(b) A circuit from the point of transmission to the point of reception 



suitable to transmit these compHcated electrical oscillations 

 in the proper magnitude without undue distortion of form 

 and without interference from electrical currents from other 

 sources. This is the telephone circuit. 



(c) An instrument which, receiving the electrical oscillations trans- 



mitted over the line from the telephone transmitter, repro- 

 duces acoustic waves of proper loudness and quality to 

 correspond with those produced by the speaker's voice, by 

 means of which waves the speech is transmitted to the 

 listener. This is the telephone receiver. 



The telephone invented by Bell in 1875 corresponded in principle 

 to the telephone receiver of today and could be used alternately as a 

 telephone transmitter and a telephone receiver. It consists of a dia- 

 phragm of magnetic material associated with a magnet and coils of 

 wire. When this instrument is placed before the speaker's mouth, 

 the variations in acoustic pressure cause the diaphragm to vibrate. 

 The instrument is so designed that this vibration in the presence of a 

 magnet produces electrical oscillations in the coils of wire. When 

 these oscillations are transmitted over the telephone circuit to the 

 coils of wire in a similar instrument, they cause variations in the 

 strength of the magnet which, in turn, cause vibrations of the dia- 

 phragm of the receiving telephone. The vibrations of the diaphragm 

 produce acoustic waves which reproduce the speech of the talker at the 

 distant end of the circuit. 



This instrument is very inefficient as a telephone transmitter and 

 from earliest days efforts were directed toward the development of 

 transmitters working on a different principle. Bell himself suggested 

 the principle most generally used. This principle is that the vibration 

 of the transmitter diaphragm shall vary the resistance of a local electri- 

 cal circuit through which current is caused to flow by a battery. The 

 variation in resistance can cause variations in the flow of current 

 sufficient to induce relatively powerful electrical oscillations in the 

 telephone circuit — in fact, the oscillations so induced may have a 

 power several hundred times as great as that of the acoustic waves 

 produced by the speaker. The telephone transmitters acting on this 

 principle are, therefore, powerful amplifiers. 



In his early work. Bell devised a transmitter working on this princi- 

 ple consisting of a small platinum wire attached to a diaphragm of 

 goldbeaters skin and dipped very slightly into acidulated water held 

 in a conducting cup. It was with this instrument that the first 



