552 Dr. H. Lester on the Variation of 



Experimental Results. 



In the beginning it was found that lack of saturation 



■characterized both positive and negative currents apparently 



without distinction. The potential-current curve showed a 



sharp bend towards the potential axis at about 40 volts. The 



increase at 400 volts over the value at 40 volts divided by the 



latter represents fairly the difficulty of attaining saturation. 



The lack of saturation will be represented by the expression 



Ai 



— , where i is the current at 40 volts and Ai is the increase 



i 



in i between 40 and 400 volts. 



This extra current may be due to ionization at the 

 surface of the cold electrode, to ionization of a gas or 

 vapour layer near the hot surface, or to phenomena at 

 the surface of the hot electrode. In order to test the first 

 possibility, an apparatus was constructed whereby it was 

 possible to rub the inner surface of the receiving electrode 

 with emery-cloth. Readings could be obtained within 

 30 seconds after rubbing. This treatment showed no 



Ai 

 appreciable change in the value of — . Another apparatus 



was designed in which it was possible to heat the cold 

 electrode to approximately 700° C. While the electrode was 

 being heated the platinum wire was cold, and was drawn 

 ■completely out of the experimental chamber. Repeated 

 heatings, some of several hours' duration, showed no change 

 in the lack of saturation. Another test was carried out as 

 follows : — Experience has shown that after considerable 



Ai 

 heating the value of — becomes very small, at least in many 



Ai . 

 cases. If — is due to the ionization of the film at the surface 

 i 



of the cold electrode, then the film having been destroyed 

 by one wire, — should be permanently small for a second 



wire not previously heated. Three or four tests of this sort 



Ai 

 showed that the value of — for the second wire was inde- 



Ai l 

 pendent of the value of — for the first wire. The series of 



experiments seemed to show conclusively that ionization at 

 the cold electrode of a volatile film could not account for 

 the observed lack of saturation. 



