<Li> Photoelectric emission: Electrons can also be emitted from the electrodes of certain metals when light of frequency greater than the threshold frequency falls on it . This effect is called photoelectric emission, and the electrons produced are called photoelectrons . This effect is used in phototubes and image intensifier tubes . </Li> <P> Cathodes can be divided into two types: </P> <P> A hot cathode is a cathode that is heated by a filament to produce electrons by thermionic emission . The filament is a thin wire of a refractory metal like tungsten heated red - hot by an electric current passing through it . Before the advent of transistors in the 1960s, virtually all electronic equipment used hot - cathode vacuum tubes . Today hot cathodes are used in vacuum tubes in radio transmitters and microwave ovens, to produce the electron beams in older cathode ray tube (CRT) type televisions and computer monitors, in x-ray generators, electron microscopes, and fluorescent tubes . </P> <P> There are two types of hot cathodes: </P>

The flow of electrons from the cathode to the anode is called