DEVELOPMENT OF A HANDSET FOR TELEPHONE STATIONS 257 



mechanical packing of this nature occurs, the initial sensitivity can be 

 restored only by tapping the transmitter until the granules assume 

 their original configuration. By filling the chamber about 5 per cent 

 less than full, it has been found possible to avoid this packing without 

 effecting an appreciable increase in the initial resistance and noise or 

 the aging rate. 



Breathing 

 In many granular carbon transmitters, variations in resistance and 

 sensitivity of considerable magnitude occur from changes in the dimen- 



Z 0.7 



0.6 



0.2 

 0,1 



4 5 6 



TIME IN MINUTES 



I 

 O 



30 z 



UJ 



< 



I- 



20 OT 



Fig. 9 — Transmitter breathing. 



sions of the various parts of the carbon chamber, caused by the heating 

 effect of the direct current. These changes in resistance usually take 

 place rather slowly and often extend over a period of several minutes. 

 It is customary to refer to them as "breathing." In a handset trans- 

 mitter, it is particularly important to eliminate any sources of variation 

 of this kind, since they are likely to be more serious in their effects. 

 Interesting means of breathing control have been provided. If the 

 diaphragm is removed and a block of lavite substituted, the resistance 

 after being connected in circuit changes w ith time in the manner shown 

 by curve A , Fig. 9. The large increase in resistance is caused by the 



