Title: Cyclopedia of Telephony and Telegraphy, Vol. 2
Author: American School of Correspondence
Release date: August 15, 2010 [eBook #33437]
Most recently updated: January 6, 2021
Language: English
Other information and formats: www.gutenberg.org/ebooks/33437
Credits: Produced by Ronald Holder, Stephen H. Sentoff and the
Online Distributed Proofreading Team at https://www.pgdp.net
THOMAS A. EDISON
Pioneer Electrical Investigator and Inventor of Numerous Telegraph, Telephone,
Lighting, and Other Electrical Devices.
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
COPYRIGHT, 1911, 1912,
BY
AMERICAN SCHOOL OF CORRESPONDENCE
COPYRIGHT, 1911, 1912
BY
AMERICAN TECHNICAL SOCIETY
Entered at Stationers' Hall, London
All Rights Reserved
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, College 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."
decorativeThe 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.
decorativeThe 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.
decorativeThe sections on Telephony cover the installation, maintenance, and operation of all standard types of telephone systems; they present without prejudice the respective merits of manual and automatic exchanges; and they give special attention to the prevention and handling of operating "troubles." The sections on Telegraphy cover both commercial service and train dispatching. Practical methods of wireless communication—both by telephone and by telegraph—are thoroughly treated.
decorativeThe 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.
decorativeThe 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.
decorativeIn conclusion, grateful acknowledgment is due to the staff of authors and collaborators, without whose hearty co-operation this work would have been impossible.
Manual Switchboards By K. B. Miller and S. G. McMeen[A] Page[B] 11
CHAPTER XXII—Common-Battery Switchboards—Line Signals—Cord Circuit—Lamps—Mechanical Signals—Relays—Jacks—Switchboard Assembly
CHAPTER XXIII—Transfer Switchboard—Transfer Lines—Handling Transfers
CHAPTER XXIV—Multiple Switchboard—Busy Test—Influence of Traffic
CHAPTER XXV—Magneto-Multiple Switchboard—Multiple Boards: Series, Branch-Terminal, Modern Magneto, Common-Battery
CHAPTER XXVI—Western Electric No. 1 Relay Board—Western Electric No. 10 Board—Types of Multiple Boards—Apparatus
CHAPTER XXVII—Trunking—Western Electric and Kellogg Trunk Circuits
Automatic Systems By K. B. Miller and S. G. McMeen Page 135
CHAPTER XXVIII—Automatic vs. Manual—Operation
CHAPTER XXIX—Selecting Switch—Line Switch—Trunking Systems—Two- and Three-Wire Systems—Subscriber's Station Apparatus—First and Second Selector Operation—Connector—Release after Conversation—Multi-Office System—Automatic Sub-Offices—Rotary Connector—Party Lines—Two-Wire Automatic System
CHAPTER XXX—Lorimer System—Central-Office Apparatus—Operation
CHAPTER XXXI—Automanual System—Operation—Subscriber's Apparatus—Operator's Equipment—Switching Equipment—Distribution of Calls—Connection—Speed
Power Plants and Buildings By K. B. Miller and S. G. McMeen Page 227
CHAPTER XXXII—Currents Employed—Types—Operator's Transmitter Supply—Ringing-Current Supply—Auxiliary Signaling Current—Primary Sources—Duplicate Apparatus—Storage Batteries—Power Switchboards—Circuits
CHAPTER XXXIII—Central-Office Building—Arrangement of Apparatus—Manual Offices—Automatic Offices
Special Service Features By K. B. Miller and S. G. McMeen Page 271
CHAPTER XXXIV—Private-Branch Exchanges—Switchboards—Supervision—With Automatic Offices—Battery Supply—Ringing Current
CHAPTER XXXV—Inter-Communicating Systems—Magneto System—Common-Battery Systems—Types
CHAPTER XXXVI—Long-Distance Switching—Operator's Orders—Trunking—Way Stations
CHAPTER XXXVII—Traffic
CHAPTER XXXVIII—Measured Service—Charging—Rates—Toll Service—Local Service
Telegraph and Railway Work By K. B. Miller and S. G. McMeen Page 321
CHAPTER XXXIX—Phantom, Simplex, and Composite Circuits—Ringing—Railway Composite
CHAPTER XL—Telephone Train Dispatching—Railroad Conditions—Transmitting Orders—Apparatus—Telephone Equipment—Types of Circuits—Test Boards—Blocking Sets—Dispatching on Electric Railways
Review Questions Page 359
Index Page 373
[A] For professional standing of authors, see list of Authors and Collaborators at front of volume.
[B] For page numbers, see foot of pages.
GROSSE POINT EXCHANGE RACK
Detroit Home Telephone Company, Detroit, Mich.
The Dean Electric Co.
LINE SIDE OF LARGE MAIN DISTRIBUTING FRAME
PORTION OF TERMINAL ROOM OF LARGE COMMON-BATTERY OFFICE
Prospect Office, New York Telephone Co.
CABLE TURNING SECTIONS, BETWEEN A AND B BOARDS
Cortlandt Office, New York Telephone Co.
CABLE RUN FROM INTERMEDIATE FRAME TO MULTIPLE
Cortlandt Office, New York Telephone Co.
WESTERN ELECTRIC COMPANY TYPICAL CHARGING OUTFIT AT DAWSON, GEORGIA
DEAN HARMONIC CONVERTER
Dry Cell Type for Magneto Exchange.
The Dean Electric Co.
POWER SWITCHBOARD FOR MEDIUM-SIZED OFFICE
Mercury Arc Rectifier Panel and Transformer at Right.
GAS ENGINE AND POWER BOARD
Citizens' Telephone Co., Racine, Wis.
The Dean Electric Co.
POWER MACHINERY
Citizens' Telephone Company, Racine, Wis.
The Dean Electric Co.
POWER APPARATUS FOR COMMON-BATTERY MANUAL OFFICE OF MEDIUM SIZE
THE POWER AND WIRE CHIEF'S ROOM OF THE EXCHANGE AT WEBB CITY, MISSOURI
RINGING AND CHARGING MACHINES AND POWER BOARD
Plaza Office, New York Telephone Co.
POWER PLANT FOR AUTOMATIC SWITCHBOARD EQUIPMENT
Bay Cities Home Telephone Company, Berkeley, Cal.
WESTERN ELECTRIC COMPANY BATTERY ROOM AT MONMOUTH, ILLINOIS
WESTERN ELECTRIC MOTOR-GENERATOR CHARGING SET
WESTERN ELECTRIC RINGING MACHINE
FRONT OF LONG-DISTANCE POWER BOARD
U.S. Telephone Company, Cleveland, Ohio.
The Dean Electric Co.
Advantages of Common-Battery Operation. The advantages of the common-battery system of operation, alluded to in Chapter XIII, may be briefly summarized here. The main gain in the common-battery system of supply is the simplification of the subscribers' instruments, doing away with the local batteries and the magneto generators, and the concentration of all these many sources of current into one single source at the central office. A considerable saving is thus effected from the standpoint of maintenance, since the simpler common-battery instrument is not so likely to get out of order and, therefore, does not have to be visited so often for repairs, and the absence of local batteries, of course, makes the renewal of the battery parts by members of the maintenance department, unnecessary. Another decided advantage in the common-battery system is the fact that the centralized battery stands ready always to send current over the line when the subscriber completes the circuit of the line at his station by removing his receiver from its hook. The common-battery system, therefore, lends itself naturally to the purposes of automatic signaling, since it is only necessary to place at the central office a device in the circuit of each line that will be responsive to the current which flows from the central battery when the subscriber removes his receiver from its hook. It is thus that the subscriber is enabled automatically to signal the central office when he desires a connection; and as will be shown, it is by the same sort of means, associated with the cord circuits used in connecting his line with some other line, that the operator is automatically notified when a disconnection is desired, the cessation of current through the subscriber's line when he hangs up his receiver being made to actuate certain responsive devices which are associated with the cord at that time connected with his line, and which convey the proper disconnect signal to the operator.
Concentration of sources of energy into a single large unit, the simplification of the subscriber's station equipment, and the ready adaptability to automatic signaling from the subscriber to the central office are, therefore, the reasons for the existence of the common-battery system.
Common Battery vs. Magneto. It must not be supposed, however, that the common-battery system always has advantages over the magneto system, and that it is superior to the magneto or local-battery system for all purposes. It is the outward attractiveness of the common-battery system and the arguments in its favor, so readily made by over-zealous salesmen, that has led, in many cases, to the adoption of this system when the magneto system would better have served the purpose of utility and economy.
To say the least, the telephone transmission to be had from common-battery systems is no better than that to be had from local-battery systems, and as a rule, assuming equality in other respects, it is not as good. It is perhaps true, however, that under average conditions common-battery transmission is somewhat better, because whereas the local batteries at the subscribers' stations in the local-battery system are not likely to be in uniformly first-class condition, the battery in a common-battery system will be kept up to its full voltage except under the grossest neglect.
The places in which the magneto, or local-battery, system is to be preferred to the common-battery system, in the opinion of the writers, are to be found in the small rural communities where the lines have a rather great average length; where a good many subscribers are likely to be found on some of the lines; where the sources of electrical power available for charging storage batteries are likely either not to exist, or to be of a very uncertain nature; and where it is not commercially feasible to employ a high-grade class of attendants, or, in fact, any attendant at all other than the operator at the central office.
In large or medium-sized exchanges it is always possible to procure suitable current for charging the storage batteries required in common-battery systems, and it is frequently economical, on account of the considerable quantity of energy that is thus used, to establish a generating plant in connection with the central office for developing the necessary electrical energy. In very small rural places there are frequently no available sources of electrical energy, and the expense of establishing a power plant for the purpose cannot be justified. But even if there is an electric light or railway system in the small town, so that the problem of available current supply does not exist, the establishment of a common-battery system with its storage battery and the necessary charging machinery requires the daily attendance at the central office of some one to watch and care for this battery, and this, on account of the small gross revenue that may be derived from a small telephone system, often involves a serious financial burden.
There is no royal road to a proper decision in the matter, and no sharp line of demarcation may be drawn between the places where common-battery systems are superior to magneto and vice versâ. It may be said, however, that in the building of all new telephone plants having over about 500 local subscribers, the common-battery system is undoubtedly superior to the magneto. If the plant is an old one, however, and is to be re-equipped, the continuance of magneto apparatus might be justified for considerably larger exchanges than those having 500 subscribers.
Telephone operating companies who have changed over the equipment of old plants from magneto to common battery have sometimes been led into rather serious difficulty, owing to the fact that their lines, while serving tolerably well for magneto work, were found inadequate to meet the more exacting demands of common-battery work. Again in an old plant the change from magneto to common-battery equipment involves not only the change of switchboards, but also the change of subscribers' instruments that are otherwise good, and this consideration alone often, in our opinion, justifies the replacing of an old magneto board with a new magneto board, even if the exchange is of such size as to demand a small multiple board.
Where the plant to be established is of such size as to leave doubt as to whether a magneto or a common-battery switchboard should be employed, the questions of availability of the proper kind of power for charging the batteries, the proper kind of help for maintaining the batteries and the more elaborate central-office equipment, the demands and previous education of the public to be served, all are factors which must be considered in reaching the decision.
It is not proper to say that anything like all exchanges having fewer than 500 local lines, should be equipped with magneto service. Where all the lines are short, where suitable power is available, and where a good grade of attendants is available—as, for instance, in the case of private telephone exchanges that serve some business establishment or other institution located in one building or a group of buildings—the common-battery system is to be recommended and is largely used, even though it may have but a dozen or so subscribers' lines. It is for such uses, and for use in those regular public-service exchange systems where the conditions are such as to warrant the common-battery system, and yet where the number of lines and the traffic are small enough to be handled by such a small group of operators that any one of them may reach over the entire face of the board, that the simple non-multiple common-battery system finds its proper field of usefulness.
Line Signals. The principles and means by which the subscriber is enabled to call the central-office operator in a common-battery system have been referred to briefly in Chapter III. We will review these at this point and also consider briefly the way in which the line signals are associated with the connective devices in the subscribers' lines.
Direct-Line Lamp. The simplest possible way is to put the line signal directly in the circuit of the line in series with the central-office battery, and so to arrange the jack of the corresponding line that the circuit through the line signal will be open when the operator inserts a plug into that jack. This arrangement is shown in Fig. 307 where the subscriber's station at the left is indicated in the simplest of its forms. It is well to repeat here that in all common-battery manual systems, the subscriber's station equipment, regardless of the arrangement or type of its talking and signaling apparatus, must have these features: First, that the line shall be normally open to direct currents at the subscriber's station; second, that the line shall be closed to direct currents when the subscriber removes his receiver from its hook in making or in answering a call; third, that the line normally, although open to direct currents, shall afford a proper path for alternating or varying currents through the signal receiving device at the sub-station. The subscriber's station arrangement shown in Fig. 307, and those immediately following, is the simplest arrangement that possesses these three necessary features for common-battery service.