THE TELEPHONE 337 



and the telephone reached a high degree of efficiency. Copper 

 wire, underground cables, long distance lines, and girl opera- 

 tors were innovations of this period. 



The period from 1896 to 1906 was one of expansion. 

 This was the period of business, when profits were made as 

 a result of previous toil and investments. During this 

 period there appeared the pay station, the farm line, and the 

 private branch exchange. 



The period from 1906 to 1916 was one of organization. 

 During this period the telephone has reached out into ever 

 broadening fields till it has become truly national in scope. 



How the telephone works. Having seen something of 

 the history and uses of the telephone, let us now try to under- 

 stand how it is constructed and how it works. Like the 

 telegraph it uses electricity, yet in principle it is quite dif- 

 ferent. In the telegraph the circuit can be opened and 

 closed, and the signals sent in this way ; but in the tele- 

 phone the voice is not strong enough to do this. A current 

 is passing through the wire all the time and the voice causes 

 variations in the strength of the current. 



In the telephone, it is not sound vibrations that travel 

 along the wire, but an electric current. 



With the telegraph, the earth can be used as a return 

 wire, thus saving the expense of one wire, but with the tele- 

 phone two wires are generally used, because the telephone 

 is so sensitive that it would be interfered with by various 

 disturbances that might develop in the earth. 



Receiver and transmitter. In the simplest form of tele- 

 phone first used, one piece of apparatus served both as 

 transmitter and receiver. Figure 138 shows the construction 

 of this. It consists of a U-shaped magnet which has around 

 each pole a coil C of many turns of very fine wire which is 

 connected with the outside circuit. Just in front of the mag- 

 net is a thin diaphragm D made of metal. As one speaks 

 into the transmitter the air waves produced by the voice 

 z 



