Thermionic Valves or Tubes. Diode.

The branch of electronics dealing with the emission of electrons from substances under the action of heat, particularly the study and designs of thermionic valves or tubes is called thermionics.

A good high vacuum is practically a perfect nonconductor, since in it no carriers of electricity are present. If two metal plates or electrodes are enclosed in a vacuum by a glass tube, we have an open circuit and no current will flow. However, one of the electrodes being heated to a high temperature, the thermal velocity of some of the conduction electrons in the metal becomes high enough for these electrons to escape. In moving through a vacuum these electrons form an electric current.

The effect of thermionic emission is due to the fact that some of the electrons in metal under certain conditions can obtain such a velocity, that their kinetic energy is great enough to overcome forces of attraction and so they may leave the metal surface. The mean velocity of the electrons in the metallic conductor is not high enough to cause more then a very small percentage of the electrons to be ejected.

Nevertheless, the electrons leaving the metal surface can produce considerable currents. Electrons can as well escape from cold metal surfaces in case they are acted upon by so high an electric field (approximately 10⁷ volt/cm or more), that the molecular forces of attraction are overcome by it.

A thermionic valve is a system of electrodes arranged in an evacuated glass or metal tube. For special purposes gas at low pressure may be introduced into the valve. The heated electrode is called the cathode (or sometimes the filament) and the cold electrode-the plate. The cathode is an electrode which is held at a negative potential to the plate. It emits electrons. The cathode of a tube may be in the form of a wire which is heated or it may be a metal tube coated with certain metallic oxides, which is heated by a separate heater. In the former case the cathode is known as “filament”, in the latter, it is called an “indirectly heated cathode”. In both cases the heating of a substance causes it to emit electrons. The device is called a diode and if we apply an alternating current to a vacuum tube it behaves as a oneway resistance, electrons move from cathode to plate but not from plate to cathode.

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