The electron gun is a device used to produce a narrow beam of electrons.Electron guns are used in electron microscopes, in mass spectrometers to ionise particles and in cathode ray oscilloscopes to produce traces of light on a screen.A small metal filament is heated by an electric current. The electrons gain kinetic energy. Some gain enough kinetic energy to escape from the surface of the metal. This is called thermionic emission.The heated filament is placed in a vacuum. The filament acts as a cathode and a high p.d. is applied between the filament and an anode. The electric field accelerates the electrons towards the anode and pass through a small hole in the anode creating a beam of electrons all with the same kinetic energy.