In cases where the distance between the plates is large, the saturation current will remain constant with diminution of pressure until the absorption is so reduced that the radiation reaches the other plate.
An interesting result follows from the rapid absorption of radiation by the gas. If the current is observed between two fixed parallel plates, distant d_{1} and d_{2} respectively from a large plane surface of active matter, the current at first increases with diminution of pressure, passes through a maximum value, and then diminishes. In such an experimental case the lower plate through which the radiations pass is made either of open gauze or of thin metal foil to allow the radiation to pass through readily.
The saturation current i is obviously proportional to
[integral]_{d_{1}}^{d_{2}} pI_{0}e^{-pλ_{1}d}, i.e. to (I_{0}/λ_{1})(e^{-pλ_{1}d_{1}} - e^{-pλ_{1}d_{2}}).
This is a function of the pressure, and is a maximum when
log_{e}(d_{1}/d_{2}) = -pλ_1(d_{2} - d_{1}).
For example, if the active matter is uranium, pλ_{1} = 1·6 for the α rays at atmospheric pressure. If d_{2} = 3, and d_{1} = 1, the saturation current reaches a maximum when the pressure is reduced to about 1/3 of an atmosphere. This result has been verified experimentally.
45. Conductivity of different gases when acted on by the rays. For a given intensity of radiation, the rate of production
of ions in a gas varies for different gases and increases
with the density of the gas. Strutt[1] has made a very complete
examination of the relative conductivity of gases exposed to the
different types of rays emitted by active substances. To avoid
correction for any difference of absorption of the radiation in the
various gases, the pressure of the gas was always reduced until
the ionization was directly proportional to the pressure, when, as
we have seen above, the ionization must everywhere be uniform
throughout the gas. For each type of rays, the ionization of
air is taken as unity. The currents through the gases were
determined at different pressures, and were reduced to a common
- ↑ Strutt, Phil. Trans. A, p. 507, 1901 and Proc. Roy. Soc. p. 208, 1903.
