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BELL SYSTEM TECHNICAL JOURNAL 



must be applied which neutralizes the Volta potential. The true 

 potential V between anode and cathode is the sum of the applied 

 potential Va and Vv or 



V= \\+ Vv. (42) 



Since 



Vv = iPc - ^a, (43) 



Since 



T^. 



Va 



V = Va-\- <Pc — <Pa- 



Vr = -V, 



- (ipc — (fa) = — Va -\- <Pa ~ ^c 



(44) 



(45) 



Vr is the true value of the retarding potential and these values of 

 Vr are to be used in equations (39), (40) and (41). V and Vr are 

 measured from the break point in Fig. 3. Figure 5 illustrates the case 



CATHODE ANODE 



Fig. 5 — Potential distribution between parallel plates; Pa < Pc- 



when ipc > (fa. For Fa = 0, V is positive and equal to <pc — <Pa. 

 To produce zero field Va must be negative and equal to <pc — ^a- The 

 dashed line gives the potential energy distribution for a somewhat 

 larger negative applied potential. 



When the contact potential is not zero, the break point in the log i 

 versus Va curve will occur when the field is zero or when 



Va = — {(fc — <Pa). 

 This is illustrated in Fig. 3 by the dashed line for a case for which 



<Pc > (fa- 



