Certain metals have a property of liberating electrons when they are heated up or subjected to light rays, the effect is known as electron emission. There are following five types of electron emission:
1. Thermionic emission: when a metal is heated up to a certain temperature, it starts emission of electrons which is called thermionic emission. The reason behind this effect is this that the electrons present in the atoms of the metal heated up acquire tremendous kinetic energy and their attraction towards their nucleus is greatly reduced. Some electrons leave the surface of the metal and come out. Pure tungsten metal emits electrons at 2400 to 2500 Kelvin.
2. Photo electric emission: certain metals are very sensitive that they start electron emission even on the incidence of light rays. The emission so caused is called photo electric emission. Sodium and potassium are photo emissive metals.
3. Secondary emission: when electrons or ions moving with fast speed strike a metallic surface they themselves enter the metallic body and they scatter some electrons out of that body. The phenomenon is called secondary emission. The action takes place in tetrode valves.
4. Field emission: when the attraction of a strong magnetic field causes emission of electrons from a metallic surface, the phenomenon is called field emission. The electron emission in a cold cathode valve is field emission.
5. Radioactive emission: there are some elements present in the nature which continue to disintegrate into new elements having lesser atomic number. Such elements are called radioactive elements. During disintegration such elements emit beta rays composed of electrons. The emission is called radioactive emission. Uranium, radium, thorium etc. are radioactive elements.
