preferred because it is tougher and less liable to breakage. Around the core, between the two heads, are then wrapped several layers of paper or specially prepared cloth in order that the wire forming the winding may be thoroughly insulated from the core. One end of the wire is then passed through a hole in one of the spool heads or washers, near the core, and the wire is then wound on in layers. Sometimes a thickness of paper is placed around each layer of wire in order to further guard against the breaking down of the insulation between layers. When the last layer is wound on, the end of the wire is passed out through a hole in the head, thus leaving both ends projecting.
Magnet Wire. The wire used in winding magnets is, of course, an important part of the electromagnet. It is always necessary that the adjacent turns of the wire be insulated from each other so that the current shall be forced to pass around the core through all the length of wire in each turn rather than allowing it to take the shorter and easier path from one turn to the next, as would be the case if the turns were not insulated. For this purpose the wire is usually covered with a coating of some insulating material. There are, however, methods of winding magnet coils with bare wire and taking care of the insulation between the turns in another way, as will be pointed out.
Insulated wire for the purpose of winding magnet coils is termed magnet wire. Copper is the material almost universally employed for the conductor. Its high conductivity, great ductility, and low cost are the factors which make it superior to all other metals. However, in special cases, where exceedingly high conductivity is required with a limited winding space, silver wire is sometimes employed, and on the other hand, where very high resistance is desired within a limited winding space, either iron or German silver or some other high-resistance alloy is used.
Page 154Wire Gauges. Wire for electrical purposes is drawn to a number of different standard gauges. Each of the so-called wire gauges consists of a series of graded sizes of wire, ranging from approximately one-half an inch in diameter down to about the fineness of a lady's hair. In certain branches of telephone work, such as line construction, the existence of the several wire gauges or standards is very likely to lead to confusion. Fortunately, however, so far as magnet wire is concerned, the so-called Brown and Sharpe, or American, wire gauge is almost universally employed in this country. The abbreviations for this gauge are B.&S. or A.W.G.
TABLE III
Copper Wire Table
Giving weights, lengths, and resistances of wire @ 68° F., of Matthiessen's Standard Conductivity.
| A.W.G. B.&S. | DIAMETER Mils | AREA Circular Mils | RESISTANCE | LENGTH | WEIGHT | |||
| Ohms per Pound | Ohms Per Foot | Feet per Pound | Feet per Ohm | Pounds per Foot | Pounds per Ohm | |||
| 0000 | 460. | 211,600. | 0.00007639 | 0.0000489 | 1.561 | 20,440. | 0.6405 | 13,090. |
| 000 | 409.6 | 167,800. | 0.0001215 | 0.0000617 | 1.969 | 16,210. | 0.5080 | 8,232. |
| 00 | 364.8 | 133,100. | 0.0001931 | 0.0000778 | 2.482 | 12,850. | 0.4028 | 5,177. |
| 0 | 324.9 | 105,500. | 0.0003071 | 0.0000981 | 3.130 | 10,190. | 0.3195 | 3,256. |
| 1 | 289.3 | 83,690. | 0.0004883 | 0.0001237 | 3.947 | 8,083. | 0.2533 | 2,048. |
| 2 | 257.6 | 66,370. | 0.0007765 | 0.0001560 | 4.977 | 6,410. | 0.2009 | 1,288. |
| 3 | 229.4 | 52,630. | 0.001235 | 0.0001967 | 6.276 | 5,084. | 0.1593 | 810.0 |
| 4 | 204.3 | 41,740. | 0.001963 | 0.0002480 | 7.914 | 4,031. | 0.1264 | 509.4 |
| 5 | 181.9 | 33,100. | 0.003122 | 0.0003128 | 9.980 | 3,197. | 0.1002 | 320.4 |
| 6 | 162.0 | 26,250. | 0.004963 | 0.0003944 | 12.58 | 2,535. | 0.07946 | 201.5 |
| 7 | 144.3 | 20,820. | 0.007892 | 0.0004973 | 15.87 | 2,011. | 0.06302 | 126.7 |
| 8 | 128.5 | 16,510. | 0.01255 | 0.0006271 | 20.01 | 1,595. | 0.04998 | 79.69 |
| 9 | 114.4 | 13,090. | 0.01995 | 0.0007908 | 25.23 | 1,265. | 0.03963 | 50.12 |
| 10 | 101.9 | 10,380. | 0.03173 | 0.0009273 | 31.82 | 1,003. | 0.03143 | 31.52 |
| 11 | 90.74 | 8,234. | 0.05045 | 0.001257 | 40.12 | 795.3 | 0.02493 | 19.82 |
| 12 | 80.81 | 6,530. | 0.08022 | 0.001586 | 50.59 | 630.7 | 0.01977 | 12.47 |
| 13 | 71.96 | 5,178. | 0.1276 | 0.001999 | 63.79 | 500.1 | 0.01568 | 7.840 |
| 14 | 64.08 | 4,107. | 0.2028 | 0.002521 | 80.44 | 396.6 | 0.01243 | 4.931 |
| 15 | 57.07 | 3,257. | 0.3225 | 0.003179 | 101.4 | 314.5 | 0.009858 | 3.101 |
| 16 | 50.82 | 2,583. | 0.5128 | 0.004009 | 127.9 | 249.4 | 0.007818 | 1.950 |
| 17 | 45.26 | 2,048. | 0.8153 | 0.005055 | 161.3 | 197.8 | 0.006200 | 1.226 |
| 18 | 40.30 | 1,624. | 1.296 | 0.006374 | 203.4 | 156.9 | 0.004917 | 0.7713 |
| 19 | 35.89 | 1,288. | 2.061 | 0.008038 | 256.5 | 124.4 | 0.003899 | 0.4851 |
| 20 | 31.96 | 1,022. | 3.278 | 0.01014 | 323.4 | 98.66 | 0.003092 | 0.3051 |
| 21 | 28.46 | 810.1 | 5.212 | 0.01278 | 407.8 | 78.24 | 0.002452 | 0.1919 |
| 22 | 25.35 | 642.4 | 8.287 | 0.01612 | 514.2 | 62.05 | 0.001945 | 0.1207 |
| 23 | 22.57 | 509.5 | 13.18 | 0.02032 | 648.4 | 49.21 | 0.001542 | 0.07589 |
| 24 | 20.10 | 404.0 | 20.95 | 0.02563 | 817.6 | 39.02 | 0.001223 | 0.04773 |
| 25 | 17.90 | 320.4 | 33.32 | 0.03231 | 1,031. | 30.95 | 0.0009699 | 0.03002 |
| 26 | 15.94 | 254.1 | 52.97 | 0.04075 | 1,300. | 24.54 | 0.0007692 | 0.1187 |
| 27 | 14.2 | 201.5 | 84.23 | 0.05138 | 1,639. | 19.46 | 0.0006100 | 0.01888 |
| 28 | 12.64 | 159.8 | 133.9 | 0.06479 | 2,067. | 15.43 | 0.0004837 | 0.007466 |
| 29 | 11.26 | 126.7 | 213.0 | 0.08170 | 2,607. | 12.24 | 0.0003836 | 0.004696 |
| 30 | 10.03 | 100.5 | 338.6 | 0.1030 | 3,287. | 9.707 | 0.0003042 | 0.002953 |
| 31 | 8.928 | 79.70 | 538.4 | 0.1299 | 4,145. | 7.698 | 0.0002413 | 0.001857 |
| 32 | 7.950 | 63.21 | 856.2 | 0.1638 | 5,227. | 6.105 | 0.0001913 | 0.001168 |
| 33 | 7.080 | 50.13 | 1,361. | 0.2066 | 6,591. | 4.841 | 0.0001517 | 0.0007346 |
| 34 | 6.305 | 39.75 | 2,165. | 0.2605 | 8,311. | 3.839 | 0.0001203 | 0.0004620 |
| 35 | 5.615 | 31.52 | 3,441. | 0.3284 | 10,480. | 3.045 | 0.00009543 | 0.0002905 |
| 36 | 5.0 | 25.0 | 5,473. | 0.4142 | 13,210. | 2.414 | 0.00007568 | 0.0001827 |
| 37 | 4.453 | 19.83 | 8,702. | 0.5222 | 16,660. | 1.915 | 0.00006001 | 0.0001149 |
| 38 | 3.965 | 15.72 | 13,870. | 0.6585 | 21,010. | 1.519 | 0.00004759 | 0.00007210 |
| 39 | 3.531 | 12.47 | 22,000. | 0.8304 | 26,500. | 1.204 | 0.00003774 | 0.00004545 |
| 40 | 3.145 | 9.888 | 34,980. | 1.047 | 33,410. | 0.9550 | 0.00002993 | 0.00002858 |
Page 155
In the Brown and Sharpe gauge the sizes, beginning with the largest, are numbered 0000, 000, 00, 0, 1, 2, and so on up to 40. Sizes larger than about No. 16 B.&S. gauge are seldom used as magnet wire in telephony, but for the purpose of making the list complete, Table III is given, including all of the sizes of the B.&S. gauge.
In Table III there is given for each gauge number the diameter of the wire in mils (thousandths of an inch); the cross-sectional area in circular mils (a unit area equal to that of a circle having a diameter of one one-thousandth of an inch); the resistance of the wire in various units of length and weight; the length of the wire in terms of resistance and of weight; and the weight of the wire in terms of its length and resistance.
It is to be understood that in Table III the wire referred to is bare wire and is of pure copper. It is not commercially practicable to use absolutely pure copper, and the ordinary magnet wire has a conductivity equal to about 98 per cent of that of pure copper. The figures given in this table are sufficiently accurate for all ordinary practical purposes.
Silk and Cotton Insulation. The insulating material usually employed for covering magnet wire is of silk or cotton. Of these, silk is by far the better material for all ordinary purposes, since it has a much higher insulating property than cotton, and is very much thinner. Cotton, however, is largely employed, particularly in the larger sizes of magnet wire. Both of these materials possess the disadvantage of being hygroscopic, that is, of readily absorbing moisture. This disadvantage is overcome in many cases by saturating the coil after it is wound in some melted insulating compound, such as wax or varnish or asphaltum, which will solidify on cooling. Where the coils are to be so saturated the best practice is to place them in a vacuum chamber and exhaust the air, after which the hot insulating compound is admitted and is thus drawn into the innermost recesses of the winding space.
Silk-insulated wire, as regularly produced, has either one or two Page 156 layers of silk. This is referred to commercially as single silk wire or as double silk wire. The single silk has a single layer of silk fibers wrapped about it, while the double silk has a double layer, the two layers being put on in reverse direction. The same holds true of cotton insulated wire. Frequently, also, there is a combination of the two, consisting of a single or a double wrapping of silk next to the wire with an outer wrapping of cotton. Where this is done the cotton serves principally as a mechanical protection for the silk, the principal insulating properties residing in the silk.
Enamel. A later development in the insulation of magnet wire has resulted in the so-called enamel wire. In this, instead of coating the wire with some fibrous material such as silk or cotton, the wire is heated and run through a bath of fluid insulating material or liquid enamel, which adheres to the wire in a very thin coating. The wire is then run through baking ovens, so that the enamel is baked on. This process is repeated several times so that a number of these thin layers of the enamel are laid on and baked in succession.
The characteristics sought in good enamel insulation for magnet wire may be thus briefly set forth: It is desirable for the insulation to possess the highest insulating qualities; to have a glossy, flawless surface; to be hard without being brittle; to adhere tenaciously and stand all reasonable handling without cracking or flaking; to have a coefficient of elasticity greater than the wire itself; to withstand high temperatures; to be moisture-proof and inert to corrosive agencies; and not to "dry out" or become brittle over a long period of time.
Space Utilization. The utilization of the winding space in an electromagnet is an important factor in design, since obviously the copper or other conductor is the only part of the winding that is effective in setting up magnetizing force. The space occupied by the insulation is, in this sense, waste space. An ideally perfect winding may be conceived as one in which the space is all occupied by wire; and this would necessarily involve the conception of wire of square cross-section and insulation of infinite thinness. In such a winding there would be no waste of space and a maximum amount of metal employed as a conductor. Of course, such a condition is not possible to attain and in practice some insulating material must be introduced between the layers of wire and between the adjacent Page 157 convolutions of wire. The ratio of the space occupied by the conductor to the total space occupied by the winding, that is, by the conductor and the insulation, is called the coefficient of space utilization of the coil. For the ideal coil just conceived the coefficient of space utilization would be 1. Ordinarily the coefficient of space utilization is greater for coarse wire than for fine wire, since obviously the ratio of the diameter of the wire to the thickness of the insulation increases as the size of the wire grows larger.
The chief advantage of enamel insulation for magnet wire is its thinness, and the high coefficient of space utilization which may be secured by its use. In good enamel wire the insulation will average about one-quarter the thickness of the standard single silk insulation, and the dielectric strength is equal or greater. Where economy of winding space is desirable the advantages of this may readily be seen. For instance, in a given coil wound with No. 36 single silk wire about one-half of the winding space is taken up with the insulation, whereas when the same coil is wound with No. 36 enameled wire only about one-fifth of the winding space is taken up by the insulation. Thus the coefficient of space utilization is increased from .50 to .80. The practical result of this is that, in the case of any given winding space where No. 36 wire is used, about 60 per cent more turns can be put on with enameled wire than with single silk insulation, and of course this ratio greatly increases when the comparison is made with double silk insulation or with cotton insulation. Again, where it is desired to reduce the winding space and keep the same number of turns, an equal number of turns may be had with a corresponding reduction of winding space where enameled wire is used in place of silk or cotton.
In the matter of heat-resisting properties the enameled wire possesses a great advantage over silk and cotton. Cotton or silk insulation will char at about 260° Fahrenheit, while good enameled wire will stand 400° to 500° Fahrenheit without deterioration of the insulation. It is in the matter of liability to injury in rough or careless handling, or in winding coils having irregular shapes, that enamel wire is decidedly inferior to silk or cotton-covered wire. It is likely to be damaged if it is allowed to strike against the sharp corners of the magnet spool during winding, or run over the edge of a hard surface while it is being fed on to the spool. Coils having other than Page 158 round cores, or having sharp corners on their spool heads, should not ordinarily be wound with enamel wire.
The dielectric strength of enamel insulation is much greater than that of either silk or cotton insulation of equal thickness. This is a distinct advantage and frequently a combination of the two kinds of insulation results in a superior wire. If wire insulated with enamel is given a single wrapping of silk or of cotton, the insulating and dielectric properties of the enamel is secured, while the presence of the silk and cotton affords not only an additional safeguard against bare spots in the enamel but also a certain degree of mechanical protection to the enamel.
Winding Methods. In winding a coil, the spool, after being properly prepared, is placed upon a spindle which may be made to revolve rapidly. Ordinarily the wire is guided on by hand; sometimes, however, machinery is used, the wire being run over a tool which moves to and fro along the length of the spool, just fast enough to lay the wire on at the proper rate. The movement of this tool is much the same as that of the tool in a screw cutting lathe.
Unless high voltages are to be encountered, it is ordinarily not necessary to separate the layers of wire with paper, in the case of silk-or cotton-insulated magnet wire; although where especially high insulation resistance is needed this is often done. It is necessary to separate the successive layers of a magnet that is wound with enamel wire, by sheets of paper or thin oiled cloth.
In Fig. 99 is shown a method, that has been used with some success, of winding magnets with bare wire. In this the various adjacent turns are separated from each other by a fine thread of silk or cotton wound on beside the wire. Each layer of wire and thread as it is placed on the core is completely insulated from the subsequent layer by a layer of paper. This is essentially a machine-wound coil, and machines for winding it have been so perfected that several coils are wound simultaneously, the paper being fed in automatically at the end of each layer.
Page 159Another method of winding the bare wire omits the silk thread and depends on the permanent positioning of the wire as it is placed on the coil, due to the slight sinking into the layer of paper on which it is wound. In this case the feed of the wire at each turn of the spool is slightly greater than the diameter of the wire, so that a small distance will be left between each pair of adjacent turns.
Upon the completion of the winding of a coil, regardless of what method is used, it is customary to place a layer of bookbinders' cloth over the coil so as to afford a certain mechanical protection for the insulated wire.
Winding Terminals. The matter of bringing out the terminal ends of the winding is one that has received a great deal of attention in the construction of electromagnets and coils for various purposes. Where the winding is of fine wire, it is always well to reinforce its ends by a short piece of larger wire. Where this is done the larger wire is given several turns around the body of the coil, so that the finer wire with which it connects may be relieved of all strain which may be exerted upon it from the protruding ends of the wire. Great care is necessary in the bringing out of the inner terminal—i.e., the terminal which connects with the inner layer—that the terminal wire shall not come in contact with any of the subsequent layers that are wound on.
Where economy of space is necessary, a convenient method of terminating the winding of the coil consists in fastening rigid terminals to the spool head. This, in the case of a fiber spool head, may be done by driving heavy metal terminals into the fiber. The connections of the two wires leading from the winding are then made with these heavy rigid terminals by means of solder. A coil having such terminals is shown in its finished condition in Fig. 100.
Winding Data. The two things principally affecting the manufacture of electromagnets for telephone purposes are the number of turns in a winding and the resistance of the wound wire. The latter governs the amount of current which may flow through the coil with a given difference of potential at its end, while the former control the amount of magnetism produced in the core by the current flowing. While a coil is being wound, it is a simple matter to count the turns by any simple form of revolution counter. When the coil has been completed it is a simple matter to measure its resistance. But it is not so simple to determine in advance how many turns of a given size wire may be placed on a given spool, and still less simple to know what the resistance of the wire on that spool will be when the desired turns shall have been wound.
Page 160TABLE IV
Winding Data for Insulated Wires—Silk and Cotton Covering
| A.W.G. B & S | 20 | 21 | 22 | 23 | 24 | 25 | 26 |
| DIAMETER Mils | 31.961 | 28.462 | 25.347 | 22.571 | 20.100 | 17.900 | 15.940 |
| AREA Circular Mils | 1021.20 | 810.10 | 642.70 | 509.45 | 404.01 | 320.40 | 254.01 |
| DIAMETER OVER INSULATION |
|||||||
| Single Cotton | 37.861 | 34.362 | 31.247 | 28.471 | 26.000 | 23.800 | 21.840 |
| Double Cotton | 42.161 | 38.662 | 35.547 | 32.771 | 30.300 | 28.100 | 26.140 |
| Single Silk | 34.261 | 30.762 | 27.647 | 24.871 | 22.401 | 20.200 | 18.240 |
| Double Silk | 36.161 | 32.662 | 29.547 | 26.771 | 24.300 | 22.100 | 20.140 |
| TURNS PER LINEAR INCH |
|||||||
| Single Cotton | 25.7 | 28.3 | 31.0 | 34.4 | 36.9 | 38.0 | 42.0 |
| Double Cotton | 22.5 | 24.5 | 26.7 | 28.97 | 31.35 | 33.92 | 36.29 |
| Single Silk | 27.70 | 30.97 | 34.39 | 38.19 | 42.37 | 47.02 | 52.06 |
| Double Silk | 26.22 | 29.07 | 32.11 | 35.53 | 39.14 | 42.94 | 46.81 |
| TURNS PER SQUARE INCH |
|||||||
| Single Cotton | 660.5 | 800.9 | 961.0 | 1183.0 | 1321.6 | 1444.0 | 1764.0 |
| Double Cotton | 506.3 | 600.2 | 712.9 | 839.2 | 982.8 | 1150.8 | 1317.0 |
| Single Silk | 767.3 | 959.1 | 1182.7 | 1458.5 | 1795.2 | 2210.9 | 2710.3 |
| Double Silk | 687.5 | 845.0 | 1031.0 | 1262.4 | 1532.0 | 1843.8 | 2191.2 |
| OHMS PER CUBIC INCH |
|||||||
| Single Cotton | .646 | .981 | 1.502 | 2.359 | 3.528 | 5.831 | 6.941 |
| Double Cotton | .533 | .795 | 1.188 | 1.772 | 2.595 | 3.802 | 5.552 |
| Single Silk | .801 | 1.261 | 1.956 | 3.049 | 4.739 | 7.489 | 9.031 |
| A.W.G. B & S | 27 | 28 | 29 | 30 | 31 | 32 | 33 |
| DIAMETER Mils | 14.195 | 12.641 | 11.257 | 10.025 | 8.928 | 7.950 | 7.080 |
| AREA Circular Mils | 201.50 | 159.79 | 126.72 | 100.50 | 79.71 | 63.20 | 50.13 |
| DIAMETER OVER INSULATION |
|||||||
| Single Cotton | 20.095 | 18.541 | 17.157 | 15.925 | 14.828 | 13.850 | 12.980 |
| Double Cotton | 24.395 | 22.841 | 21.457 | 20.225 | 19.128 | 18.150 | 17.280 |
| Single Silk | 16.495 | 14.941 | 13.557 | 12.325 | 11.228 | 10.250 | 9.380 |
| Double Silk | 18.395 | 16.841 | 15.457 | 14.225 | 13.128 | 12.150 | 11.280 |
| TURNS PER LINEAR INCH |
|||||||
| Single Cotton | 48.0 | 53.0 | 56.5 | 59.66 | 64.125 | 68.600 | 73.050 |
| Double Cotton | 38.95 | 41.61 | 44.27 | 46.93 | 49.78 | 52.34 | 55.10 |
| Single Silk | 57.67 | 63.36 | 70.11 | 77.14 | 84.64 | 92.72 | 101.65 |
| Double Silk | 51.59 | 56.43 | 61.56 | 66.79 | 72.39 | 78.19 | 84.17 |
| TURNS PER SQUARE INCH |
|||||||
| Single Cotton | 2304.0 | 2809.9 | 3192.3 | 3359.2 | 4112.2 | 4692.5 | 5333.5 |
| Double Cotton | 1517.2 | 1731.0 | 1959.9 | 2202.5 | 2478.0 | 2739.5 | 3036.1 |
| Single Silk | 3326.0 | 4014.5 | 4915.5 | 5950.2 | 7164.0 | 8597.5 | 10332.0 |
| Double Silk | 2661.6 | 3184.5 | 3789.8 | 4461.0 | 5240.0 | 6114.0 | 7085.0 |
| OHMS PER CUBIC INCH |
|||||||
| Single Cotton | 10.814 | 17.617 | 25.500 | 34.800 | 48.5 | 73.8 | 104.5 |
| Double Cotton | 8.078 | 11.54 | 16.47 | 23.43 | 32.83 | 46.19 | 64.30 |
| Single Silk | 13.92 | 26.86 | 41.29 | 62.98 | 95.70 | 144.70 | 217.8 |
| A.W.G. B & S | 34 | 35 | 36 | 37 | 38 | 39 | 40 |
| DIAMETER Mils | 6.304 | 5.614 | 5.000 | 4.453 | 3.965 | 3.531 | 3.144 |
| AREA Circular Mils | 39.74 | 31.52 | 25.00 | 19.83 | 15.72 | 12.47 | 9.89 |
| DIAMETER OVER INSULATION |
|||||||
| Single Cotton | 12.204 | 11.514 | 1090.0 | 10.353 | 9.865 | 9.431 | 9.044 |
| Double Cotton | 16.504 | 15.814 | 15.200 | 14.653 | 14.165 | 13.731 | 13.344 |
| Single Silk | 8.504 | 7.914 | 7.300 | 6.753 | 6.265 | 5.831 | 5.344 |
| Double Silk | 10.504 | 9.814 | 9.200 | 8.653 | 8.165 | 7.731 | 7.344 |
| TURNS PER LINEAR INCH |
|||||||
| Single Cotton | 77.900 | 82.600 | 87.100 | 91.870 | 95.000 | 100.700 | 106.000 |
| Double Cotton | 57.57 | 60.04 | 62.51 | 64.70 | 66.80 | 68.80 | 71.20 |
| Single Silk | 112.11 | 119.7 | 130.15 | 140.6 | 151.05 | 163.04 | 177.65 |
| Double Silk | 90.44 | 96.90 | 103.55 | 110.20 | 116.85 | 122.55 | 129.20 |
| TURNS PER SQUARE INCH |
|||||||
| Single Cotton | 6068.5 | 6773.3 | 7586.5 | 8440.0 | 9025.0 | 10140.5 | 11236.0 |
| Double Cotton | 3314.2 | 3605.0 | 3907.5 | 4186.1 | 4462.2 | 4733.6 | 5069.8 |
| Single Silk | 8179.5 | 9389.5 | 16940.0 | 19770.0 | 22820.0 | 26700.0 | 31559.0 |
| Double Silk | 8179.5 | 9389.5 | 10772.0 | 12145.0 | 13665.0 | 15018.0 | 16692.0 |
| OHMS PER CUBIC INCH |
|||||||
| Single Cotton | 151.4 | 202.0 | 298.8 | 418.0 | 567.0 | 811.0 | 1113.0 |
| Double Cotton | 70.58 | 125.9 | 166.3 | 225.6 | 305.5 | 409.8 | 545.5 |
| Single Silk | 342.1 | 489.0 | 721.1 | 1062.0 | 1557.0 | 2266.0 | 3400.0 |
Page 161
If the length and the depth of the winding space of the coil as well as the diameter of the core are known, it is not difficult to determine how much bare copper wire of a given size may be wound on it, but it is more difficult to know these facts concerning copper wire which has been covered with cotton or silk. Yet something may be done, and tables have been prepared for standard wire sizes with definite thicknesses of silk and cotton insulation. As a result of facts collected from a large number of actually wound coils, the number of turns per linear inch and per square inch of B.&S. gauge wires from No. 20 to No. 40 have been tabulated, and these, supplemented by a tabulation of the number of ohms per cubic inch of winding space for wires of three different kinds of insulation, are given in Table IV.
Bearing in mind that the calculations of Table IV are all based upon the "diameter over insulation," which it states at the outset for each of four different kinds of covering, it is evident what is meant by "turns per linear inch." The columns referring to "turns per square inch" mean the number of turns, the ends of which would be exposed in one square inch if the wound coil were cut in a plane passing through the axis of the core. Knowing the distance between the head, and the depth to which the coil is to be wound, it is easy to select a size of wire which will give the required number of turns in the provided space. It is to be noted that the depth of winding space is one-half of the difference between the core diameter and the complete diameter of the wound coil. The resistance of the entire volume of wound wire may be determined in advance by knowing the total cubic contents of the winding space and multiplying this by the ohms per cubic inch of the selected wire; that is, one must multiply in inches the distance between the heads of the spool by the difference between the squares of the diameters of the core and the winding space, and this in turn by .7854. This result, times the ohms Page 162 per cubic inch, as given in the table, gives the resistance of the winding.
There is a considerable variation in the method of applying silk insulation to the finer wires, and it is in the finer sizes that the errors, if any, pile up most rapidly. Yet the table throughout is based on data taken from many samples of actual coil winding by the present process of winding small coils. It should be said further that the table does not take into account the placing of any layers of paper between the successive layers of the wires. This table has been compared with many examples and has been used in calculating windings in advance, and is found to be as close an approximation as is afforded by any of the formulas on the subject, and with the further advantage that it is not so cumbersome to apply.
Winding Calculations. In experimental work, involving the winding of coils, it is frequently necessary to try one winding to determine its effect in a given circuit arrangement, and from the knowledge so gained to substitute another just fitted to the conditions. It is in such a substitution that the table is of most value. Assume a case in which are required a spool and core of a given size with a winding of, say No. 25 single silk-covered wire, of a resistance of 50 ohms. Assume also that the circuit regulations required that this spool should be rewound so as to have a resistance of, say 1,000 ohms. What size single silk-covered wire shall be used? Manifestly, the winding space remains the same, or nearly so. The resistance is to be increased from 50 to 1,000 ohms, or twenty times its first value. Therefore, the wire to be used must show in the table twenty times as many ohms per cubic inch as are shown in No. 25, the known first size. This amount would be twenty times 7.489, which is 149.8, but there is no size giving this exact resistance. No. 32, however, is very nearly of that resistance and if wound to exactly the same depth would give about 970 ohms. A few turns more would provide the additional thirty ohms.
Similarly, in a coil known to possess a certain number of turns, the table will give the size to be selected for rewinding to a greater or smaller number of turns. In this case, as in the case of substituting a winding of different resistance, it is unnecessary to measure and calculate upon the dimensions of the spool and core. Assume a spool wound with No. 30 double silk-covered wire, which requires Page 163 to be wound with a size to double the number of turns. The exact size to do this would have 8922. turns per square inch and would be between No. 34 and No. 35. A choice of these two wires may be made, using an increased winding depth with the smaller wire and a shallower winding depth for the larger wire.
Impedance Coils. In telephony electromagnets frequently serve, as already stated, to perform other functions than the producing of motion by attracting or releasing their armatures. They are required to act as impedance coils to present a barrier to the passage of alternating or other rapidly fluctuating currents, and at the same time to allow the comparatively free passage of steady currents. Where it is desired that an electromagnet coil shall possess high impedance, it is usual to employ a laminated instead of a solid core. This is done by building up a core of suitable size by laying together thin sheets of soft iron, or by forming a bundle of soft iron wires. The use of laminated cores is for the purpose of preventing eddy currents, which, if allowed to flow, would not only be wasteful of energy but would also tend to defeat the desired high impedance. Sometimes in iron-clad impedance coils, the iron shell is slotted longitudinally to break up the flow of eddy currents in the shell.
Frequently electromagnetic coils have only the function of offering impedance, where no requirements exist for converting any part of the electric energy into mechanical work. Where this is the case, such coils are termed impedance, or retardation, or choke coils, since they are employed to impede or to retard or to choke back the flow of rapidly varying current. The distinction, therefore, between an impedance coil and the coil of an ordinary electromagnet is one of function, since structurally they may be the same, and the same principles of design and construction apply largely to each.
Number of Turns. It should be remembered that an impedance coil obstructs the passage of fluctuating current, not so much by ohmic resistance as by offering an opposing or counter-electromotive force. Other things being equal, the counter-electromotive force of self-induction increases directly as the number of turns on a coil and directly as the number of lines of force threading the coil, and this latter factor depends also on the reluctance of the magnetic circuit. Therefore, to secure high impedance we need many Page 164 turns or low reluctance, or both. Often, owing to requirements for direct-current carrying capacity and limitations of space, a very large number of turns is not permissible, in which case sufficiently high impedance to such rapid fluctuations as those of voice currents may be had by employing a magnetic circuit of very low reluctance, usually a completely closed circuit.
Kind of Iron. An important factor in the design of impedance coils is the grade of iron used in the magnetic circuit. Obviously, it should be of the highest permeability and, furthermore, there should be ample cross-section of core to prevent even an approach to saturation. The iron should, if possible, be worked at that density of magnetization at which it has the highest permeability in order to obtain the maximum impedance effects.
Types. Open-Circuit:—Where very feeble currents are being dealt with, and particularly where there is no flow of direct current, an open magnetic circuit is much used. An impedance coil having an open magnetic circuit is shown in section in Fig. 101, Fig. 102 showing its external appearance and illustrating particularly the method of bringing out the terminals of the winding.
Closed-Circuit:—A type of retardation coil which is largely used in systems of simultaneous telegraphy and telephony, known as composite systems, is shown in Fig. 103. In the construction of this coil the core is made of a bundle of fine iron wires first bent into U-shape, and then after the coils are in place, the free ends of the core are brought together to form a closed magnetic circuit. The coils have a large number of turns of rather Page 165 coarse wire. The conditions surrounding the use of this coil are those which require very high impedance and rather large current-carrying capacity, and fortunately the added requirement, that it shall be placed in a very small space, does not exist.
Toroidal:—Another type of retardation coil, called the toroidal type due to the fact that its core is a torus formed by winding a continuous length of fine iron wire, is shown in diagram in Fig. 104. The two windings of this coil may be connected in series to form in effect a single winding, or it may be used as a "split-winding" coil, the two windings being in series but having some other element, such as a battery, connected between them in the circuit. Evidently such a coil, however connected, is well adapted for high impedance, on account of the low reluctance of its core.
This coil is usually mounted on a base-board, the coil being enclosed in a protecting iron case, as shown in Fig. 105. The terminal wires of both windings of each coil are brought out to terminal punchings on one end of the base-board to facilitate the making of the necessary circuit connections.
The usual diagrammatic symbol for an impedance coil is shown in Fig. 106. This is the same as for an ordinary bar magnet, except that the parallel lines through the core may be taken as indicating that the core is laminated, thus conveying the idea of high impedance. The symbol of Fig. 104 is a good one for the toroidal type of impedance coil.
Induction Coil. An induction coil consists of two or more windings of wire interlinked by a common magnetic circuit. In an induction coil having two windings, any change in the strength of the current flowing in one of the windings, called the primary, will cause corresponding changes in the magnetic flux threading the magnetic circuit, and, therefore, changes in flux through the Page 166 other winding, called the secondary. This, by the laws of electromagnetic induction, will produce corresponding electromotive forces in the secondary winding and, therefore, corresponding currents in that winding if its circuit be closed.
Current and Voltage Ratios. In a well-designed induction coil the energy in the secondary, i.e., the induced current, is for all practical purposes equal to that of the primary current, yet the values of the voltage and the amperage of the induced current may vary widely from the values of the voltage and the amperage of the primary current. With simple periodic currents, such as the commercial alternating lighting currents, the ratio between the voltage in the primary and that in the secondary will be equal to the ratio of the number of turns in the primary to the number of turns in the secondary. Since the energy in the two circuits will be practically the same, it follows that the ratio between the current in the primary and that in the secondary will be equal to the ratio of the number of turns in the secondary to the number of turns in the primary. In telephony, where the currents are not simple periodic currents, and where the variations in current strength take place at different rates, such a law as that just stated does not hold for all cases; but it may be stated in general that the induced currents will be of higher voltage and smaller current strength than those of the primary in all coils where the secondary winding has a greater number of turns than the primary, and vice versâ.
Functions. The function of the induction coil in telephony is, therefore, mainly one of transformation, that is, either of stepping up the voltage of a current, or in other cases stepping it down. The induction coil, however, does serve another purpose in cases where no change in voltage and current strength is desired, that is, it serves as a means for electrically separating two circuits so far as any conductive relation exists, and yet of allowing the free transmission by induction from one of these circuits to the other. This is a function that in telephony is scarcely of less importance than the purely transforming function.
Design. Induction coils, as employed in telephony, may be divided into two general types: first, those having an open magnetic circuit; and, second, those having a closed magnetic circuit. In the design of either type it is important that the core should be Page 167 thoroughly laminated, and this is done usually by forming it of a bundle of soft Swedish or Norway iron wire about .02 of an inch in diameter. The diameter and the length of the coil, and the relation between the number of turns in the primary and in the secondary, and the mechanical construction of the coil, are all matters which are subject to very wide variation in practice. While the proper relationship of these factors is of great importance, yet they may not be readily determined except by actual experiment with various coils, owing to the extreme complexity of the action which takes place in them and to the difficulty of obtaining fundamental data as to the existing facts. It may be stated, therefore, that the design of induction coils is nearly always carried out by "cut-and-try" methods, bringing to bear, of course, such scientific and practical knowledge as the experimenter may possess.
Use and Advantage. The use and advantages of the induction coil in so-called local-battery telephone sets have already been explained in previous chapters. Such induction coils are nearly always of the open magnetic circuit type, consisting of a long, straight core comprised of a bundle of small annealed iron wires, on which is wound a primary of comparatively coarse wire and having a small number of turns, and over which is wound a secondary of comparatively fine wire and having a very much larger number of turns. A view of such a coil mounted on a base is shown in Fig. 107, and a sectional view of a similar coil is shown in Fig. 108. The method of bringing out the winding terminals is clearly indicated in this figure, the terminal wires 2 and 4 being those of the primary winding and 1 and 3 those of the secondary winding. It is customary to bring out these wires and attach them by solder to suitable Page 168 terminal clips. In the case of the coil shown in Fig. 108 these clips are mounted on the wooden heads of the coil, while in the design shown in Fig. 107 they are mounted on the base, as is clearly indicated.
Repeating Coil. The so-called repeating coil used in telephony is really nothing but an induction coil. It is used in a variety of ways and usually has for its purpose the inductive association of two circuits that are conductively separated. Usually the repeating coil has a one to one ratio of turns, that is, there are the same number of turns in the primary as in the secondary. However, this is not always the case, since sometimes they are made to have an unequal number of turns, in which case they are called step-up or step-down repeating coils, according to whether the primary has a smaller or a greater number of turns than the secondary. Repeating coils are almost universally of the closed magnetic circuit type.
Ringing and Talking Considerations. Since repeating coils often serve to connect two telephones, it follows that it is sometimes necessary to ring through them as well as talk through them. By this is meant that it is necessary that the coil shall be so designed as to be capable of transforming the heavy ringing currents as well as the very much smaller telephone or voice currents. Ringing currents ordinarily have a frequency ranging from about 16 to 75 cycles per second, while voice currents have frequencies ranging from a few hundred up to perhaps ten thousand per second. Ordinarily, therefore, the best form of repeating coil for transforming voice currents is not the best for transforming the heavy ringing currents and vice versâ. If the comparatively heavy ringing currents alone were to be considered, the repeating coil might well be of heavy construction with a large amount of iron in its magnetic circuit. On the other hand, for carrying voice currents alone it is usually made with a small amount of iron and with small windings, in order to prevent waste of energy in the core, and to give a high degree of responsiveness with the least amount of distortion of wave form, so that the voice currents will retain as far as possible their original characteristics. When, therefore, a coil is required to carry both ringing and talking currents, a compromise must be effected.
Types. The form of repeating coil largely used for both ringing and talking through is shown in Fig. 109. This coil comprises a Page 169 soft iron core made up of a bundle of wires about .02 inch in diameter, the ends of which are left of sufficient length to be bent back around the windings after they are in place and thus form a completely closed magnetic path for the core. The windings of this particular coil are four in number, and contain about 2,400 turns each, and have a resistance of about 60 ohms. In this coil, when connected for local battery work, the windings are connected in pairs in series, thus forming effectively two windings having about 120 ohms resistance each. The whole coil is enclosed in a protecting case of iron. The terminals are brought out to suitable clips on the wooden base, as shown. An external perspective view of this coil is shown in Fig. 110. By bringing out each terminal of each winding, eight in all, as shown in this figure, great latitude of connection is provided for, since the windings may be connected in circuit in any desirable way, either by connecting them together in pairs to form virtually a primary and a secondary, or, as is frequently the case, to split the primary and the secondary, connecting a battery between each pair of windings.
Page 170Fig. 111 illustrates in section a commercial type of coil designed for talking through only. This coil is provided with four windings of 1,357 turns each, and when used for local battery work the coils are connected in pairs in series, thus giving a resistance of about 190 ohms in each half of the repeating coil. The core of this coil consists of a bundle of soft iron wires, and the shell which forms the return path for the magnetic lines is of very soft sheet iron. This shell is drawn into cup shape and its open end is closed, after the coil is inserted, by the insertion of a soft iron head, as indicated. As in the case of the coil shown in Figs. 109 and 110, eight terminals are brought out on this coil, thus providing the necessary flexibility of connection.
Still another type of repeating coil is illustrated in diagram in Fig. 112, and in view in Fig. 113. This coil, like the impedance coil shown in Fig. 104, comprises a core made up of a bundle of soft iron wires wound into the form of a ring. It is usually provided with two primary windings placed opposite each other upon the core, and with two secondary windings, one over each primary. In practice these two primary windings are connected in one circuit and the two secondaries in another. This is the standard repeating coil now used by the Bell companies in their common-battery cord circuits.
Page 171Conventional Symbols. The ordinary symbol for the induction coil used in local battery work is shown in Fig. 114. This consists merely of a pair of parallel zig-zag lines. The primary winding is usually indicated by a heavy line having a fewer number of zig-zags, and the secondary by a finer line having a greater number of zig-zags. In this way the fact that the primary is of large wire and of comparatively few turns is indicated. This diagrammatic symbol may be modified to suit almost any conditions, and where a tertiary as well as a secondary winding is provided it may be shown by merely adding another zig-zag line.
The repeating coil is indicated symbolically in the two diagrams of Fig. 115. Where there is no necessity for indicating the internal connections of the coil, the symbol shown in the left of this figure is usually employed. Where, however, the coil consists of four windings rather than two and the method of connecting them is to be indicated, the symbol at the right hand is employed. In Fig. 116 another way of indicating a four-winding repeating coil or induction coil is shown. Sometimes such windings may be combined by connection to form merely a primary and a secondary winding, and in other cases the four windings all act separately, in which case one may be considered the primary and the others, respectively, the secondary, tertiary, and quaternary.
Where the toroidal type of repeating coil is employed, the diagram of Fig. 112, already referred to, is a good symbolic representation.
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It is often desired to introduce simple ohmic resistance into telephone circuits, in order to limit the current flow, or to create specific differences of potential at given points in the circuit.
Temperature Coefficient. The design or selection of resistance devices for various purposes frequently involves the consideration of the effect of temperature on the resistance of the conductor employed. The resistance of conductors is subject to change by changes in temperature. While nearly all metals show an increase, carbon shows a decrease in its resistance when heated.
The temperature coefficient of a conductor is a factor by which the resistance of the conductor at a given temperature must be multiplied in order to determine the change in resistance of that conductor brought about by a rise in temperature of one degree.
TABLE V
Temperature Coefficients
| Pure Metals | Temperature Coefficients | |
| Centigrade | Fahrenheit | |
| Silver (annealed) | 0.00400 | 0.00222 |
| Copper (annealed) | 0.00428 | 0.00242 |
| Gold (99.9%) | 0.00377 | 0.00210 |
| Aluminum (99%) | 0.00423 | 0.00235 |
| Zinc | 0.00406 | 0.00226 |
| Platinum (annealed) | 0.00247 | 0.00137 |
| Iron | 0.00625 | 0.00347 |
| Nickel | 0.0062 | 0.00345 |
| Tin | 0.00440 | 0.00245 |
| Lead | 0.00411 | 0.00228 |
| Antimony | 0.00389 | 0.00216 |
| Mercury | 0.00072 | 0.00044 |
| Bismuth | 0.00354 | 0.00197 |
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Positive and Negative Coefficients. Those conductors, in which a rise in temperature produces an increase in resistance, are said to have positive temperature coefficients, while those in which a rise in temperature produces a lowering of resistance are said to have negative temperature coefficients.
The temperature coefficients of pure metals are always positive and for some of the more familiar metals, have values, according to Foster, as in Table V.
Iron, it will be noticed, has the highest temperature coefficient of all. Carbon, on the other hand, has a large negative coefficient, as proved by the fact that the filament of an ordinary incandescent lamp has nearly twice the resistance when cold as when heated to full candle-power.
Certain alloys have been produced which have very low temperature coefficients, and these are of value in producing resistance units which have practically the same resistance for all ordinary temperatures. Some of these alloys also have very high resistance as compared with copper and are of value in enabling one to obtain a high resistance in small space.
One of the most valuable resistance wires is of an alloy known as German silver. The so-called eighteen per cent alloy has approximately 18.3 times the resistance of copper and a temperature coefficient of .00016 per degree Fahrenheit. The thirty per cent alloy has approximately 28 times the resistance of copper and a temperature coefficient of .00024 per degree Fahrenheit.
For facilitating the design of resistance coils of German silver wire, Tables VI and VII are given, containing information as to length, resistance, and weight of the eighteen per cent and the thirty per cent alloys, respectively, for all sizes of wire smaller than No. 20 B. & S. gauge.
Special resistance alloys may be obtained having temperature coefficients as low as .000003 per degree Fahrenheit. Other alloys of nickel and steel are adapted for use where the wire must carry heavy currents and be raised to comparatively high temperatures thereby; for such use non-corrosive properties are specially to be desired. Such wire may be obtained having a resistance of about fifty times that of copper.
Page 174TABLE VI
18 Per Cent German Silver Wire
| No. B. & S. Gauge |
Diameter Inches |
Weight Pounds per Foot |
Length Feet per Pound |
Resistance Ohms per Foot |
| 21 | .02846 | .002389 | 418.6 | .2333 |
| 22 | .02535 | .001894 | 527.9 | .2941 |
| 23 | .02257 | .001502 | 665.8 | .3710 |
| 24 | .02010 | .001191 | 839.5 | .4678 |
| 25 | .01790 | .0009449 | 1058. | .5899 |
| 26 | .01594 | .0007493 | 1335. | .7438 |
| 27 | .01419 | .0005943 | 1683. | .9386 |
| 28 | .01264 | .0004711 | 2123. | 1.183 |
| 29 | .01126 | .0003735 | 2677. | 1.491 |
| 30 | .01003 | .0002962 | 3376. | 1.879 |
| 31 | .008928 | .0002350 | 4255. | 2.371 |
| 32 | .007950 | .0001864 | 5366. | 2.990 |
| 33 | .007080 | .0001478 | 6766. | 3.771 |
| 34 | .006304 | .0001172 | 8532. | 4.756 |
| 35 | .005614 | .00009295 | 10758. | 5.997 |
| 36 | .005000 | .00007369 | 13569. | 7.560 |
| 37 | .004453 | .00005845 | 17108. | 9.532 |
| 38 | .003965 | .00004636 | 21569. | 12.02 |
| 39 | .003531 | .00003675 | 27209. | 15.16 |
| 40 | .003145 | .00002917 | 34282. | 19.11 |
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TABLE VII
30 Per Cent German Silver Wire
| No. B. & S. Gauge |
Diameter Inches |
Weight Pounds per Foot |
Length Feet per Pound |
Resistance Ohms per Foot |
| 21 | .02846 | .002405 | 415.8 | .3581 |
| 22 | .02535 | .001907 | 524.4 | .4513 |
| 23 | .02257 | .001512 | 661.3 | .5693 |
| 24 | .02010 | .001199 | 833.9 | .7178 |
| 25 | .01790 | .0009513 | 1051. | .9051 |
| 26 | .01594 | .0007544 | 1326. | 1.141 |
| 27 | .01419 | .0005983 | 1671. | 1.440 |
| 28 | .01264 | .0004743 | 2108. | 1.815 |
| 29 | .01126 | .0003761 | 2659. | 2.287 |
| 30 | .01003 | .0002982 | 3353. | 2.883 |
| 31 | .008928 | .0002366 | 4227. | 3.638 |
| 32 | .007950 | .0001876 | 5330. | 4.588 |
| 33 | .007080 | .0001488 | 6721. | 5.786 |
| 34 | .006304 | .0001180 | 8475. | 7.297 |
| 35 | .005614 | .00009358 | 10686. | 9.201 |
| 36 | .005000 | .00007419 | 13478. | 11.60 |
| 37 | .004453 | .00005885 | 16994. | 14.63 |
| 38 | .003965 | .00004668 | 21424. | 18.45 |
| 39 | .003531 | .00003700 | 27026. | 23.26 |
| 40 | .003145 | .00002937 | 34053. | 29.32 |
Inductive Neutrality. Where the resistance unit is required to be strictly non-inductive, and is to be in the form of a coil, special designs must be employed to give the desired inductive neutrality.
Provisions Against Heating. In cases where a considerable amount of heat is to be generated in the resistance, due to the necessity of carrying large currents, special precautions must be taken as to the heat-resisting properties of the structure, and also as to the provision of sufficient radiating surface or its equivalent to provide for the dissipation of the heat generated.
Types. Mica Card Unit. One of the most common resistance coils used in practice is shown in Fig. 117. This comprises a coil of fine, bare German silver wire wound on a card of mica, the windings being so spaced that the loops are not in contact with each other. The winding is protected by two cards of mica and the whole is bound in place by metal strips, to which the ends of the winding are attached. Binding posts are provided on the extended portions of the terminals to assist in mounting the resistance on a supporting frame, and the posts terminate in soldering terminals by which the resistance is connected into the circuit.
Differentially-Wound Unit. Another type of resistance coil is that in which the winding is placed upon an insulating core of heat-resisting material and wound so as to overcome inductive effects. In order to accomplish this, the wire to be bound on the core is doubled back on itself at its middle portion to form two strands, and these are wound simultaneously on the core, thus forming two spirals of equal number of turns. The current in traversing the entire coil must flow through one spiral in one direction with relation to the core, and in the opposite direction in the other spiral, thereby nullifying the inductive effects of one spiral by those of the other. This is called a non-inductive winding and is in reality an example of differential winding.
Page 176Lamp Filament. An excellent type of non-inductive resistance is the ordinary carbon-filament incandescent lamp. This is used largely in the circuits of batteries, generators, and other sources of supply to prevent overload in case of short circuits on the line. These are cheap, durable, have large current-carrying capacities, and are not likely to set things afire when overheated. An additional advantage incident to their use for this purpose is that an overload on a circuit in which they are placed is visibly indicated by the glowing of the lamp.
Obviously, the carbon-filament incandescent lamp, when used as a resistance, has, on account of the negative temperature coefficient of carbon, the property of presenting the highest resistance to the circuit when carrying no current, and of presenting a lower and lower resistance as the current and consequent heating increases. For some conditions of practice this is not to be desired, and the opposite characteristic of presenting low resistance to small currents and comparatively high resistance to large currents would best meet the conditions of practice.
Page 177Iron-Wire Ballast. Claude D. Enochs took advantage of the very high positive temperature coefficient of iron to produce a resistance device having these characteristics. His arrangement possesses the compactness of the carbon-filament lamp and is shown in Fig. 118. The resistance element proper is an iron wire, wound on a central stem of glass, and this is included in an exhausted bulb so as to avoid oxidation. Such a resistance is comparatively low when cold, but when traversed by currents sufficient to heat it considerably will offer a very large increase of resistance to oppose the further increase of current. In a sense, it is a self-adjusting resistance, tending towards the equalization of the flow of current in the circuit in which it is placed.
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Charge. A conducting body insulated from all other bodies will receive and hold a certain amount of electricity (a charge), if subjected to an electrical potential. Thus, referring to Fig. 119, if a metal plate, insulated from other bodies, be connected with, say, the positive pole of a battery, the negative pole of which is grounded, a current will flow into the plate until the plate is raised to the same potential as that of the battery pole to which it is connected. The amount of electricity that will flow into the plate will depend, other things being equal, on the potential of the source from which it is charged; in fact, it is proportional to the potential of the source from which it is charged. This amount of electricity is a measure of the capacity of the plate, just as the amount of water that a bath-tub will hold is a measure of the capacity of the bath-tub.
Capacity. Instead of measuring the amount of electricity by the quart or pound, as in the case of material things, the unit of electrical quantity is the coulomb. The unit of capacity of an insulated conductor is the farad, and a given insulated conductor is said to have unit capacity, that is, the capacity of one farad, when it will receive a charge of one coulomb of electricity at a potential of one volt.
Referring to Fig. 119, the potential of the negative terminal of the battery may be said to be zero, since it is connected to the earth. If the battery shown be supposed to have exactly one volt potential, then the plate would be said to have the capacity of one farad if one coulomb of electricity flowed from the battery to the plate before the plate was raised to the same potential as that of the positive pole, that is, to a potential of one volt above the potential of the earth; it being assumed that the plate was also at zero potential before the connection was made. Another conception of this quantity may be had by remembering that a coulomb is such a quantity of current as will result from one ampere flowing one second.
Page 179The capacity of a conductor depends, among other things, on its area. If the plate of Fig. 119 should be made twice as large in area, other things remaining the same, it would have twice the capacity. But there are other factors governing the capacity of a conductor. Consider the diagram of Fig. 120, which is supposed to represent two such plates as are shown in Fig. 119, placed opposite each other and connected respectively with the positive and the negative poles of the battery. When the connection between the plates and the battery is made, the two plates become charged to a difference of potential equal to the electromotive force of the battery. In order to obtain these charges, assume that the plates were each at zero potential before the connection was made; then current flows from the battery into the plates until they each assume the potential of the corresponding battery terminal. If the two plates be brought closer together, it will be found that more current will now flow into each of them, although the difference of potential between the two plates must obviously remain the same, since each of them is still connected to the battery.
Theory. Due to the proximity of the plates, the positive electricity on plate A is drawn by the negative charge on plate B towards plate B, and likewise the negative electricity on plate B is drawn to the side towards plate A by the positive charge on that plate. These two charges so drawn towards each other will, so to speak, bind each other, and they are referred to as bound charges. The charge on the right-hand side of plate A and on the left-hand side of plate B will, however, be free charges, since there is nothing to attract them, and these are, therefore, neutralized by a further flow of electricity from the battery to the plate.
Obviously, the closer together the plates are the stronger will be the attractive influence of the two charges on each other. From this Page 180 it follows that in the case of plate A, when the two plates are being moved closer together, more positive electricity will flow into plate A to neutralize the increasing free negative charges on the right-hand side of the plate. As the plates are moved closer together still, a new distribution of charges will take place, resulting in more positive electricity flowing into plate A and more negative electricity flowing into plate B. The closer proximity of the plates, therefore, increases the capacity of the plates for holding charges, due to the increased inductive action across the dielectric separating the plates.
Condenser Defined. A condenser is a device consisting of two adjacent plates of conducting material, separated by an insulating material, called a dielectric. The purpose is to increase by the proximity of the plates, each to the other, the amount of electricity which each plate will receive and hold when subjected to a given potential.
Dielectric. We have already seen that the capacity of a condenser depends upon the area of its plates, and also upon their distance apart. There is still another factor on which the capacity of a condenser depends, i.e., on the character of the insulating medium separating its plates. The inductive action which takes place between a charged conductor and other conductors nearby it, as between plate A and plate B of Fig. 120, is called electrostatic induction, and it plays an important part in telephony. It is found that the ability of a given charged conductor to induce charges on other neighboring conductors varies largely with the insulating medium or dielectric that separates them. This quality of a dielectric, by which it enables inductive action to take place between two separated conductors, is called inductive capacity. Usually this quality of dielectrics is measured in terms of the same quality in dry air, this being taken as unity. When so expressed, it is termed specific inductive capacity. To be more accurate the specific inductive capacity of a dielectric is the ratio between the capacity of a condenser having that substance as a dielectric, to the capacity of the same condenser using dry air at zero degrees Centigrade and at a pressure of 14.7 pounds per square inch as the dielectric. To illustrate, if two condensers having plates of equal size and equal distance apart are constructed, one using air as the dielectric Page 181 and the other using hard crown glass as the dielectric, the one using glass will have a capacity of 6.96 times that of the one using air. From this we say that crown glass has a specific inductive capacity of 6.96.
Various authorities differ rather widely as to the specific inductive capacity of many common substances. The values given in Table VIII have been chosen from the Smithsonian Physical Tables.
TABLE VIII
Specific Inductive Capacities
| Dielectric | Referred to Air as 1 |
| Vacuum | .99941 |
| Hydrogen | .99967 |
| Carbonic Acid | 1.00036 |
| Dry Paper | 1.25 to 1.75 |
| Paraffin | 1.95 to 2.32 |
| Ebonite | 1.9 to 3.48 |
| Sulphur | 2.24 to 3.90 |
| Shellac | 2.95 to 3.73 |
| Gutta-percha | 3.3 to 4.9 |
| Plate Glass | 3.31 to 7.5 |
| Porcelain | 4.38 |
| Mica | 4.6 to 8.0 |
| Gutta-percha | 3.3 to 4.9 |
| Glass—Light Flint | 6.61 |
| Glass—Hard Crown | 6.96 |
| Selenium | 10.2 |
This data is interesting as showing the wide divergence in specific inductive capacities of various materials, and also showing the wide divergence in different observations of the same material. Undoubtedly, this latter is due mainly to the fact that various materials differ largely in themselves, as in the case of paraffin, for instance, which exhibits widely different specific inductive capacities according to the difference in rapidity with which it is cooled in changing from a liquid to a solid state.
We see then that the capacity of a condenser varies as the area of its plates, as the specific inductive capacity of the dielectric employed, and also inversely as the distance between the plates.
Obviously, therefore, in making a condenser of large capacity, it is important to have as large an area of the plate as possible; to Page 182 have them as close together as possible; to have the dielectric a good insulating medium so that there will be practically no leakage between the plates; and to have the dielectric of as high a specific inductive capacity as economy and suitability of material in other respects will permit.
Dielectric Materials. Mica. Of all dielectrics mica is the most suitable for condensers, since it has very high insulation resistance and also high specific inductive capacity, and furthermore may be obtained in very thin sheets. High-grade condensers, such as are used for measurements and standardization purposes, usually have mica for the dielectric.
Dry Paper. The demands of telephonic practice are, however, such as to require condensers of very cheap construction with large capacity in a small space. For this purpose thin bond paper, saturated with paraffin, has been found to be the best dielectric. The conductors in condensers are almost always of tinfoil, this being an ideal material on account of its cheapness and its thinness. Before telephony made such urgent demands for a cheap compact condenser, the customary way of making them was to lay up alternate sheets of dielectric material, either of oiled paper or mica and tinfoil, the sheets of tinfoil being cut somewhat smaller than the sheets of dielectric material in order that the proper insulation might be secured at the edges. After a sufficient number of such plates were built up the alternate sheets of tinfoil were connected together to form one composite plate of the condenser, while the other sheets were similarly connected together to form the other plate. Obviously, in this way a very large area of plates could be secured with a minimum degree of separation.
Page 183There has been developed for use in telephony, however, and its use has since extended into other arts requiring condensers, what is called the rolled condenser. This is formed by rolling together in a flat roll four sheets of thin bond paper, 1, 2, 3, and 4, and two somewhat narrower strips of tinfoil, 5 and 6, Fig. 121. The strips of tinfoil and paper are fed on to the roll in continuous lengths and in such manner that two sheets of paper will lie between the two strips of tinfoil in all cases. Thin sheet metal terminals 7 and 8 are rolled into the condenser as it is being wound, and as these project beyond the edges of the paper they form convenient terminals for the condenser after it is finished. After it is rolled, the roll is boiled in hot paraffin so as to thoroughly impregnate it and expel all moisture. It is then squeezed in a press and allowed to cool while under pressure. In this way the surplus paraffin is expelled and the plates are brought very close together. It then appears as in Fig. 122. The condenser is now sealed in a metallic case, usually rectangular in form, and presents the appearance shown in Fig. 123.
A later method of condenser making which has not yet been thoroughly proven in practice, but which bids fair to produce good results, varies from the method just described in that a paper is used which in itself is coated with a very thin conducting material. This conducting material is of metallic nature and in reality forms a part of the paper. To form a condenser of this the sheets are merely rolled together and then boiled in paraffin and compressed as before.
Sizes. The condensers ordinarily used in telephone practice range in capacity from about 1/4 microfarad to 2 microfarads. When larger capacities than 2 microfarads are desired, they may be obtained by connecting several of the smaller size condensers in multiple. Table IX gives the capacity, shape, and dimensions of a variety of condensers selected from those regularly on the market.
Page 184TABLE IX
Condenser Data
| Capacity | Shape | Dimensions in Inches | ||
| Height | Width | Thickness | ||
| 2 m. f. | Rectangular | 9–1/6 | 4–3/4 | 11/16 |
| 1 m. f. | " | 9–1/6 | 4–3/4 | 11/16 |
| 1 m. f. | " | 4–3/4 | 2–3/32 | 13/16 |
| 1/2 m. f. | " | 2–3/4 | 1–1/4 | 3/4 |
| 1 m. f. | " | 4–13/16 | 2–1/32 | 25/32 |
| 1/2 m. f. | " | 4–3/4 | 2–3/32 | 13/16 |
| 3/10 m. f. | " | 4–3/4 | 2–3/32 | 13/16 |
| 1 m. f. | " | 2–3/4 | 3 | 1 |
Conventional Symbols. The conventional symbols usually employed to represent condensers in telephone diagrams are shown in Fig. 124. These all convey the idea of the adjacent conducting plates separated by insulating material.
Functions. Obviously, when placed in a circuit a condenser offers a complete barrier to the flow of direct current, since no conducting path exists between its terminals, the dielectric offering a very high insulation resistance. If, however, the condenser is connected across the terminals of a source of alternating current, this current flows first in one direction and then in the other, the electromotive force in the circuit increasing from zero to a maximum in one direction, and then decreasing back to zero and to a maximum in the other direction, and so on. With a condenser connected so as to be subjected to such alternating electromotive forces, as the electromotive force begins to rise the electromotive force at the condenser terminals will also rise and a current will, therefore, flow into the condenser. When the electromotive force reaches its maximum, the condenser will have received its full charge for that potential, and the current flow into it will cease. When the electromotive force begins to fall, the condenser can no longer retain Page 185 its charge and a current will, therefore, flow out of it. Apparently, therefore, there is a flow of current through the condenser the same as if it were a conductor.
Means for Assorting Currents. In conclusion, it is obvious that the telephone engineer has within his reach in the various coils—whether non-inductive or inductive, or whether having one or several windings—and in the condenser, a variety of tools by which he may achieve a great many useful ends in his circuit work. Obviously, the condenser affords a means for transmitting voice currents or fluctuating currents, and for excluding steady currents. Likewise the impedance coil affords a means for readily transmitting steady currents but practically excluding voice currents or fluctuating currents. By the use of these very simple devices it is possible to sift out the voice currents from a circuit containing both steady and fluctuating currents, or it is possible in the same manner to sift out the steady currents and to leave the voice currents alone to traverse the circuit.
Great use is made in the design of telephone circuits of the fact that the electromagnets, which accomplish the useful mechanical results in causing the movement of parts, possess the quality of impedance. Thus, the magnets which operate various signaling relays at the central office are often used also as impedance coils in portions of the circuit through which it is desired to have only steady currents pass. If, on the other hand, it is necessary to place a relay magnet, having considerable impedance, directly in a talking circuit, the bad effects of this on the voice currents may be eliminated by shunting this coil with a condenser, or with a comparatively high non-inductive resistance. The voice currents will flow around the high impedance of the relay coil through the condenser or resistance, while the steady currents, which are the ones which must be depended upon to operate the relay, are still forced in whole or in part to pass through the relay coil where they belong.
In a similar way the induction coil affords a means for keeping two circuits completely isolated so far as the direct flow of current between them is concerned, and yet of readily transmitting, by electromagnetic induction, currents from one of these circuits to the other. Here is a means of isolation so far as direct current is concerned, with complete communication for alternating current.
ToC
Page 186
The methods by which current is supplied to the transmitter of a telephone for energizing it, may be classified under two divisions: first, those where the battery or other source of current is located at the station with the transmitter which it supplies; and second, those where the battery or other source of current is located at a distant point from the transmitter, the battery in such cases serving as a common source of current for the supply of transmitters at a number of stations.
The advantages of putting the transmitter and the battery which supplies it with current in a local circuit with the primary of an induction coil, and placing the secondary of the induction coil in the line, have already been pointed out but may be briefly summarized as follows: When the transmitter is placed directly in the line circuit and the line is of considerable length, the current which passes through the transmitter is necessarily rather small unless a battery of high potential is used; and, furthermore, the total change in resistance which the transmitter is capable of producing is but a small proportion of the total resistance of the line, and, therefore, the current changes produced by the transmitter are relatively small. On the other hand, when the transmitter is placed in a local circuit with the battery, this circuit may be of small resistance and the current relatively large, even though supplied by a low-voltage battery; so that the transmitter is capable of producing relatively large changes in a relatively large current.
To draw a comparison between these two general classes of transmitter current supply, a number of cases will be considered in connection with the following figures, in each of which two stations connected by a telephone line are shown. Brief reference to the local battery method of supplying current will be made in order to make this chapter contain, as far as possible, all of the commonly used methods of current supply to transmitters.
Page 187Local Battery. In Fig. 125 two stations are shown connected by a grounded line wire. The transmitter of each station is included in a low-resistance primary circuit including a battery and the primary winding of an induction coil, the relation between the primary circuits and the line circuits being established by the inductive action between the primary and the secondary windings of induction coils, the secondary in each case being in the line circuits with the receivers.
Fig. 126 shows exactly the same arrangement but with a metallic circuit rather than a grounded circuit. The student should become accustomed to the replacing of one of the line wires of a metallic circuit by the earth, and to the method, employed in Figs. 125 and 126, of indicating a grounded circuit as distinguished from a metallic circuit.
In Fig. 127 is shown a slight modification of the circuit shown in Fig. 126, which consists of connecting one end of the primary winding to one end of the secondary winding of the induction coil, thus linking together the primary circuit and the line circuit, a portion of each of these circuits being common to a short piece of the local wiring. There is no difference whatever in the action of the circuits shown in Figs. 126 and 127, the latter being shown merely Page 188 for the purpose of bringing out this fact. It is very common, particularly in local-battery circuits, to connect one end of the primary and the secondary windings, as by doing so it is often possible to save a contact point in the hook switch and also to simplify the wiring.
The advantages to be gained by employing a local battery at each subscriber's station associated with the transmitter in the primary circuit of an induction coil are attended by certain disadvantages from a commercial standpoint. The primary battery is not an economical way to generate electric energy. In all its commercial forms it involves the consumption of zinc and zinc is an expensive fuel. The actual amount of current in watts required by a telephone is small, however, and this disadvantage due to the inexpensive method of generating current would not in itself be of great importance. A more serious objection to the use of local batteries at subscribers' stations appears when the subject is considered from the standpoint of maintenance. Batteries, whether of the so-called "dry" or "wet" type, gradually deteriorate, even when not used, and in cases where the telephone is used many times a day the deterioration is comparatively rapid. This makes necessary the occasional renewals of the batteries with the attendant expense for new batteries or new material, and of labor and transportation in visiting the station. The labor item becomes more serious when the stations are scattered in a sparsely settled community, in which case the visiting of the stations, even for the performance of a task that would require but a few minutes' time, may consume some hours on the part of the employes in getting there and back.
Common Battery. Advantages. It would be more economical if all of the current for the subscribers' transmitters could be supplied from a single comparatively efficient generating source instead of Page 189 from a multitude of inefficient small sources scattered throughout the community served by the exchange. The advantage of such centralization lies not only in more economic generating means, but also in having the common source of current located at one place, where it may be cared for with a minimum amount of expense. Such considerations have resulted in the so-called "common-battery system," wherein the current for all the subscribers' transmitters is furnished from a source located at the central office.
Where such a method of supplying current is practiced, the result has also been, in nearly all cases, the doing away with the subscriber's magneto generators, relying on the central-office source of current to furnish the energy for enabling the subscriber to signal the operator. Such systems, therefore, concentrate all of the sources of energy at the central office and for that reason they are frequently referred to as central-energy systems.
NOTE. In this chapter the central-energy or common-battery system will be considered only in so far as the supply of current for energizing the subscribers' transmitters is concerned, the discussion of the action of signaling being reserved for subsequent chapters.
Series Battery. If but a single pair of lines had to be considered, the arrangement shown in Fig. 128 might be employed. In this the battery is located at the central office and placed in series with the two grounded lines leading from the central office to the two subscribers' stations. The voltage of this battery is made sufficient to furnish the required current over the resistance of the entire line circuit with its included instruments. Obviously, changes in resistance in the transmitter at Station A will affect the flow of current in the entire line and the fluctuations resulting from the vibration of the transmitter diaphragm will, therefore, reproduce these sounds in the receiver at Station B, as well as in that at Station A.
An exactly similar arrangement applied to a metallic circuit is shown in Fig. 129. In thus placing the battery in series in the circuit Page 190 between the two stations, as shown in Figs. 128 and 129, it is obvious that the transmitter at each station is compelled to vary the resistance of the entire circuit comprising the two lines in series, in order to affect the receiver at distant stations. This is in effect making the transmitter circuit twice as long as is necessary, as will be shown in the subsequent systems considered. Furthermore, the placing of the battery in series in the circuit of the two combined lines does not lend itself readily to the supply of current from a common source to more than a single pair of lines.
Series Substation Circuit. The arrangement at the substations—consisting in placing the transmitter and the receiver in series in the line circuit, as shown in Figs. 128 and 129—is the simplest possible one, and has been used to a considerable extent, but it has been subject to the serious objection, where receivers having permanent magnets were used, of making it necessary to so connect the receiver in the line circuit that the steady current from the battery would not set up a magnetization in the cores of the receiver in such a direction as to neutralize or oppose the magnetization of the permanent magnets. As long as the current flowed through the receiver coils in such a direction as to supplement the magnetization of the permanent magnets, no harm was usually done, but when the current flowed through the receiver coils in such a way as to neutralize or oppose the magnetizing force of the permanent magnets, the action of the receiver was greatly interfered with. As a result, it was necessary to always connect the receivers in the line circuit in a certain way, and this operation was called poling.
In order to obviate the necessity for poling and also to bring about other desirable features, it has been, until recently, almost universal practice to so arrange the receiver that it would be in the Page 191 circuit of the voice currents passing over the line, but would not be traversed by direct currents, this condition being brought about by various arrangements of condensers, impedance coils, or induction coils, as will be shown later. During the year 1909, however, the adoption by several concerns of the so-called "direct-current" receiver has made it necessary for the direct current to flow through the receiver coils in order to give the proper magnetization to the receiver cores, and this has brought about a return to the very simple form of substation circuit, which includes the receiver and the transmitter directly in the circuit of the line. This illustrates well an occurrence that is frequently observed by those who have opportunity to watch closely the development of an art. At one time the conditions will be such as to call for complicated arrangements, and for years the aim of inventors will be to perfect these arrangements; then, after they are perfected, adopted, and standardized, a new idea, or a slight alteration in the practice in some other respect, will demand a return to the first principles and wipe out the necessity for the things that have been so arduously striven for.
Bridging Battery with Repeating Coil. As pointed out, the placing of the battery in series in the line circuit in the central office is not desirable, and, so far as we are aware, has never been extensively used. The universal practice, therefore, is to place it in a bridge path across the line circuit, and a number of arrangements employing this basic idea are in wide use. In Fig. 130 is shown the standard arrangement of the Western Electric Company, employed by practically all the Bell operating companies. In this the battery at the central office is connected in the middle of the two sides of a repeating coil so that the current from the battery is fed out to the two connected lines in multiple.
Page 192Referring to the middle portion of this figure showing the central-office apparatus, 1 and 2 may be considered as the two halves of one side of a repeating coil divided so that the battery may be cut into their circuit. Likewise, 3 and 4 may be considered as the two halves of the other side of the repeating coil similarly divided for the same purpose. The windings of this repeating coil are ordinarily alike; that is, 1 and 2 combined have the same resistance, number of turns, and impedance as 3 and 4 combined. The two sides of this coil are alternately used as primary and secondary, 1 and 2 forming the primary when Station A is talking, and 3 and 4, the secondary; and vice versâ when Station B is talking.
As will be seen, the current flowing from the positive pole of the battery will divide and flow through the windings 2 and 4; thence over the upper limb of each line, through the transmitter at each station, and back over the lower limbs of the line, through the windings 1 and 3, where the two paths reunite and pass to the negative pole of the battery. It is evident that when neither transmitter is being used the current flowing through both lines will be a steady current and that, therefore, neither line will have an inductive effect on the other. When, however, the transmitter at Station A is used the variations in the resistance caused by it will cause undulations in the current. These undulations, passing through the windings 1 and 2 of the repeating coil, will cause, by electromagnetic induction, alternating currents to flow in the windings 3 and 4, and these alternating currents will be superimposed on the steady currents flowing in that line and will affect the receiver at Station B, as will be pointed out. The reverse conditions exist when Station B is talking.
Bell Substation Arrangement. The substation circuits at the stations in Fig. 130 are illustrative of one of the commonly employed methods of preventing the steady current from the battery from flowing through the receiver coil. This particular arrangement is that employed by the common-battery instruments of the various Bell companies. Considering the action at Station B, it is evident that the steady current will pass through the transmitter and through the secondary winding of the induction coil, and that as long as this current is steady no current will flow through the telephone receiver. The receiver, transmitter, and primary Page 193 winding of the induction coil are, however, included in a local circuit with the condenser. The presence of the condenser precludes the possibility of direct current flowing in this path. Considering Station A as a receiving station, it is evident that the voice currents coming to the station over the line will pass through the secondary winding and will induce alternating currents in the primary winding which will circulate through the local circuit containing the receiver and the condenser, and thus actuate the receiver. The considerations are not so simple when the station is being treated as a transmitting station. Under this condition the steady current passes through the transmitter in an obvious manner. It is clear that if the local circuit containing the receiver did not exist, the circuit would be operative as a transmitting circuit because the tran
