Thermionic constants of graphite

Abstract

Some years ago, one of us (K. S. K.) suggested a new method of determining the thermionic constants of solids. The method is based on the determination of the saturation vapour pressure of the electron gas in equilibrium with the substance at different known temperatures. In practice, this is done by finding the rate of effusion into vacuum of electrons out of a small hole in a thin wall of a chamber scooped out of the substance. Using a well-known thermodynamic relation, analogous to the Clapeyron-Clausius equation connecting the temperature variation of the saturation vapour pressure of a given substance with its latent heat of evaporation, one obtains readily the thermionic constants. The work function of graphite was determined by this method by Dr. A. S. Bhatnagar, and of a few other substances by Dr. S. B. L. Mathur; but the A coefficients in Richardson's equation were not determined by these authors, owing to uncertainties in some of the absolute measurements.