Page:Radio-activity.djvu/63

 velocity proportional to the strength of the field. On the application of an electric field, the ions almost instantly attain the velocity corresponding to the field and then move with a uniform speed.

Zeleny first drew attention to the fact that the positive and negative ions had different velocities. The velocity of the negative ion is always greater than that of the positive, and varies with the amount of water vapour present in the gas.

The results, previously discussed, of the variation of the current with voltage and of the rate of recombination of the ions do not of themselves imply that the ions produced in gases by the radiations from active bodies are of the same size as those produced by Röntgen rays under similar conditions. They merely show that the conductivity under various conditions can be satisfactorily explained by the view that charged ions are produced throughout the volume of the gas. The same general relations would be observed if the ions differed considerably in size and velocity from those produced by Röntgen rays. The most satisfactory method of determining whether the ions are identical in the two cases is to determine the velocity of the ions under similar conditions.

In order to compare the velocity of the ions, the writer has used an apparatus similar to that shown in Fig. 6 on p. 40.

The ions were carried with a rapid constant stream of air past the charged electrode A, and the conductivity of the gas tested immediately afterwards at an electrode B, which was placed close to A. The insulated electrodes A and B were fixed centrally in the metal tube L, which was connected with earth.

For convenience of calculation, it is assumed that the electric field between the cylinders is the same as if the cylinders were infinitely long.

Let a and b be the radii of the electrode A, and of the tube L respectively, and let V = potential of A.

The electromotive intensity X (without regard to sign) at a distance r from the centre of the tube is given by

X = V/(r log_{e}(b/a)).