A cathode in which thermionic emission provides the source of electrons. A directly heated cathode is one that acts as its own source of heat: the cathode is in the form of a filament and the heater current is passed through it, superimposed on the normal cathode potential (usually earth potential). An indirectly heated cathode is one that has a separate heater. The heater is usually a coil of wire formed around the cathode and to which a current is applied.

The electron emission from the cathode may be increased by coating it with a thin layer of a suitable material. A coated cathode usually consists of a cylinder of platinum coated with an oxide of barium, strontium, or calcium. It operates at a lower temperature than the clean metal.