•556 Dr. H. Lester on the Variation of 



effect by Richardson and Cooke*, have further established the 

 fact of the potential discontinuity. It is probable that this 

 drop of potential takes place in a film at the surface of the 

 metal which has different properties in case of a new wire as 

 •compared with one which has been heated for some time. 

 There is considerable evidence for the existence of surface- 

 films. Langmuirt, for example, showed that films exist, and 

 that it is extremely difficult to get rid of them. I have given 

 •evidence in another paper of the existence of ionized films, 

 such that an escaping electron must do work against the field 

 produced by the film J. It seems to me that a consideration 

 of the conditions in such a film will give an explanation of 

 the peculiarities of initial thermionic currents. 



There is evidence to support the idea that a double layer 

 •exists in the film even when the metal is cold and has not 

 been heated §. However, during heating the escape of 

 volatile impurities brings new material to the surface, and 

 this material coming in contact with active gases forms 

 chemical compounds, some of which are solid, and add to an 

 old or actually form a new film. 



If a positive potential is applied to the hot metal the more 

 loosely held positive ions escape and form the initial large 



Ai 

 current which shows small values for — and negligible over- 

 shooting. This initial current decays rapidly to a smaller 

 value that maybe fairly constant for a long time. 



The conditions under which the current shows slow decay 

 maybe very complicated. It is probable that an equilibrium 

 condition results between the escaping ions and circum- 

 stances tending to prevent their escape. The simplest con- 

 dition that occurs to me is a state in which the ions have to 

 move through neutral material by thermal energy plus the 

 force of the applied field before they are in a favourable 

 position to evaporate. We should then have equilibrium 

 conditions between the rate of formation of ions, the rate of 

 drift to the film surface, and the rate of escape under the 

 accelerating potential. The rate of formation represents an 

 equilibrium condition between the rate at which dissociation 

 occurs and the rate of removal of the ions. It might be 



* Phil. Mag. July 1910 and April 1911. 



t Journ. Am. Chem. Soc. xxxv. p. 105. 



X Phil. Mag. [6] xxxi. p. 197 (1916). 



§ The fact that both the heating effect and the cooling effect are 

 increased when active gases are present indicates that similar films exist 

 ■at both surfaces. 



