Page:Radio-activity.djvu/151

 If the electricity carried by the electron is supposed to be distributed uniformly over a sphere of radius a, for speeds slow compared with the velocity of light, the apparent mass m_{0} = (2/3)(e^2/a).

Therefore a = (2/3)(e/m_{0}). e.

Taking the value of e as 1·13 × 10^{-20}, a is 1·4 × 10^{-13} cms.

Thus the diameter of an electron is minute compared with the diameter of an atom.

83. Distribution of velocity amongst the β particles. Some interesting experiments have been recently made by Paschen to determine the relative number of β particles which are expelled from radium at the different speeds. The experimental arrangement is shown in Fig. 29.

Fig. 29.

A small thin silvered glass tube b, containing 15 mgrs. of radium bromide, was placed in the axis of a number of lead vanes arranged round a cylinder of diameter 2 cms. and length 2·2 cms.