Abstract :
The paper describes experiments upon a granular carbon microphone, conducted at known amplitudes of vibration of the diaphragm. The results show that the output is determined by six variables, namely:¿Amplitude of vibration; frequency of vibration; resistance of microphone; temperature of microphone; feed current of microphone; arrangement of granules. It is shown that the effect of the last is very much less than has previously been supposed, being of the order of ± 10 percent at 50 mA feed current, and increasing with the feed current. The curves obtained are found to be constant for long periods of time, and they therefore represent characteristics by which microphones may be compared. An experiment upon the conductivity/separation relation for two granules, indirectly supported by recent theoretical work in wave mechanics, together with the other experiments, leads to the hypothesis that the action of the granules is to be regarded as a simple make-and-break effect at each contact. A complete bibliography is appended.
