Thermionic Emission

Any metal from which we are trying to remove electrons is referred as metal electrode. In thermionic emission, the metal electrode is heated by an electric heater just to raise it's temperature above the room temperature. This will result in the thermal or heat energy given to the surface electrons to attain higher and sufficient speed to escape from neighbouring atoms at the surface of the electrode. Because of the resulting stream of electrons coming from the electrode surface, the electrode is now referred to as an emitter. The greater the number of free electrons that will be jettisoned. From the surface of the metal electrode or emitter.

   In thermionic emission of electrons, the source of heat can be obtained by the direct heating or indirect heating methods. In the direct heating method, metal electrode is heated directly by connecting electric current to the metal electrode. The metal electrode can itself be in form of a wire. In indirect heating method, we heat up a separate heater element, which may be located inside the metal electrode called cathode, which is already coated with the electron emissive materials. The cathode is then heated up indirectly by the method of heat transfer from the Hester elements. The current needed for heating up the emitter or cathode may be direct current from a battery or AC from an alternating current source like NEP supply or a  generating plant. In the directly heated emitters, the materials used ad the metal electrode are tungsten, thoriated tungsten, or oxide coated emitters. In the indirectly heated system, oxide coated emitters are employed.