Space Charges

Author: J.B. Hoag

Ordinarily, radio tubes are operated with the plate voltage adjusted to a value which is insufficient to draw over all of the electrons which are emitted by the filament. Thus, in usual operation, there is a cloud of electrons or so-called space charge surrounding the filament, and limiting the plate current to a value appreciably less than the saturation value.

The electrons which make up the space charge are not at rest in the region just outside of the filament, but are actually in rapid motion. Electrons in transit from the filament to the plate repel other electrons which are behind them and drive some of the newly emitted electrons back into the filament. Thus the space charge is continually pouring some of its electrons onward to the plate, the while they are being supplanted by new electrons just fresh from the filament. Despite this state of dynamic equilibrium, the net result is the same as though there existed a fixed cloud of electrons in the space between the filament and the plate.

The number of electrons in a unit volume, i.e., the density of space charge, is greatest near the filament and decreases progressively as the plate is approached.

When the filament is quite hot and the plate voltage is quite small, the space charge will keep the plate current down to a very small value. But, if the filament is cooler and the plate voltage is high, the plate current will be nearly equal to that of the saturation of total emission value.