VIII. On the origin of the electron emission from glowing solids

Abstract

Recent researches on the negative ionisation from glowing solids have thrown doubt upon the accuracy of the view of the origin of this ionisation which has been developed by O. W. Richardson. This view is based upon the electron theory of metallic conduction, and supposes that an electron entering the surface layer with a normal velocity component greater than a certain amount is able to escape into the surrounding space and to take part in carrying the thermionic current. On this view, as Richardson has shown, the electron emission increases rapidly with the temperature, the connection between these quantities being given by an exponential formula, i = Aθ ½ ϵ -Q/2θ , where i is the thermionic current, θ the absolute temperature, and A and Q are constants. That this formula represents, with fair accuracy, the observed results in the case of platinum and of certain other substances for tempera­tures up to about 1500°C. has been shown by the experiments of Richardson,'' by H. A. Wilson, by the author, and by other experimenters. Experiments made at higher temperatures, however, have generally shown that the current increases less rapidly with the temperature than is required by this formula. Even if in all cases the temperature variation of the thermionic current were in agreement with the above formula, this could not be taken as a proof of the theory of the origin of the effect, for it has been shown by H. A. Wilson that the current measured at any tempera­ture depends very largely upon the nature of the residual gas present in the apparatus. § This has led to the view that the ionisation is entirely due to chemical action between these gases and the heated solid—a view which is also in agreement with the exponential relation between the current and the temperature, for such chemical changes as might occur would probably increase with the temperature in this manner. This chemical action theory has received considerable support from the experiments of Haber and Just, and of Fredenhagen on the alkali metals, and of Pring and Parker on carbon. The experiments with the alkali metals have shown that by repeated purification (re-distillation) and continued improvement of the vacuum the electron emission from these substances can be continually reduced. If, however, a small amount of air or of any gas having chemical action upon the metal under test is allowed to enter the apparatus a large increase in the thermionic current at once occurs.