TELEPHONES 



443 



simple telephones were so feeble that the range of possible 

 operation was very much restricted. This line of investi- 

 gation had its inception in the fact that the investigators 

 who were experi- 

 menting with the 



first telephones Fig. 335. The simplest possible telephone system. 

 observed that ^^ e current * 5 produced by inductive effect only. 



movements of the connections in the conductors forming 

 the circuits produced sounds in the telephones. 



Various substances were used in the experiments, and it 

 was found that, while conducting bodies generally, when in 

 loose contact, would vary the resistance in a circuit when 

 subjected to the vibration of a diaphragm caused by sound 

 waves set up by audible speech, carbon was most positive 

 and uniform in its performance in this regard. 



Carbon transmitters of many designs have been pro- 

 duced. The single-contact Blake transmitter was in very 



general use a few years ago. 

 Several forms of multiple-contact 

 transmitters employing carbon 

 pencils or carbon balls were used 

 to some extent. The type most 

 popular at present is the granular 

 carbon type, of which the White 

 solid back is a good example. 



Series circuit. The trans- 

 mitters and receivers of a tele- 

 phone circuit may be placed 

 directly in series with the battery 

 and satisfactory results may be 

 obtained where the lines are short, 

 but when the receivers and 

 transmitters are so connected, 

 the receiver must be so constructed as to offer the least 

 possible resistance to the battery current as well as the voice 

 currents. For this reason some of them are known as 



Jhelf 



Permanent 

 maynefe 



Po/e p/ece* 



O/aphram 



Fig. 336. Showing details of 

 transmitter construction 



