the Ions in Gases at Loic Pressures. 561 



per cub. centim., e the charge on an ion, it the velocity of the 

 ion under unit potential gradient, then the quantity of nega- 

 tive electricity lost by the plate in one second is 



Aneu Q aD/b. 



But the plate is observed to fall in potential by ah per second, 

 and the capacity of the system attached to the plate is C : 

 hence the loss of electricity by the plate per second is 



Cad. 



Equating these two expressions for the loss of electricity, wo 

 get 



Aneu xD/b = Cad 



b Cd 

 or e = — x t\ • 



nu A \J 



Hence knowing b, C, A, and u , if we measure n and d/D we 

 can determine e. 



To calculate n we begin by finding the volume of water 

 deposited in consequence of the expansion in each cub. centim. 

 of the expansion. In my previous paper I show how this 

 can be determined if we know the ratio of the final to the 

 initial volumes and the temperature before expansion. In the 

 present experiments the final volume was 1*36 times the initial 

 volume, and the temperature before expansion was 18°'5 C. 

 It follows from this that 50 x 10 -7 cub. centim. of water were 

 deposited in each cub. centim. of the expansion chamber. 



If a is the radius of one of the drops, the volume of a drop 



is 47ra 3 /3, and hence //= j — « ;here n' is the number 



of ions per cub. centim. of the expanded gas. 

 If v is the velocity of fall 



v 9 ii 

 Since for air fx=lS x 10~ 4 , we find 



1-1 x 10 s 



and 



i7r« :! = 3-Uy 3 2 xlO- 9 , 

 o 



, 5000 



n — 



3-14 1>! 



