Letter | Published:

The Energy of Photo-electrons produced by Soft X-rays


IN a series of experiments carried out in Prof. O. W. Richardson's laboratory at King's College, London, I have attempted to measure the velocity of the photo-electrons set free from a metal under the influence of soft X-rays, excited in the usual way by electrons from a hot filament falling on the radiator through an accelerating potential difference of 50–800 volts. The results obtained with the stopping potential method—in which a varied retarding potential is applied to a surrounding electrode, thus preventing all electrons with a kinetic energy below the corresponding value to escape—are difficult to interpret because of the masking effect of scattered radiation on the measurements for higher voltages. Various forms of magnetic methods have also been tried, of which the last one has yielded definite results. In this apparatus the photo-electrons are emitted from a narrow rod forming the axis of a short cylindrical ring, connected to the electrometer and kept at the same potential as the rod. A variable magnetic field, applied in the direction of the axis, prevents electrons with velocity less than a certain value from reaching the cylinder.

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