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VII. The Diffusion of Ions produced in Air by the Action of a Radio-active Sub¬ 
stance, Ultra-violet Light and Point Discharges. 
By John S. Townsend, M.A., Clerk-Maxwell Student, Cavendish Laboratory, 
Fellow of Trinity College, Cambridge. 
Communicated by Professor J. J. Thomson, F.R.S. 
Received May 17,—Read June 14, 1900. 
A general method of finding the rate of diffusion of ions into a gas has heen 
described in a previous paper,* and an account was there given of the results obtained 
with ions produced by Rontgen rays. The present paper gives the results obtained 
with ions produced by a radio-active substance, by point discharges, and by ultra¬ 
violet light, 'fhe principle of the method consists in calculating the rate of diffusion 
from observations on the loss of conductivity of a gas as it passes along metal 
tubing. 
The experiments were arranged so that the loss due to diffusion should be much 
greater than the loss due to other causes. In order to ensure this, there are two 
effects which must be considered in fixing; the dimensions of the tubing : the recom- 
bination which occurs when there are both positive and negative ions present in the 
gas ; and the effect due to the mutual repulsion of the ions which takes place when 
most of the ions are charged with electricity of the same sign. It is therefore 
necessary either to correct for these sources of error or to arrange the conditions of 
the experiments so that the loss of conductivity due to these causes is negligible. 
The present paper is divided into five sections. The first section contains an 
investigation of the relative importance of the processes of diffusion, recombination, 
and mutual repulsion in causing loss of conductivity. The descriptions of apparatus, 
and the results of the experiments made on ions produced by a radio-active substance, 
by ultra-violet light, and by point discharges, are given in Sections II., III., and IV. 
respectively. The conclusions to be drawn from the experiments are discussed in 
Section V. 
Section I. 
In the previous paper we have shown that when a number of ions, A, are uniformly 
distributed throughout' a gas, B, which is entering metal tubing, the ratio R, of the 
* John S. Townsend, ‘Phil. Trans.,’ A, vol. 193, 1899, p. 129. 
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