Bell's Telephonic Researches 



the telephone shown in Fig. 8, and I sought, by 

 changing the size and tension of the membrane, 

 the diameter and thickness of the steel spring, 

 the size and power of the magnet, and the coils of 

 insulated wire around their poles, to discover 

 empirically the exact effect of each element of 

 the combination, and thus to deduce a more 

 perfect form of apparatus. It was found that a 

 marked increase in the loudness of the sounds 



Fig. 10 



resulted from shortening the length of the coils 

 of wire, and by enlarging the iron diaphragm 

 which was glued to the membrane. In the latter 

 case, also, the distinctness of the articulation was 

 improved. Finally, the membrane of gold 

 beaters' skin was discarded entirely, and a simple 

 iron plate was used instead, and at once intelligi- 

 ble articulation was obtained. The new form 

 of instrument is that shown in Fig. 10, and, as 

 had been long anticipated, it was proved that the 

 only use of the battery was to magnetize the iron 

 75 



