40 IONIZATION THEORY OF GASES [CH. 
velocity proportional to the strength of the field. On the appli- 
cation of an electric field, the ions almost instantly attain the 
velocity corresponding to the field and then move with a uniform 
speed. 
Zeleny? first drew attention to the fact that the positive and 
negative ions had different velocities. The velocity of the negative 
ion is always greater than that of the positive, and varies with the 
amount of water vapour present in the gas. 
The results, previously discussed, of the variation of the current 
with voltage and of the rate of recombination of the-ions do not of 
themselves imply that the ions produced in gases by the radiations 
from active bodies are of the same size as those produced by 
Rontgen rays under similar conditions. They merely show that 
the conductivity under various conditions can be satisfactorily 
explained by the view that charged ions are produced throughout 
the volume of the gas. The same general relations would be 
observed if the ions differed considerably in size and velocity from 
those produced by Réntgen rays. The most satisfactory method 
of determining whether the ions are identical in the two cases is 
to determine the velocity of the ions under similar conditions. 
In order to compare the velocity of the ions?, the writer has 
used an apparatus similar to that shown in Fig. 6 on p. 37. 
The ions were carried with a rapid constant stream of air 
past the charged electrode A, and the conductivity of the gas tested 
immediately afterwards at an electrode 5, which was placed close 
to A. The insulated electrodes A and B were fixed centrally in 
the metal tube Z, which was connected with earth. 
For convenience of calculation, it 1s assumed that the electric 
field between the cylinders is the same as if the cylinders were 
infinitely long. 
Let a and b be the radi of the electrode A, and of the tube Z 
respectively, and let V = potential of A. 
The electromotive intensity X (without regard to sign) at a 
distance r from the centre of the tube is given by 
Wa 
MS Top 
r log, — 
Beg 
1 Phil. Mag. July, 1898. 2 Phil. Mag. Feb. 1899. 
