CHAPTER XXII 

 TELEPHONES 



Invention. In the year 1875 Alexander Graham Bell 

 made a discovery which led to the electrical transmission of 

 speech. While carrying on experiments with a view to 

 developing an improved system of telegraphy, he observed 

 that a thin strip of iron could be caused to vibrate if sup- 

 ported with its free end in proximity to one of the poles of 

 an electromagnet, the winding of the electromagnet being 

 connected to another and similar one, having associated with 

 it an iron reed which could be set in motion by hand. 



From this beginning it was but a short step indeed to the 

 development of devices which had diaphragms instead of 

 reeds, and permanent magnets instead of electromagnets, 

 which would transmit articulate speech. 



Such devices were 

 exhibited at the 

 Centennial Ex- 

 position in Phila- 

 delphia in 1876. 

 The telephone 

 receivers in use 

 today are essen- 

 tially the same as 

 those early "tele- 

 phones." 



Transmitters. 

 Quite early in the 

 course of experi- 

 mental work on the telephone it became apparent that a 

 better method of generating the telephonic or voice currents 

 would be necessary, because the currents produced by the 



Cover- 



Rubber 

 D/aphram 



Moufhp/ec* 



Case 



Carbon c/z'sc 

 e/ecrroo'es 



Grana/a/eo^ 



Fig. 334. 



Typical receiver as used on induction 

 coil circuits. 



442 



