3. Vacuum Tubes

Abstract

This chapter describes the thermionic emission, as the phenomenon, by which solids emit electrons or ions, when heated to a sufficiently high temperature. It is concerned with the emission of electrons in an evacuated region. The chapter discusses the Richardson-Dushman equation; the Schottky effect. The maximum current density that can be emitted, from the surface of a solid, is a function of its absolute temperature and the material substance of the solid. It is also dependent to a lesser degree on the component of electric field normal to the emitting surface. Experiments indicate that the work function is slightly dependent on temperature. It also explains different types of cathodes: pure metals, monolayer emitters, oxide emitters, and dispenser cathodes. Cathodes are of two types, the directly heated and the indirectly heated. Usually the pure metals and monolayer metals are directly heated and the oxide and dispenser cathodes indirectly heated. If the cathode temperature is to be above 1500°C the directly heated type is used due to the practical difficulty of getting the heater hot enough to achieve this temperature on the cathode surface in the indirect type.