258 BELL SYSTEM TECHNICAL JOURXAL 



expansion of the walls of the chamber and a reduction in the contact 

 pressure between the granules. An increase of this magnitude would 

 be objectionable in commercial use, for it would not only make it 

 more difficult to meet the resistance requirements but would also 

 increase the contact voltages, especially in aged instruments, to a 

 point where undesirable carbon noise would result. To counteract 

 the effect of the expansion of the walls of the carbon chamber, the por- 

 tion of the diaphragm in contact with the carbon is coned slightly, so 

 that, as the walls expand, the diaphragm moves inward sufficiently to 

 maintain substantially constant contact pressures between the gran- 

 ules. The extent to which this feature is effective in reducing breath- 

 ing is shown by curve B, Fig. 9. 



Cohering 



In common with a number of other materials, granular carbon is 

 susceptible to cohering. When cohering occurs the resistance and 

 sensitivity of the carbon are lowered and remain so until the transmitter 

 is subjected to mechanical agitation. Experience has shown that 

 cohering will greatly reduce the output of the handset transmitter and 

 that fairly loud talking or a sharp mechanical shock is required to re- 

 store it to its initial sensitivity. Not infrequently cohering results 

 from breaking the circuit connecting the transmitter to the battery, 

 as for example, when the subscriber depresses the switch in the mount- 

 ing in order to attract the attention of the operator. A study of the 

 electrical conditions responsible for cohering under these circumstances 

 has shown that the distributed capacity and the inductance of the 

 component parts of the station set are such that transient oscillations 

 of a frequency of several thousand kilocycles per second are set up by 

 the breaking of the circuit. Further investigation has shown that the 

 transmitter can be protected from the cohering effect of these oscilla- 

 tions, without introducing a transmission loss at voice frequencies, by 

 connecting a condenser of a few thousandths of a microfarad capacity 

 across the transmitter to by-pass these transient currents. 



In the case of the deskstand transmitter, these cohering effects have 

 not been important for several reasons. Probably the principal one is 

 that the mechanical impact incident to switchhook operation is carried 

 directly to the transmitter and prevents appreciable cohering. 



Ca rbon Prepa ra t io 7i 

 Substantial improvements in the technique of granular carbon man- 

 ufacture have been made in conjunction with the development of the 

 new transmitter. Roasting processes have been adopted which sub- 



