by a separate filament, allowing it to be maintained at a uniform electrical potential. The development of oxide-coated cathodes permitted substantial emission at comparatively low temperatures and became standard in receiving practice.

The interior of the tube is evacuated so that electrons may travel from cathode to anode without significant collision with gas molecules. When a positive voltage is applied to the anode (plate), electrons are attracted across the intervening space. The resulting current is governed not only by the emission