THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



SEP TEMB.ER 1915. 



r 



XXXII. The Mobility of Negative Ions at Low Pressures. 

 By Sir J. J. Thomson, O.M., P.E.S* 



FROM the experiments made by Franck and Pohl on the 

 mobility of the negative ions in argon and nitrogen 

 carefully freed from oxygen, and also from the very interest- 

 ing investigation on the mobility of the negative ions in air, 

 recently published by "Wellisch f, it seems clear that the 

 electron can traverse in a free state distances which are 

 large compared with the mean free path. In a recent 

 communication to the Cambridge Philosophical Society, 

 I showed that tl lis result would explain many of: the 

 peculiarities shown by the negative ion at low pressures, 

 such, for example, as its abnormally large mobility and the 

 lack of proportionality between its velocity and the electric 

 force acting upon it. In this paper I wish to find expressions 

 for the magnitude of effects due to this cause and the factors 

 on which they depend. 



The case considered is when the charged particles are 

 moving under a uniform electric iield through gas enclosed 

 by two parallel plates at right angles jjo the lines of electric 

 force : this corresponds to the conditions under which the 

 mobility of the negative ion is usually investigated. All the 

 negative ions are supposed to begin as electrons, though 

 some of them ultimately unite with the molecules of the 

 gas and become negative ions. It is probable that when 



* Communicated by the Author. 



t American Journal of Science, May L915. 



Phil Mag. S. 6. Vol. 30. No. 17 7. Sept. 1915. Y 



