442 Mr. S. Ratner on the 



v 



of — , where X is the electric force and p is the pressure, 



P X 



and that for large values of — the mobility begins to 



diminish with increase of the force. Experiments were 

 carried out in order to determine whether the mobility of the 

 positive ion also departs from the inverse pressure law. 

 Todd * has found that at pressures below 1 mm. the mobility 

 of the positive ion also becomes abnormally great ; a com- 

 paratively small increase of the mobility of the positive ion 

 was observed also by J. S. Townsend f, at higher pressures, 

 but these results are contradicted by other experimenters. 



The study of the mobility of ions leads us to results which 

 may throw considerable light upon the nature of an ion and 

 should be thoroughly carried out in all possible directions. 

 Unfortunately the methods usually employed are but little 

 adapted for this purpose when the mobility becomes ab- 

 normally great. Rutherford's, Langevin^s, and Zeleny's 

 methods and their modifications serve only in the case when 



cm. 

 the velocity of the ion does not exceed the order of 10 3 — - 



J sec. 



and therefore can hardly be used for a complete study of 

 the mobility of the negative ion. Chattock's method involves 

 other difficulties, as in this case the mobility has to be 

 investigated under very unfavourable conditions, i. e. when 

 the electric force acting on the ion is very strong and not 

 uniform. The only suitable method for measuring large 

 ionic velocities is that given by J. S. Townsend, depending 

 on the action of a magnetic force on the motion of a stream 

 of ions. 



In the present paper a new method of measuring ionic 

 mobilities is described and some results are given. 



Method and Apparatus. 



The method may be considered as a modification of that 

 given by Chattock J, and is based on the production of a 

 wind in an ionized gas when acted on by an electric field. 

 In the case of the discharge from a point to a plate, Chattock 

 deduced the following expression for the mobility (k) of an 

 ion in terms of the discharge current (c), the corresponding 

 wind-pressure (P), and the distance between the plates (d) : 



k = d^. 



* G. Todd, Phil. Mag. xxii. p. 791 (1911). 



t J. S. Townsend, Phil. Mag. July, 1914. 



J Chattock, Phil. Mag. (5) xlviii. p. 401 (1899). 



