Spontaneous Resistance Fluctuations in Carbon Microphones and Other Granular Resistances

l W H E N a direct current is passed through certain types of resistance elements a small potential fluctuation between the terminals of the resistance can be observed in addition to that caused by the thermal agitation of electric charge. The resistances in which this effect is particularly noted are granular carbon microphones and commercial grid leaks which are granular in nature, such as sputtered or evaporated metal films, and any of a number of composite materials containing carbon in a finely divided state. If such a resistance element is in a current-carrying circuit associated with a telephone receiver or loud speaker, particularly when amplification is present, a steady hissing noise which sounds like that due to shot effect or thermal agitation of electric charge is heard. It is this noise which sets a practical limit to the use of the carbon microphone in sound fields of low intensity, and of commercial grid leaks in circuits carrying direct current and working at low signal levels, 197