624 BELL SYSTEM TECHNICAL JOURNAL 



resonances in the enclosed cavities which are associated with these 

 diaphragms, such, for example, as the mouthpiece of the transmitter 

 and the cases in which the transmitter and receiver units are placed. 

 In the present type of deskstand transmitter the several resonances 

 are so located as to give a fairly broad maximum response in the range 

 between 1,000 and 2,000 cycles and the resonance of the receiver has 

 been placed so that its maximum response is around 1,000 cycles. 

 It is seen then that the inherent resonances of these instruments have 

 been located in the more important part of the voice- frequency range 

 and have been utilized to increase their efficiencies in that range. 



The remarkable performance of the granular carbon type of trans- 

 mitter merits some indication of its accomplishment. Its conversion 

 of the complex speech waves into equivalent electrical waves has been 

 improved from time to time and now the most efficient type of trans- 

 mitter which is in general use, when energized with the direct current 

 which it gets on short loops, has an electrical output which is more than 

 a thousand times the magnitude of the acoustical power which is 

 delivered by the speaker. Furthermore, it provides this conversion 

 and large amplification at a low cost. Since the average energy given 

 out by a speaker in carrying on a telephone conversation is of the order 

 of 10 microwatts, the large stepup in power from the acoustic waves 

 entering the transmitter to the electrical waves leaving it is of vital 

 importance in affording telephone service at a reasonable cost and also 

 in rendering the telephone system less susceptible to the effects of 

 interference currents. 



While large improvements have been made in the receiver, the effi- 

 ciency of the present instrument is very low in comparison with many 

 other types of energy converters which it is considered practicable 

 to use. For the receiver, the average ratio of the acoustic power out- 

 put to the electrical power input is below 1 per cent. It is possible 

 to increase materially the efficiency of the receiver used in commercial 

 telephony but this would bring up the noises on the telephone circuit. 

 Also there are limitations upon the maximum efficiency of the combi- 

 nation of transmitter and receiver due to crosstalk between telephone 

 circuits and to the fact that with loud talkers over short telephone con- 

 nections the combination of present instruments is close to the point of 

 giving uncomfortably loud sounds in the ear of the listener. 



In considering the performance of the transmitter in the plant, it is 

 customary for many reasons, important among which is the battery 

 supply circuit, to take the combination of a transmitter, a station set, 

 a typical loop connecting the set to the central office and the cord 

 circuit from which is supplied the direct current for energizing the 



