Page:Radio-activity.djvu/158

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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[1]. 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.

  1. It is presumed that the results were corrected, if necessary, for the discharging action due to the ionized gas, although no direct mention of this is made in the paper by Seitz.