A survey of present knowledge of thermionic emitters

In Sections 1 and 2, after an account of factors influencing the work function of metals, values are quoted and the usefulness of a number of metals as practical thermionic emitters is discussed. The effect of adsorbed films is described in Section 3 and illustrated in connection with emitters with monatomic layers of thorium, caesium and barium. The emission from compounds, and in particular from semiconductors, is considered in Section 4. Sections 5 and 6 deal with the behaviour of certain metallic borides and carbides. Some of these, such as lanthanum boride and thorium carbide, are likely to be valuable as practical emitters. The general properties of oxides are discussed in Sections 8 and 9, where the groups in the periodic table are compared. The barium-strontium-oxide type of cathode is treated in Section 10. This is the most important practical cathode, and the behaviour and limitations are dealt with in some detail. Section 11 describes the performance of thoria as a thermionic emitter.