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The thermionic diode

As you may know, if a metal is heated it may emit electrons. The hotter it is the more electrons are emitted. Of course some materials are better than others for emitting electrons.

This emission of electrons due to the heating of a surface is called thermionic emission.

This is the basis of the thermionic diode.

A thermionic diode is an evacuated glass globe. Into this is put one plate (the cathode), that will emit electrons easily when heated, a heater and another plate to collect the electrons (the anode).

When the heater is switched on the cathode is heated and electrons are emitted. If the anode is now given a positive charge by connecting a high voltage power supply between the cathode and anode the electrons will be pulled across the tube.

A meter will measure this flow of electrons as a small current. The size of the current depends on the diode and the voltages but it will probably be a few milliamps (mA).

There are a number of things that can affect the size of the current through a diode:

1. If the heater is made hotter by turning up the heater voltage, the current is increased.
2. If the anode voltage is increased the current is increased.
3. If the anode voltage is switched off or made negative the current stops.
4. If the heater is switched off the current stops.

© Keith Gibbs 2013