Thermionic emission

A double filament light bulb (car headlamp bulb) can be used as a diode. Electrons boiled off one hot filament (cathode) can be collected at the other filament (cold anode) by applying a positive voltage to the anode. The cathode-anode current is small (< one milliamp) but will flow only in the one direction.

Coating a purpose built hot cathode with cesium oxide greatly increases the available thermionic emission and the possible maximum anode current.

Vacuum diodes and triodes (above) were important in the development of radio 1905-1945.

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