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.
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.
