214 APPLIED SCIENCE 



is a comparatively simple one, involving merely the carrying of 

 sound waves by means of an electric current, but in a large 

 city with thousands of telephones and many exchanges, the 

 problem of proper connection and transmission becomes a 

 complicated one. 



The transmitting and receiving instruments are identical 

 in nature, each consisting of a coil of insulated wire con- 

 nected with the line. 



In transmitting, the message is spoken into the mouth- 

 piece at one end. The to-and-fro motion thus imparted to 

 the metallic diaphragm attached to the mouthpiece produces 

 induction currents in the coil. These impulses passing 

 over the main line produce similar movements in the dia- 

 phragm of the receiving instrument and thus cause the latter 

 to reproduce the message in articulate sound to the one 

 listening. 



262. Making a Connection. In order to understand how 

 a call is made through a large city exchange, it is necessary 

 to have in mind a distinct picture of a switchboard and to 

 understand the functions of the various operators. (See 

 Fig. 94.) For the sake of clearness it will be well to take a 

 single typical case. 



Between fifty and ninety subscribers' lines run to each 

 operator's switchboard. Operator A, for instance, receives 

 all the calls from the subscribers on the Audubon exchange 

 whose numbers are from 1 to 50. At the bottom of her 

 switchboard there is a hole, called an answering jack, for 

 each of these lines. Should one of these subscribers, Mr. 

 Smith, take his receiver from the hook in order to call, a 

 small supervisory lamp lights below the answering jack in 

 which Mr. Smith's line ends. Operator A is thus notified 



