The thermionic emission constant A

Abstract 1. Summary of Paper.—The main object of this paper is to apply Nordheim’s theory of the emission coefficient of electrons from metals so as to explain the remarkable relation between the constants A and X of the thermionic emission formula, first recorded by O. W. Richardson and recently reformulated by Du Bridge. This theory regards the emission as due to the passage of electrons through simple surface potential steps and double layers, to be calculated according to the wave mechanics. We conclude that the theory gives a completely satisfactory account of the facts whenever it can be properly applied. The necessary conditions for its proper application are formulated. Certain apparent discrepancies in absolute magnitude between the observed and theoretical values of A are left outstanding and it is necessary to discuss possible explanations of these in some detail. It is concluded that though their precise cause cannot yet be specified they can certainly be explained and their existence is in no way antagonistic to the main theory. 2. The Experimental Facts.—Much attention has been devoted recently to the values of the constant A in the formula for the saturated thermionic (electronic) current I, namely, I = AT2e-x/kT, where T is the absolute temperature, X the work function and k Boltzmann’s constant. Particularly interesting measurements of A (and X) have been recorded by Du Bridge for platinum when very severely outgassed. Du Bridge seems definitely to have established that for really clean platinum the photoelectric threshold and the thermionic work function agree at or near 6·35 volts, and that the corresponding values of A for various specimens range from 10,000 to 20,000 amp./cm.2.