Page:Radio-activity.djvu/58

 Now the greater the value of V required to obtain a given value of ρ (supposed small compared with unity), the greater the potential required to produce saturation.

It thus follows from the equation that:

(1) For a given intensity of radiation, the saturation increases with the distance between the plates. In the equation, for small values of ρ, V varies as l^2. This is found to be the case for uniform ionization, but it only holds approximately for non-uniform ionization.

(2) For a given distance between the plates, the saturation is greater, the greater the intensity of ionization between the plates. This is found to be the case for the ionization produced by radio-active substances. With a very active substance like radium, the ionization produced is so intense that very large voltages are required to produce approximate saturation. On the other hand, only a fraction of a volt per cm. is necessary to produce saturation in a gas where the ionization is very slight, for example, in the case of the natural ionization observed in a closed vessel, where no radio-active substances are present.

For a given intensity of radiation, the saturation decreases rapidly with the lowering of the pressure of the gas. This is due to two causes operating in the same direction, viz. a decrease in the intensity of the ionization and an increase in the velocity of the ions. The ionization varies directly as the pressure, while the velocity varies inversely as the pressure. This will obviously have the effect of causing more rapid saturation, since the rate of recombination is slower and the time taken for the ions to travel between the electrodes is less.

The saturation curves observed for the gases hydrogen and carbon dioxide are very similar in shape to those obtained for air. For a given intensity of radiation, saturation is more readily obtained in hydrogen than in air, since the ionization is less than in air while the velocity of the ions is greater. Carbon dioxide on the other hand requires a greater to produce saturation than does air, since the ionization is more intense and the velocity of the ions less than in air.