500 Messrs. Wood, de Long, and Compton on 



pressure, Mr. Jenkinson interpreted his results by the 

 equation 



E=^(2*-l)logg, (1) 



where p 1 and p 2 are the partial pressures of the ions of either 

 kind just outside the electrodes, x is the proportion o£ the 

 total current carried by the positive ions, and E is the electro- 

 motive force resulting from the unequal ionic concentrations 

 at the two electrodes. According to this equation the sign 

 of the electromotive force should depend on the value of x y 

 i. e. on the kind of ion, positive or negative, which effects 

 the greater transfer of electricity. 



From the sign of the electromotive force observed in his 

 experiments (the electrode exposed to the ionizing radiations 

 always acquiring a positive charge), Mr. Jenkinson concluded 

 that it must be the positive ions which determine the potential 

 step between a metal and the surrounding gas. He takes 

 from his experiments support of the view that a layer of 

 occluded gas at the surface of a metal is essential to the 

 production of a contact potential effect, and that positive ions 

 only pass from this layer to the surrounding gas. 



In view of the increasing importance of discovering the 

 conditions which exist at the surface of a metal, we feel 

 justified in pointing out a much more probable interpretation 

 of Mr. Jenkinson's experiments which involves neither the 

 formation of an occluded gas layer nor the passage of 

 electricity to and from a metal by the sole agency of 

 positive ions. 



Theory. — Consider the simplest case, that of two infinite 

 parallel metal plates and a source of ionizing radiations 

 which ionizes the intervening gas more strongly near one 

 plate than the other. If the two plates are originally at tl e 

 same potential, the ions will move by diffusion from the 

 regions of greater to those of less concentration ; but this 

 process will never produce uniform ionic concentration in all 

 parts of the gas because of recombinations which take place 

 simultaneously with diffusion. In this respect the present 

 case differs from the similar case of an electrolyte. 



If the positive and negative ions diffuse at the same rate 

 the plates will obviously remain at equal potentials ; but if 

 the negative ions diffuse at a more rapid rate than the 

 positive ions, as is usually the case, the plate in the region 

 of less ionic concentration will acquire a negative charge 

 and a negative potential with respect to the other plate. If 

 the plates are insulated, this process will continue until the 



