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The effect of thermionic emission is due to the fact that some of the free electrons in metal under certain conditions can obtain such a velocity, that their kinetic energy is great enough to overcome the molecular 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 than 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 a 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 with respect 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 one-way resistance, electrons flowing from cathode to plate but not from plate to cathode.

With polarity reversed, there can be no current of electrons at all. Suppose an a-c voltage is applied to terminal A. During each positive cycle current will flow through the diode and the voltage drop across the re­sistance R will be the value of the diode current times R. Note, that although the voltage of A is negative 50% of the time, the voltage of В is never negative. The voltage of В can be used to charge up a capacitor. This application of the diode called a diode rectifier can con­vert an a-c voltage into d-c voltage, all a-c operated radio and TV sets having diode rectifiers to convert the alter­nating current into direct current.

The vacuum tube was invented in 1904 by Sir John Anbrose Fleming, a British electrical engineer. It was the heart of the electronic circuit until 1947. For 40 years

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this device underwent constant improving and by 1947 it had come in so many different shapes and sizes and had performed so many different functions, that it seemed absurd to suppose it could ever be replaced.