Free Barium in and Evaporated from Oxide Cathodes

Abstract

DURING 1945, in connexion with experiments on thermionic emission from electrodes other than the cathode in small radio valves, we had occasion to make measurements on the quantity of free barium evaporated from the cathode, and also the amount actually adsorbed on or distributed in the barium–strontium oxide coating. In view of the theoretical importance of the latter in connexion with the electrical conductivity of the coating treated as a semi-conductor (investigated by D. A. Wright, whose results are at present in the course of publication), we feel that our results have become of more general interest. Previous measurements by Berdennikowa¹ and Clausing² did not distinguish between the barium in the cathode coating and that evaporated, while Prescott and Morrison³, who made the distinction, used cathodes with free nickel distributed through the coating and activated them in an unusual manner by heating in methane. This may have produced excess barium, which could alloy with the nickel in the coating.