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When a metal plate is "illuminated" by Röntgen rays, part of the energy of these incident rays is coverted into high-speed secondary cathode particles, which are ejected from the plate in all directions. The mere detection of this emission of negative electricity is an easy matter, since the illuminated plate, if insulatedin vacuo charges up positively. By measuring the rate of charging, it would be possible to determine the number of particles ejected per second, while by applying an electric force of such a magnitude and in such a direction as to stop the emission, it might be thought that their speed could be measured. In practice, however, this method of measuring velocities is restricted to slowly moving rays, such as are produced, for example, by ultra-violet light. The particles ejected by Röntgen rays have in most cases a speed corresponding to a potential fall through many thousands of volts, and at present it is not possible to perform accurate experiments with potentials of this magnitude. Quite the best way of determining high velocities is the magnetic deflection method, in which a magnetic force of known strength is applied to a fine beam of the moving particles in such a way as to act at right angles to their direction of motion; form observations of the curvature of the path described by the particles their speed can be calculated. This method, unfortunately, is at present impossible in the case of secondary cathode particles, since their intensity is never sufficient to allow of their being formed into anything like a narrow beam. There is, however, a method of attacking the problem which it is the object of this paper to outline. The experimental data available are somewhat incomplete, and the conclusions drawn from them of necessity share this incompleteness.

The velocity of the secondary cathode particles ejected by the characteristic Röntgen rays