CONDENSER AND CARBON MICROPHONES 61 



of the output meter is recorded. The sound source and carbon 

 microphone circuit are then short circuited and the output from the 

 attenuator again applied to the measuring circuit. The attenuator 

 is adjusted until the reading of the output meter is the same as was 

 previously obtained with the carbon microphone in circuit. In this 

 way the voltage applied to the measuring circuit when the carbon 

 microphone is in operation is determined. The open circuit voltage 

 developed by the carbon microphone may then be computed from the 

 voltage and the constants of the microphone circuit. At the locations 

 where these measurements were made a certain amount of interference 

 from 60-cycle circuits and low frequency acoustic disturbances was 

 encountered. The high-pass filter in the measuring circuit was intro- 

 duced to facilitate the measurements under these conditions. The 

 adjustable low-pass filter was used to confine the measurements to 

 the fundamental frequency. Only that portion of the apparatus to 

 the left of the dotted line was mounted in the damped room. 



The two buttons of the carbon microphone are identical in their 

 dimensions and if the granular carbon is in the same mechanical state 

 have substantially the same electrical characteristics. They are also 

 practically free from the cyclic variations in resistance known as 

 "breathing" which result from the temperature changes caused by 

 the power dissipated in the granular carbon. It is, however, a matter 

 of every day experience that a given mass of granular material will 

 occupy different volumes, depending upon the configuration of the 

 particles. In the case of microphone carbon this change in configura- 

 tion of the granules results in changes in the contact forces of sufficient 

 magnitude to affect the resistance and sensitivity. If these changes 

 occur in unequal amounts in the buttons electrical unbalance results. 

 When complete balance exists the electrical output is free from all 

 harmonics introduced by the circuit. Hence, in using the microphone 

 care should be taken to see that a fair degree of balance between the 

 buttons is maintained. 



The performance of a carbon microphone may be affected adversely 

 by cohering of the granules. Severe cohering is accompanied by a 

 serious reduction in resistance and sensitivity which persists for an 

 extended period unless the instrument is tapped or agitated mechan- 

 ically. One of the common causes of cohering is breaking the circuit 

 when current is flowing through the microphone. Experiment has 

 shown that the insertion of a simple filter consisting of two .02 mf. 

 condensers and three coupled retardation coils each having a self- 

 inductance .0014 henry, will effectively protect the microphone button 

 from cohering influences without introducing an appreciable trans- 



