INSTRUMENTS FOR THE NEW TELEPHONE SETS 343 



1. Its performance is less affected by angular position, 



2. There is less aging under the conditions encountered in service. 



3. The electrical output is higher and the response more uniform. 



4. The modulation products resulting from non-linearity are materially 



reduced. 



Effect of Angular Position. — In order to insure good contact between 

 the carbon granules and the diaphragm in the positions in which the 

 handset is most likely to be held in service, the carbon chamber of the 

 earlier transmitter was placed in front instead of the conventional 

 location in back of the diaphragm.^ The positional characteristics of 

 this transmitter were further improved by the use of a "barrier" type 

 of variable resistance element in which the electrodes are stationary 

 and form the walls of the carbon chamber, and in which the surface of 

 the diaphragm in contact with the granules is insulated and serves 

 only as means for changing the contact forces between the granules in 

 response to the variations in sound pressure at the diaphragm surface. 

 While this transmitter represented a distinct advance in handset 

 performance from a transmission standpoint and was quite effective in 

 reducing undesirable positional effects, particularly in the "horizontal 

 face-up" position, it was somewhat complicated mechanically and 

 involved the problem of providing a closure between the diaphragm 

 and the adjacent electrode which would be sufficiently resilient to 

 meet the transmission requirements and at the same time prevent 

 carbon leakage. In addition, there was some degradation in quality 

 when it was held in the "horizontal face-down" position where the 

 carbon granules tended to fall away from the diaphragm. While this 

 condition occurred only infrequently in service, it was one which it was 

 considered desirable to eliminate if this could be accomplished without 

 making the structure mechanically complex or difficult to manufacture 

 or maintain. A tendency also was observed in the field for the re- 

 sistance to increase sufficiently under certain conditions to react ad- 

 versely on the operation of the associated signaling apparatus. Owing 

 to the inherently small areas of the sound passages leading to the 

 diaphragm the moisture condensed from the breath could not be 

 excluded by a membrane without complicating the structure and 

 adding sufficient mechanical impedance to impair transmission. 



Following the introductory work on the barrier transmitter, an 

 intensive study of the direct action type of carbon element was made to 

 determine whether the limitations of the earlier structures of this type, 

 which arose from the non-fluid character of the carbon, could be over- 

 come. This study resulted in the transmitter unit shown on Fig. 2. 

 This unit eliminates the undesirable features of the inverted type with- 

 out sacrificing its desirable characteristics. 



