Spark image

Thermionic diode

This animation shows the action of a thermionic diode. When the heater is switched on the temperature of the cathode is increased and electrons are emitted from its surface. These electrons are attracted across the diode towards the positively charged anode.

Most of the acceleration takes place close to the cathode and so once they have left the cathode the elctrons move through the vaccum at a constant velocity.
© Keith Gibbs/John Bourne 2009