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Thermionic effect

This effect is the engine of a vacuum tube.
A heated metal lets electrons escape from its surface and form a cloud around it.
Each electron push back the others but stay close to the cathode which is positive due to their loss.
If you connect a battery between this metal and another electrode, you'll see a current flowing. The electrons which have left the heated metal are replaced by those coming from the battery which, itself, receives the electrons coming from the second electrode.
This is a diode: the heated metal is called cathode, the second electrode anode or plate and the current flows from the cathode to the anode.

Two heating methods are used: the direct heating where the filament itself is the cathode and the indirect heating where the cathode is heated by a separate filament close to it.

The main metals used for a direct heated tube are the pure tungsten (for very high voltages), thoriated tungsten (tungsten doped with thorium oxyde impurities where a thermic treatment makes the thorium atoms come up to the surface and create an extremely thin layer) and for the indirect heated tubes, the cathode is made of nickel or nickel alloy covered by layers of barium and strontium oxydes which gives a very high emission while allowing a lower temperature.

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