Fig. 8.
For the purpose of comparison, the current at atmospheric pressure and temperature in each case is taken as unity. The actual value of the current was greatest in carbonic acid and least in hydrogen. In hydrogen, where the absorption is small, the current over the whole range is nearly proportional to the pressure. In carbonic acid, where the absorption is large, the current diminishes at first slowly with the pressure, but is nearly proportional to it below the pressure of 235 mms. of mercury. The curve for air occupies an intermediate position.
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 d1 and d2 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
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.