The Linear Theory of Fluctuations Arising from Diffusional Mechanisms - An Attempt at a Theory of Contact Noise

H E N a direct current is passed through a granular resistance such as a carbon microphone or a metallic-film grid leak, or through a single contact, there is produced a voltage fluctuation possessing a component called contact noise which is differentiated from the familiar thermal noise component by the fact that its r.m.s. value in any frequency band is roughly proportional to the magnitude of the average applied voltage, and is differentiated from shot noise by the strong frequency dependence of its spectral density. One may regard this component of the voltage fluctuation as arising from the spontaneous resistance fluctuations of the element in question if one is willing to allow the resistance to have a slight voltage dependence. This effect has been the subject of numerous experimental investigations, 1-8 among which we mention in particular that of Christensen and Pearson 9 on granular resistance elements. These authors (henceforth abbreviated as CP) arrived at an empirical formula, to be discussed presently, connecting the contact noise power per unit frequency band with the applied voltage, the resistance, and the frequency for several types of granular resistance. Their measurements covered a range of frequency from 60 to 10,000 cps, and involved the variation of several other parameters, i.e., pressure. More recently, Wegel and Montgomery 10 have measured the noise power arising H. A. Frederick, Bell Telephone Quarterly 10, 164 (1931). A. W. Hull and N. H. Williams, Pliys.