m 



TELEPHONES 445 



and permanent injury. (3) To increase the sizes of the 

 current-carrying parts is not feasible, since their size and 

 weight must be kept within rather well-established limits, to 



insure that their inertia 



and natural periods of 

 vibrations will be such 

 that the instrument will 



faithfully react under the 



influence of the sound 



Waves impressed Upon Fig. 338. Circuits for common battery 

 .-, induction coil telephone 



them. 



It was found that to establish telephonic connection 

 between any two telephones in the same system, the telephone 

 instruments must be universal in order that they may be 

 used for conversation over lines of practically any length. 

 Induction coil circuit. To extend the range of the 

 battery transmitter and receiver combination, the induction 

 coil is utilized principally in telephones of the local battery 

 or magneto type, where the transmitter current is supplied 

 by a battery individual to each instrument. 



Call bells. To complete the telephone instrument, it was 

 necessary to find some form of electrically operated device 

 which would produce a signal sufficiently loud to call the 

 attention of persons in the vicinity of a telephone instrument 

 to the fact that some one desired to communicate with them. 



At first, when the 

 \\^ try lines were short, an 

 ordinary vibrating 

 doorbell gave satis- 

 factory service, but as 



F JG - 339' Induction coil local battery stations -j^e lines Were ex- 

 using metallic circuit . 



tended it was neces- 

 sary to use the telegraph for signaling purposes. Although 

 efficient, the telegraph was too expensive and cumbersome 

 for extensive telephone systems, and further experiments 

 were made to obtain a satisfactory signaling device for 



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