The liberation of electrons from metal surfaces by positive ions. Part I.—Experimental

Abstract

Abstract Modern theories of the glow and arc discharges require that electrons should be set free from the cathode surface as a result of the bombardment by positive ions. The conditions in the neighbourhood of the negative electrode are exceedingly complex, and it is only by systematic examination of each reaction which we believe to be present, isolated from the disturbing effects of the others, that we can hope for a complete understanding of this important region. This paper describes some experiments which have been carried out in an endeavour to explain the precise nature of the reaction between the positive ions and the surface of the conductor. The literature of this subject is fairly extensive, and many of the experiments have been very carefully thought out. It seems to be quite definitely estab­lished that ions of velocity greater than about 100 volts set free electrons from metal surfaces which are not rigorously freed from gas. Most of the experi­ments have been carried out with alkali ions from thermionic sources, and owing to the abnormally low ionisation potentials of the alkalis the results are scarcely applicable to the case of discharges in ordinary gas. We are not concerned with the liberation of electrons by very fast canal rays or alpha particles, but with the effect produced by positive ions of less than about 1000 volts energy. Penning has established by a series of experiments that neon ions of as little as 7 volts energy, drifting up against a metal surface, are able to set free electrons from it.