540 



Mr. A. Schuster. 



is independent of the current. When the glow has completely 

 covered the kathode, a rise in the intensity of the current is accom- 

 panied by an increased difference of potential between the kathode 

 and the surrounding portions of gas. 



Warburg* has studied the fall of potential near the kathode with 

 electrodes made of different material and at different pressures. It 

 is known that the negative glow consists of three parts : a luminous 

 layer directly adjacent to the kathode, the so-called dark space, and 

 the glow proper. As the density of the gas diminishes these layers 

 gradually expand ; but the difference of potential only changes slowly. 

 It has been stated that the difference of potential between a kathode 

 which i s not completely covered with the glow and some point in the 

 glow is independent of the intensity of the current ; Warburg calls it 

 the normal fall of potential. While the fall is normal a change of 

 current is only accompanied by a change in the extent to which the 

 kathode is covered with the glow. Warburg has shown how the 

 normal fall depends on the material of the electrode, the nature of 

 the gas, and the density, and his results are of considerable interest 

 f rom our point of view. Slight quantities of moisture add ed to nitrogen 

 produce a considerable change in the difference of potential. Thus, 

 with platinum electrodes nitrogen containing a small quantity of 

 aqueous vapour gave a normal fall of 260 volts, while dry nitrogen 

 gave 410 volts. This is not due to the fact that aqueous vapour is a 

 better conductor than nitrogen, for, if more than traces of moisture 

 are p resent, the fall of potential is as great or greater than with dry 

 nitrogen. We must, on the other hand, find in these observations a 

 strong support of our view, that the difficulty of passage of positive 

 electricity from the gas to the electrode is much reduced by chemical 

 actions at the electrodes. The effects of small traces of moisture are 

 so similar to those investigated by Dixon and others in chemical re- 

 action b etween gases and between a solid and a gas, that in my 

 opinion we really have no choice but to adopt some such view as I 

 h ave taken of the matter. Warburg draws the same conclusion from 

 his experiments. 



The effects of deoxidation of the electrodes, where zinc or steel 

 kathodes are used in hydrogen, makes itself apparent by con- 

 s iderable reduction of the fall of potential near the kathode, which 

 disappears when the layer of oxide on the kathode has been removed 

 by disintegration of its surface. A change of pressure, according to 

 Warburg, is only accompanied by a slight change in the normal fall 

 of potential. 



* ' Wiedemann, Annalen,' vol. 31, p. 592 (1887). 



