Fluctuation noise in vacuum tubes which are not temperature-limited

This paper is concerned chiefly with a discussion of whether or not the anode stream of a thermionic valve produces a component of fluctuation voltage equal to that which would be produced by a metallic conductor of equal resistance at the cathode temperature. In a recent paper, G. L. Pearson has shown that when the anode current is very small, the fluctuation voltage is equal to that in a metallic conductor of equal resistance at a temperature about half that of the cathode. In this paper it is shown that when the anode current is very small a familiar expression for the shot voltage assumes a form suggestive of a thermal-agitation voltage in a resistance at half the cathode temperature. This form is due to the dependence of anode current on cathode temperature when the anode is negative. Thus it would seem that Pearson´s work does not necessarily prove that the anode stream is the seat of a thermal-agitation voltage: his results can equally well be interpreted as a shot-voltage effect, and such interpretation gives a direct explanation of why the relevant temperature is only half that of the cathode.