Television

Principle of Operation of cathode Ray Tube ( CRT)

Thermion Emission

A hot cathode is used to produce a supply of electrons. This may consists of a fine tansand wire or filament which is heated to high temperature .Metals content free electrons if the temperature of the metal is raised, a thermal velocities of the electrons will be increased.

The chance of electrons escaping from the attraction of the positive Irons will then also be raised. Therefore by heating a metal to a high temperature electrons can be boiled off. This is called Thermion Emission.

Once an electron escaped from the cathode it is subjected to a retarding force set up by the negatively charged electrons that have already escaped. An electron is slowed down by this force and may be return to the cathode an emitted electron can also be slowed down by coming in to collision with gas molecules. To minimize this effect cathode is placed within an evacuated glass envelope.

Electrons are continually emitted from the surface of a heated cathode. There are subjected to forces which tend to return them cathode. The cathode is surrounded by a cloud of electrons. The cloud of electrons around the cathode is known as the space charge. Space charge is negative.If another electrode the anode is introduced into the evacuated space and is maintained at positive potential with respect to the cathode it will exert and attractive force on the emitted electrons due to the influence of that attractive force electrons are rapidly accelerated to the anode.A value with just a cathode and anode is known as a diode. A triode has a third electrode known as the control grid inserted between cathode and the anode .the tetrode value has another grid inserted between the control grid and the anode. The pentod value has three control grids.