Ans. An electron gun is used for producing a beam of fast moving electrons called cathode-rays.
In the process of thermionic emission, electrons from the hot metal surface are emitted. Metals contain a large number of free electrons. At room temperature, electrons cannot escape the metal surface due to attractive forces of the atomic 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. For this purpose, typical values of the voltage and current- used are 6 V and 0.3 A respectively.