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BELL SYSTEM TECHNICAL JOURNAL 



cathodes are in the same position and have the same temperatures and 

 that the retarding potential is sufficiently great. This theorem was 

 verified experimentally by Davisson.-^ 



-4.5 



-5.0 



-5.5 



-6.0 



-6.5 



-7.0 



0-7.5 

 O 



-8.5 



-9.0 



-9.5 



-10.0 



-10.5 



-1.5 



-1.2 



-0.9 -0.6 -0.3 O 0.3 0.6 0.9 1.2 1.5 



Va~ POTENTIAL APPLIED TO ANODE IN VOLTS 



1.8 



2.1 



Fig. 6 — Retarding potential curves for cylindrical electrodes. 



The curves shown in Fig. 6 are for ideal conditions. Experimental 

 conditions frequently dififer from ideal conditions in at least five 

 respects: (1) The various portions of the cathode are not at the same 

 potential ; (2) the work function of the cathode is non-uniform ; (3) the 

 work function of the anode is non-uniform ; (4) the temperature of the 

 cathode is non-uniform; and (5) the current is limited by space charge 

 rather than by the applied potential. The last two conditions are 

 discussed in other treatises. The first condition is usually due to the 

 fact that the cathode is a filament and is heated by passing a current 

 through it. As a result the observed curve is the sum of a series of 



