Thermionic constants of the iron group of metals

Abstract

In a recent communication, we gave an account of a new method of determining the thermionic constants of graphite, which is based on the determination of the saturation vapour pressure of the electron gas in equilibrium with the substance at different known temperatures, and application of the well-known thermodynamic relation of Clausius and Clapeyron. In the actual experiments, the saturation vapour pressure of the electron gas in a graphite chamber is determined by finding the rate of effusion of the gas into vacuum through a small hole in a thin wall of the chamber. The chamber is in the form of a thin-walled tube, and can be heated to any desired high temperature by sending a suitable heavy electric current through it. In order to eliminate the electrons emitted by the graphite surface adjoining the effusion hole, the surface is covered by a thin sheet of mica, with a small hole punctured in it; this comes just in front of a bigger one in the graphite wall and serves as the effective effusion hole.