Thermionic Current

Author: J.B. Hoag

Suppose we heat the filament of the tube in Fig. 10 A hotter and hotter, measuring its temperature with an optical pyrometer, the while we observe the plate current i. We will find that the thermionic current increases very rapidly as the filament grows hotter and hotter, as indicated by the line OA in Fig. 10 B, provided only that the voltage of the battery is sufficiently large to draw over to the plate all of the electrons as they are emitted from the filament.

Fig. 10 B. Plate current at different filament temperatures. (From E. & N. P.)

O. W. Richardson, using the mathematics of evaporating liquids, was the first to derive a mathematical equation connecting these currents and the temperatures of the filament. The fact that his theoretical equation agrees with the measured values lends great weight to the picture of free electrons existing inside of conductors.