Page:Radio-activity.djvu/158

 the relative number of electrons which are stopped in their passage through different thicknesses of matter. The experimental arrangement is shown in Fig. 31.

The radium was placed outside a glass vessel containing an insulated brass plate P, the connection of which with a wire leading to the electrometer could be made or broken by a simple electromagnetic device. The β rays from the radium R, after passing through openings in a brass plate A, covered with thin aluminium foil, were absorbed in the plate P. The glass vessel was exhausted, and the charge communicated to P by the β rays was measured by an electrometer.

In a good vacuum, the magnitude of the current observed is a measure of the number of β particles absorbed by the upper plate. The following table shows the results obtained when different thicknesses of tin foil were placed over the radium. The second table gives the ratio I/I_{0} where I_{0} is the rate of discharge observed before the absorbing screen is introduced. The mean value of the absorption constant λ was deduced from the equation I/I_{0} = e^{-λd} where d is the thickness of matter traversed.

Fig. 31.

The values included in the brackets have not the same accuracy as the others. There is thus a wide difference in penetrating power of the β particles emitted from radium, and some of them are very readily absorbed.