﻿268 Prof. 0. W. Richardson on the Electron 



the writer that the balance of the evidence is now strongly 

 in favour of the view that the Volta contact electromotive 

 force is an intrinsic property of the pure metals, and not, as 

 is perhaps more widely believed, an accidental development 

 caused by traces of chemical action. At the same time there 

 is no reason for doubting that the actual measured value of 

 the contact electromotive force is very sensitive to actions 

 of a chemical nature, as it should be on the view we have 

 been discussing. 



The case for the chemical origin of contact electromotive 

 force rests largely on the results of a number of experiments 

 which show that under certain circumstances, which are 

 usually interpreted as involving the absence of oxygen or 

 water-vapour from the surface film*, the contact electromotive 

 force, as measured by the condensing electroscope or an 

 equivalent method, is found to either vanish or drop to a 

 comparatively small value. The circumstances referred to are 

 rather varied, and include boiling in heavy oil at about 

 140° C. (J. Brown *), exposure to the temperature of liquid 

 air (Majorana f ), and heating in a vacuum in the presence 

 of drying agents (Spiers t, Greinacher §). In all these 

 cases it seems quite likely that the surface of the metals 

 under examination becomes covered with an insulating layer 

 which entangles the escaping electrons. This would give 

 rise to a double layer at the surface of each metal, and the 

 seat of the w T hole of the potential difference between the 

 metals would lie in these double layers. The surface of each 

 metal would then act like a condenser positively charged on 

 the metal side and negatively on the other. On separating 

 the plates there would be no change in the capacity of the 

 system and no free charges would be developed. An expla- 

 nation along similar lines accounts for the null effects obtaiued 

 by Greinacher by the ionization method. It seems quite 

 clear that when the plates are immersed in insulating oil the 

 apparent contact difference of potential will vanish on the 

 electron theory. In the case of Majorana's experiment it is 

 necessary to suppose either that a minute film of condensed 

 moisture developed on the plates, or that the hydroxide films 

 always present become non-conducting at the low temperatures. 

 In any case a film a few molecules in thickness is all that is 

 required. It is significant that Greinacher using the ioniza- 

 tion method was unable to confirm Majorana's result. In 



* Phil. Maor. [5] vol. v. p. 591 (1903). 

 f Atti de Lincei, vol. ix. p. 162 (1900). 

 % Phil. Mag. [4] vol. xlix. p. 70 (1900). 

 § Ann. der Physih, vol.. xvi. p. 708 (1906). 



