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XI. The Mobilities of the fans produced by Rimtgen Rays in Gases and Vapours. 



By E. M. WELHSCH, M.A. (Sydney], Emmanuel College, Cambridge; Barker 

 Graduate Scholar of the University of Sydney. 



Communicated by Prof. Sir J. J. THOMSON, F.R.S. 

 Keceived December 19, 1908, Eead January 21, 1909. 



1. Introductory. 



FOR various reasons the determination of the velocities in an electric field of the ions 

 produced in gases by the action of Ro'utgen rays is of fundamental importance in 

 electrical theory. A knowledge' of the ionic mobilities (i.e. the velocities under unit 

 electric intensity) affords information with regard to the structure of the ion ; if, in 

 addition, the diffusion coefficients of the ions in various gases are known, the charge 

 associated with the ion can be compared with that carried by the monovalent ion in 

 the electrolysis of solutions. 



Experimental methods of determining the mobilities of the positive and negative 

 ions were devised not long after the ionising action of the Rontgen rays was known. 

 RUTHERFORD* determined the values of the sum of the positive and negative 

 mobilities in a series of gases. ZELENY,! by comparing the velocity acquired by the 

 ions in an electric field with that of a gaseous current parallel to the field, succeeded 

 in deducing the values of the difference of the ionic mobilities and also their ratio. 

 In later experiments ZEL,ENY| employed a current of gas in a direction perpendicular 

 to the electric field and deduced the absolute values of the mobilities in air, oxygen, 

 carbon dioxide, and hydrogen. 



No determinations, however, were made of the ionic mobilities in vapours. The 

 determination of the physical constants of vapours opened out a considerable field for 



* ' Phil. Mag.,' vol. 44, p. 422 (1897). 

 t ' Phil. Mag.,' vol. 46, p. 120 (1898). 

 J ' Phil. Trans.,' A, vol. 195, p. 193 (1900). 



For a concise account of the experiments on ionic mobilities vide J. J. THOMSON, " Conduction of 

 Electricity through Gases," 2nd edition. 



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