Thermionic effects caused by vapours of alkali metals

Abstract Early in 1923 it was shown that a tungsten filament heated to 1200° K or more in saturated cæsium vapour converts all cæsium atoms which strike it into cæsium ions. Thus when the filament is surrounded by a negatively charged cylinder a positive ion current flows from the filament, which is independent of the filament temperature (above 1200° K) and independent of the applied potential, if this is sufficient to overcome the space charge effect of the positive ions. At lower voltages the currents follow the 3/2 power law, and the currents are smaller than the corresponding electron currents obtainable from the same filament in the ratio of the square roots of the masses of the electrons and cæsium ions. The reason that the cæsium atoms lose their valence electrons so readily upon contact with the filament, is merely that the electron affinity of tungsten (Richardson work function) is 4·53 volts, while the electron affinity of a cæsium atom (ionising potential) is only 3·88 volts. Experiments showed in fact that if the work function for the filament is lowered to 2·69, by allowing a monatomic layer of thorium atoms to accumulate on the surface (by diffusion from the interior of a thoriated tungsten filament), the positive ion emission becomes negligible.