Metals contain a lot of electrons free to move throughout the metal. Normally these electrons move completely randomly. They are held within the wire by net attractive forces produced by the positively charge atoms. If the metal is heated however, the electrons will gain energy and may gain enough to leave the metal. Heating a metal to release electrons is known as thermionic emission - 'thermal ion emission'.
An electron gun uses thermionic emission to produce an electron beam.
A low voltage suuply powers a heater to heat the cathode where the electrons are released. Once released the electrons are accelerated towards a positively charge anode, which is shaped to focuse the beam. The speed of the electrons is determined by the accelerating voltage. If the accelerating voltage is
then by the time the electrons reach the anode, they will have lost
of electrical energy. Equating this to the kinetic energy gained gives
where
and
are the mass and charge of an electron.
An electric current is merely a collection of moving charge, so the beam of electrons is a moving current. We can use
where
is the current and
is the number of electrons released per second
and the number of electrons released per second can be found by measuring the current.