260 BELL SYSTEM TECHNICAL JOURNAL 



The higher efficiency of the handset receiver is, however, of appre- 

 ciable indirect value, and has been used in two ways, at the same time 

 maintaining the same intensity of the received speech as is obtained 

 with the deskstand. 



By lowering the impedance to about half that of the deskstand 

 receiver, the loss in the sidetone path and the transmitting efficiency of 

 the set have been increased. The lower sidetone tends to increase the 

 speaker's talking level, giving a further effective gain in transmitting 

 efficiency. The reduction in the room noise appearing in the sidetone 

 also results in an effective receiving gain as compared to the desk- 

 stand. 



The separation between the diaphragm and the pole pieces has been 

 increased so as to minimize the "freezing" of the diaphragm on the 

 pole pieces, which is likely to occur when clamped diaphragm receivers 

 are subjected to sudden changes in temperature of considerable mag- 

 nitude. 



Assembled Handset 

 The dimensions and proportioning of the handle and the assembled 

 handset are the result of rather extensive investigation. They repre- 

 sent an effort both to meet the technical requirements and to produce 

 a mechanically rugged structure, light in weight and comfortable to 

 hold, as well as attractive in appearance and in harmony with the 

 mounting. 



Prevention of Howling 



One of the major objections to the handset in the past has been the 

 tendency to howl. While a certain degree of howling control can be 

 realized in the electrical coupling by the design of the station circuit, 

 such control cannot be depended upon to take care of extreme circuit 

 conditions where the coupling is greatest and howling most likely to 

 occur. 



From an economic standpoint, moreover, it is desirable that the 

 handset should be completely interchangeable with the deskstand 

 without the necessity for changes in the set or other parts of the tele- 

 phone plant. If this is to be realized, the handset must be so designed 

 as to be Inherently free from the danger of howling under any conditions 

 encountered. 



Mechanical disturbances set up by the receiver produce transverse 

 vibration In the handle which moves the transmitter as a whole. The 

 solid phenol plastic handle employed in the present handset has a 

 relatively high resonant frequency and provides a comparatively in- 

 efficient medium for the transmission of mechanical disturbances from 



