Saturation Currents

Author: J.B. Hoag

Suppose we have a pan of water out in the open where the wind can blow away the water molecules as fast as they evaporate. Then, by way of contrast, suppose we have a pan of water with a box over it, so that the water molecules that have evaporated cannot escape, but remain hovering above the water surface, sometimes returning to become part of the liquid again. In the first case, the liquid will evaporate much more rapidly than in the latter. Similarly, in the case of the electrons evaporating from the filament in Fig. 10 A; if the plate P is very positive with respect to the filament (by use of a large battery V), then all of the electrons emitted by the filament will be drawn over to the plate. But if the battery voltage is small, electrons will collect in the space between the filament and the plate, like the cloud of water vapor above the liquid inside the box. Then the electrical current registered on the meter i will not be as large as that discussed by Richardson. This state of affairs is shown by the dashed lines in Fig. 10 B. The higher the plate voltage, the more nearly will the currents approximate the full saturation value of curve OA.

Fig. 10 A. A high vacuum two-electrode tube or diode.		Fig. 10 B. Plate current at different filament temperatures.