612 BELL SYSTEM TECHNICAL JOURNAL 



In a diode, the tube noise may be expressed equally well and with 

 equal correctness either as a modified shot noise or as a thermal 

 resistance noise. In this paper, the thermal resistance viewpoint was 

 taken for two reasons. First, the coefficient "X," used in the thermal 

 resistance noise equation 



£/ = ^krp{\T)df, 



is practically always a constant equal to 0.644, whereas, the factor, 

 " F," used by Schottky and Spenke in their modified shot noise 

 equation 



7? = 2eFUodf 



is always a function of the operating condition. That is, for the 

 operating conditions for which X is a constant, F has the following 

 value: 



1.39 



F = 



^""^T-W^^ 



The second reason for the selection of the thermal resistance noise 

 relation is that power from the motion of the atoms in the cathode is 

 actually transferred to the plate electrode and external circuit through 

 the mechanism of the initial electron velocities. Hence, the tube 

 noise in a diode with space charge is very similar to a thermal resistance 

 noise. 



Part III — Effect of Transit Time 



The analysis, in Part I, while giving the correct results for all 

 operating conditions in the ordinary frequency range, is extremely 

 long and cumbersome. It shows, however, that only the limiting 

 values of the effective temperature of the plate resistance are required 

 for most practical cases, and therefore it points the way to make 

 simplifying assumptions which result in a much shorter analysis, and 

 moreover, which allow the analysis to be extended to frequencies so 

 high that electron transit time phenomena become of importance. 



Thus the final noise equation in Part I shows that for moderately 

 high anode potentials and for the usual excess of cathode emission, 

 a very good approximation may be had by a consideration of the 

 current-voltage relations existing in the jS-region between potential 

 minimum and anode without the necessity of encumbering the analysis 

 by including the a-region between potential minimum and cathode. 

 Moreover, for a large anode potential, the terminal velocities of the 



