266 MR, J. S. TOWNSEND ON THE DIFFUSION OF IONS PRODUCED IN AIR 
cock S } was open. If the velocity so found was too big or too small, the length of the 
capillary tubing was changed so as to bring the velocity V within the required limits. 
Any alteration in the tubing leading into the diffusion apparatus necessitates a fresh 
determination of Y before experiments on the conductivity are made. 
The tube G is connected to the gasometer only for the purpose of determining Y. 
Since the pressure inside the gasometer is the same as the atmospheric pressure, 
the air from the room may be allowed to enter G directly during experiments on the 
conductivity. 
The electrical arrangements were the same as have been already described. 
C5 o 
Method of Conducting the Experiments with Air at Low Pressures. 
The apparatus is arranged as shown in fig. 2, and the pressure is reduced until it 
is a little lower than the pressure P at which the experiment is to be made. The 
stopcock S x is turned on and the air is allowed to run through the apparatus for about 
a minute before the quadrants, to which F x is connected, are insulated. When the 
manometer shows that the pressure is about 2 millims. lower than P, the quadrants 
are insulated and the deflection n Y divisions per minute on the electrometer scale is 
observed. The observations are continued until the pressure is about 2 millims. 
above P, and the mean taken. The difference between the deflections in the first 
and last half minute was scarcely perceptible. 
Having determined n lt the stopcock So was closed, and the air is admittted into the 
diffusion apparatus through S l5 the tube B 1 was then removed and Ik put in its 
place. Before making observations with F 2 connected to the electrometer, it is 
necessary to test whether the joint between the two discs is air-tight. For this 
purpose S, was closed and So opened so as to let some of the air from the diffusion 
apparatus into Wj. The stopcock S 2 was then closed and the manometer was observed. 
It was found that the air did not get into the apparatus at one-thousandth the rate 
at which it entered when Sj was open. 
The determination of n 2 is then made in the same way as n v Particular care was 
taken in all cases to make the observations over the same part of the electrometer 
scale. 
Generally the deflection n. 2 was obtained from two half-minute observations. 
Since ?q is much smaller than n. 2 , its value was taken as the mean of a number of 
observations made while the spot of light was passing the same part of the scale as 
was used for the determination of n 3 . 
The following tables give the results of experiments at different pressures. Tables 
I. and II. refer to positive and negative ions respectively in dry air. Tables III. 
and IV. are the corresponding observations for moist air. The numbers in the 
columns n x and n 2 are the deflections obtained per minute; P is the pressure of the 
