486 TATE AND FOOTE : ANODE FALL OF POTENTIAL 



the voltmeter were 20 per cent less than those by the potenti- 

 ometer. If the voltmeter readings were a correct indication of 

 the potential of the probe, it would require that 20 per cent of 

 the total energy be expended between the probe and cathode 

 and radiated back to the crucible. Even if we make the most 

 favorable assumption that all of this energy is expended at the 

 probe, since the crucible subtends a solid angle of 2ir at the 

 probe while the radiation from the probe must take place 

 through 4-7T, it is evident that only half of this energy, or 10 per 

 cent of the total energy, could reach the anode, still leaving 

 10 per cent to be accounted for. This amount of energy was 

 several times greater than possible errors of observation. 

 Furthermore, the bright tin surface possesses such a high re- 

 flection coefficient that undoubtedly very little energy could be 

 absorbed were it present in the form of radiation. Still further, 

 one would have to explain how a probe carrying a thousandth 

 or less of the main current could produce a potential fall for 

 the entire current in a space already completely ionized. 



Conclusion. A film of high resistance forms at the surface 

 of a probe placed in the anode glow. If a voltmeter is employed 

 for the measurement of anode fall, the readings must be cor- 

 rected to allow for the potential drop through this high-resis- 

 tance film. Such a correction would be impracticable on ac- 

 count of the varying nature of the film. 



Since the probe functions as an anode during voltmeter read- 

 ings, evidence has been obtained for the existence of an anode 

 polarization similar to the polarization at the cathode in a 

 highly ionized gas, as observed by Skinner. The magnitude of 

 the anode polarization is very much less than the cathode polar- 

 ization. This explains the fact observed by Skinner that in 

 an auxiliary transverse discharge through the cathode glow the 

 resistance and the current potential curves are characteristic 

 of the auxiliary cathode employed. 



