Thermionic Emission

In the 1850's physicists started to examine the passage of electricity through a vacuum by putting two electrodes in a sealed vacuum tube.Some kind of rays were emitted from the cathode or the negative electrode.These rays were called cathode rays.J.J Thormson in 1897 observed the deflection of cathode rays by both electric and magnetic fields.From these deflection experiments,he concluded that cathode rays must carry a negative charge.These negative charged particles were given the name electrons.

The process of emission of electrons from the hot metal surface is called thermionic emission. Metals contain a large number of free electrons.At room temperature electrons can not escape the metal surface due to attractive forces of nucleus.If the metal is heated to a high temperature,some of the free electrons may gain sufficient energy to escape the metal surface. 

Thermionic emission can also be produced by electrically heating a fine tungsten filament.Typically values of voltage and current used are 6V And 0.3A respectively.