56 IONIZATION THEORY OF GASES [CH. 
volts—a value considerably greater than that observed by Town- 
send from data of ionization by collision. The ionization in the two © 
cases, however, is produced under very different conditions, and it 
is impossible to estimate how much of the energy of the rays is 
dissipated in the form of heat. 
42. Variations are found in the saturation current through gases, 
exposed to the radiations from active bodies, when the pressure 
and nature of the gas and the distance between the electrodes are 
varied. Some cases which are of special importance in measure- 
ments will now be considered. With unscreened active material 
the ionization of the gas is, to a large extent, due to the a rays, which 
are absorbed in their passage through a few centimetres of air. 
In consequence of this rapid absorption, the ionization decreases 
rapidly from the surface of the active body, and this gives rise to 
conductivity phenomena different in character from those observed 
with Réntgen rays, where the ionization is in most cases uniform. 
43. Variation of the current with distance between the 
plates. It has been found experimentally! that the intensity of 
the ionization, due to a large plane surface of active matter, falls 
off approximately in an exponential law with the distance from the 
plate. On the assumption that the rate of production of ions at 
any point is a measure of the intensity J of the radiation, the 
value of IJ at that point is given by Fae, where A is a 
0 
constant, x the distance from the plate, and J, the intensity of the — 
radiation at the surface of the plate. This result can be deduced 
theoretically on the assumption that the ionization at any point is 
proportional to the intensity of the radiation, and that the energy 
of the rays is used up in producing ions. 
With an infinite plane of active matter, the mtensity of the 
radiation would be constant for all distances from the plane if 
there were no absorption of the radiation in the gas. 
Let gq be the number of ions produced per second per unit 
volume when the intensity of radiation is J. 
Let J = Kq, where K is a constant. 
If w is the average energy required to produce an ion, the 
1 Rutherford, Phil. Mag. Jan. 1899. 
