692 Prof. E. P. Adams and Mr. A. K. Chapman 



the other hand, the positive metal ions can scarcely be supposed 

 to be the sole carriers o£ electricity across the surface. 

 What then are the ions in the metal which have the effective 

 pressure P ? 



A suggestion only to an answer can be given. Every thing- 

 points to the conclusion that the presence of an occluded 

 layer of gas in a metal surface is an essential feature in the 

 production of the P.D. between the metal and air. We may 

 imagine that this layer exists at a potential different from 

 that of the air, the transference of electricity from it to 

 external air being effected by + air ions. (It may also 

 possibly be at a potential different from that of the metal, the 

 total step up from metal to air would then be the sum of 

 that from metal to layer and from layer to air. It is then 

 possible that the potential step from metal to layer is controlled 

 by corpuscles in the way described above.) According to 

 this view, P would represent the ionic pressure in this 

 occluded layer, and the results of the present experiments 

 suggest first, that the production of this layer is a reversible 

 process in a thermodynamical sense, and secondly, that 

 positive ions must be looked upon as the controlling agent in 

 the production of the potential step between it and the 

 external air. 



These experiments were carried out some five years ago in 

 the Physical Laboratory of the University of Sheffield. It 

 was hoped at the time that the effect might be made the basis 

 of a method of measuring the ranges of ionization in air of 

 ol rays, as if one of the wires is a little behind the other, when 

 one is in the range and the other out of it an E.M.F. would 

 be developed. Pressure of other work has, however, 

 prevented the experiments being carried on in this direction. 



I -have to thank very sincerely Dr. Hicks and especially 

 Dr. Milner, at whose suggestion the work was undertaken, 

 for much kind help, advice and encouragement. 



LXXII. The Corbino Effect. i%E. P. Adams, Professor of 

 Physics, and Albert K. Chapman, Fellow in Physics, 

 Princeton University*. 



WHEN a uniform radial electric current flows through a 

 circular metallic disk in a magnetic field normal to the 

 plane of the disk, there is produced a circular electric current 

 such that the current density is inversely proportional to the 

 radius. This effect was predicted by Corbino and confirmed 

 by him by experiments with a bismuth disk. In a paper 

 * Communicated bv the Authors. 



