INSTRUMENTS FOR THE NEW TELEPHONE SETS 351 



during the development of the new unit. As previously mentioned, 

 this cannot be fully explained by a non-liaear relationship between 

 input and output. The available data indicate that it is due to the 

 manner in which the resistance changes cyclically when pressure waves 

 of two frequencies are impressed simultaneously on the transmitter. 

 Similar effects also haye been observed in the microphonic action of 

 carbon contacts themselves. Hence it is not unlikely that this is a 

 fundamental characteristic of the carbon itself. If this proves to be 

 true the extent to which an improvement can be effected in the per- 

 formance of the transmitter will depend upon the ultimate control 

 which can be exerted over the basic properties of the granular material. 



Receiver Unit 



The new receiver unit is of the bipolar permanent magnet type. 

 The magnetic circuit consists of pole-pieces of 45 per cent permalloy, 

 two straight bar magnets of remalloy and a permendur diaphragm,^ 

 The magnets are welded to the pole-pieces to form a unit which is 

 mounted on projecting lugs on the die-cast frame. The coils are 

 wound with enamel insulated wire interleaved with cellulose acetate. 

 The pole tips project through a phenol fibre plate which is fastened at 

 the edge to the frame to form a cavity in back of the diaphragm. 

 This cavity is connected to the recess in the handset handle or receiver 

 shell by a hole in the plate. A disc of specially prepared silk covers 

 this hole and provides the required amount of acoustical resistance. 

 The silk fabric is so woven that it does not change in resistance with 

 wetting and drying. The front of the unit is protected by a perforated 

 metal grid which is assembled to the frame by means of a thin ferrule. 

 A disc of impregnated silk is mounted between the grid and the frame 

 to form a screen which prevents the transfer of foreign material from 

 the front to the back of the diaphragm when the receiver is dropped. 

 The grid and ferrule are anodically finished to resist corrosion. The 

 spring contact surfaces are silver plated. 



Prior to the introduction of this unit the receivers in general use for 

 telephone purposes in this country and abroad employed simple 

 resonant diaphragms and as a result had response characteristics which 

 were characterized by prominent resonance peaks. As a rule the peak 

 due to the first overtone, as well as that due to the fundamental 

 resonance, fell within the important frequency range. This resonance 

 not only introduced frequency distortion but increased the intensity 

 with which circuit disturbances such as clicks were reproduced. 

 Furthermore, the diaphragms of these receivers were rigidly clamped 

 between surfaces which differed in temperature coefficients of expansion 



