TELEPHONES 



44 7 



Fig. 



After Western Electric Co. 



341. Common battery 

 wall set 



an essential element of most telephones in common use, 

 was devised, and when the elements so far described are 

 combined the "local battery telephone," with which most 

 people in the United States are 

 familiar, is obtained. 



Telephone system. Two such 

 telephones, when connected by a 

 pair of wires, or by a single wire 

 and the earth, provide facilities 

 for communicating between the 

 points where the instruments are 

 located. 



Most of the telephones in use 

 at the present time form parts of 

 telephone-exchange systems, facili- 

 ties being provided at the central 

 offices for connecting each tele- 

 phone with others in the system. 



For telephone-exchange service in cities and towns it has 

 been found to be possible and desirable to concentrate the 



battery supply for the 



transmitters and the 

 current supply for 

 operating the bells at 

 the central office. 

 Also, it is the practice 

 to arrange the signals 

 which are employed 

 for attracting the 

 attention of the 

 operators at the cen- 

 tral office so that 

 they are responsive 

 to the movements of the hook switch in the telephone set. 

 The telephone instruments employed in such systems are 

 similar to the local battery instruments except that magnetos 



After Western Electric Co. 



Fig. 342. Same as Fig. 341, open 



