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
This is a function of the pressure, and is a maximum when
For example, if the active matter is uranium, pλ1 = 1·6 for the α rays at atmospheric pressure. If d2 = 3, and d1 = 1, the saturation current reaches a maximum when the pressure is reduced to about ⅓ of an atmosphere. This result has been verified experimentally.
**45. Conductivity