The representation and computation of fluctuation voltages

Much has been written concerning the fundamental physics of fluctuation phenomena, but the methods to be used in computing signal/noise ratio in the complex circuits of modern communication engineering have never been fully described. This paper gives particulars of two methods by which such computations can be made by the ordinary processes of circuit analysis, no reference to the obscure physical mechanism of the effects being necessary. In the first method, voltage representation, the fluctuations associated with valves and resistances are represented by generators injecting voltages of specified mean-square value in series with each valve or resistance. In the second method, current representation, the representation is by a current injected by a hypothetical generator, having infinite internal impedance, connected in parallel with the valve or resistance. Either method may be used, and their relative values in given circumstances are discussed. Both representations appear devoid of physical significance and neither has been rigorously justified theoretically, but extensive experiments have shown that exact calculations of fluctuations in complex networks can be made by their use.