Page:Radio-activity.djvu/85

 This difference of conductivity in gases is due to unequal absorptions of the radiations. The writer has shown that the total number of ions produced by the α rays for uranium, when completely absorbed by different gases, is not very different. The following results were obtained:

Gas             Total Ionization Air                       100 Hydrogen                   95 Oxygen                    106 Carbonic acid              96 Hydrochloric acid gas     102 Ammonia                   101

The numbers, though only approximate in character, seem to show that the energy required to produce an ion is probably not very different for the various gases. Assuming that the energy required to produce an ion in different gases is about the same, it follows that the relative conductivities are proportional to the relative absorption of the radiations.

A similar result has been found by M^cLennan for cathode rays. He proved that the ionization was directly proportional to the absorption of the rays in the gas, thus showing that the same energy is required to produce an ion in all the gases examined.

46. Potential Gradient. The normal potential gradient between two charged electrodes is always disturbed when the gas is ionized in the space between them. If the gas is uniformly ionized between two parallel plates, Child and Zeleny have shown that there is a sudden drop of potential near the surface of both plates, and that the electric field is sensibly uniform for the intermediate space between them. The disturbance of the potential gradient depends upon the difference of potential applied, and is different at the surface of the two plates.

In most measurements of radio-activity the material is spread over one plate only. In such a case the ionization is to a large extent confined to the volume of the air close to the active plate. The potential gradient in such a case is shown in Fig. 9. The