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those which have been in use for a sufficient length of time by the Bell and Independent companies to have proved their practical value and become thoroughly standardized. No partiality is shown the equipments and methods of either organization, except in so far as they are superior electrically, magnetically, or mechanically, or in illustrating the points to be impressed upon the reader.

Much new matter has been added to the Second Revised edition and the work is thoroughly upto-date. The material on construction has been enlarged by description of underground conduits and cable laying, bracing and preservation of poles, improvements in line wire practice, etc. A complete explanation of the latest practice in automatic selection at the central exchange is included, and the lamp annunciator system now used so largely is described. The storage battery is explained and many time and labor saving tools, appliances and processes have been treated. THE AUTHORS.

April, 1914.

INTRODUCTION

Telephone Engineering, although one of the youngest of the engineering professions, and perhaps the most handicapped at the start, stands today among those most active and advanced. Starting with a simple invention of Prof. Alexander Graham Bell in 1876, the history of the telephone extends over a period of only thirty-eight years. For several years after its birth, the telephone was looked upon as a mere toy, and, until the patents covering its important parts were bought by a corporation and a working system established, none realized the great future in store for it.

Telephone Companies operating up to the year 1895 were each a part of the corporation just mentioned, and known as the Bell Telephone Company. Upon the expiration in 1895 of the original patents governing the construction of the telephone, its essential features became public property, and many companies independent of the original one were formed to manufacture, sell, and operate telephone apparatus. The Bell Telephone Company, however, continued to operate as before, and the organizations conducted irrespective of or in opposition to them, became known as Independent Companies.

The Development of the Telephone Industry during the last dozen years has been most remarkable, and is largely due to the competition between the Bell and Independent companies. Each year has witnessed improved apparatus, more efficient and less expensive service, with the result that the number of telephone subscribers has increased from 281,700 in 1895, the date of the expiration of the original patents, to over 10,000,000. These telephones are about equally divided between the Bell and Independent organizations, the former of whom do most of their business in our large cities and towns, while the latter have exerted their efforts chiefly in developing trade in villages and smaller cities throughout the United States.

CONSTRUCTION, OPERATION,

AND INSTALLATION OF
TELEPHONE INSTRUMENTS

The Receiver. The receiver in use to-day differs but slightly from the original magnet telephone brought out in 1876. Its principle of operation is precisely the same, and in explaining its action reference will be made to the simple device

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FIG. 1.-Simplest Form of Telephone Circuit, Showing Principles of Operation

at first employed. Fig. 1 shows one of these magnet telephones at A. The principal parts are: a permanent bar magnet ns, a coil of insulated wire c mounted on one end of ns, and a thin iron diaphragm or disk e supported in front of the magnet.

The Principle of the Magnet Telephone is as fol

lows: The iron bar n s, being a magnet, is assumed to have lines of magnetic force issuing from one end and curving backward, entering the opposite end as indicated by the dotted lines in Fig. 1. The magnetic circuits thus formed are completed through the iron bar. On account of the iron diaphragm e in the path of these lines of force offering less resistance to them than the surrounding air, many lines will traverse the diaphragm in completing their circuits outside the magnet. If, now, the diaphragm be vibrated slightly to and from the end of the magnet, there will be produced a change in the lines of force; when the diaphragm is near the bar there will be a greater number of magnetic lines passing through the bar because the resistance of the magnetic circuits has been decreased by shortening the paths of the lines of force, and when the diaphragm is not so near there will be less lines passing through the bar. The change in the magnetic condition of n s induces a varying electromotive force or electrical pressure in the coil of wire c.

Suppose, now, the magnet telephone just considered be connected to an exactly similar telephone B, by joining the terminals of the coils c and r together as in Fig. 1. The variation in the electromotive force produced in the coil c will now cause a varying current of electricity in the coil of telephone B, and this in turn will cause a varying attraction of the diaphragm i, making it vibrate in unison with the diaphragm e of tele

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