at Low Pressures. 
263 
At one millimetre pressure this gives 
k 2 -h= 1 42 x 10 5 — . 
sec. 
The actual difference between the velocities of the negative 
and positive ions in the positive column is given by 
v 2 -v 1 = X(k 2 - h) = 34-9 Vi? x 1-42 x UPp-™ 
_ 4-95 x 10 6 
P 
These experiments were carried out in the Cavendish 
Laboratory and my best thanks are due to Prof. J. J. Thomson 
for his kindly interest and advice throughout the course of the 
work. 
