
66 IONIZATION THEORY OF GASES [cH. 1 
The results showed 
(1) That the current through the gas for small voltages 
increased more rapidly than the potential difference applied, but 
not as rapidly as the square of that potential difference. 
(2) The current through the gas depended on the direction of 
the electric field; the current was always smaller when the active 
plate was charged positively on account of the smaller mobility of 
the positive ion. The difference between 7, and 7 was greatest 
when the gas was dry, which is the condition for the greatest 
difference between the velocities of the ions. 
An interesting result follows from the above theory. For given 
values of V and d, the current cannot exceed a certain definite 
value, however much the ionization may be increased. In a 
similar way, when an active preparation of radium is used as a 
source of surface ionization, it is found that, for a given voltage 
and distance between the plates, the current does not increase 
beyond a certain value however much the activity of the material 
is increased. 
In this chapter an account of the ionization theory of gases has 
been given to the extent that is necessary for the interpretation of 
the measurements of radio-activity by the electric method. It 
would be out of place here to discuss the development of that 
theory in detail, to explain the passage of electricity through 
flames and vapours, the discharge of electricity from hot bodies, 
and the very complicated phenomena observed in the passage of — 
electricity through a vacuum tube. This chapter was written 
before the publication of J. J. Thomson’s recent book Conduction 
of Electricity through Gases (Cambridge University Press, 1903), 
to which the reader is referred for further information on this. 
important subject. 
