Glass Envelope Devices
The invention of the thermionic valve in the early years of the twentieth century is credited to Fleming, although the emission of electrons from heated materials was already known. The important aspect, however, was the appreciation of the valve as unidirectional conductor or rectifier. In any substance free electrons are present and in a state of continuous movement which is entirely random. These electrons have escaped from neutral atoms which are therefore left with a n equivalent positive charge. Normally the electrons remain within the body of the material, which appears electrically neutral. However, at the surface some electrons will escape into space thus leaving a positively charged surface which promptly encourages the electrons to return, and therefore some additional energy is required before the electrons can escape altogether. The amount of energy required varies with the substance and is known as the work function of the material. In order to assist the electrons to escape various methods may be employed: a low work function material may be chosen; the surface of the material may be brought under the influence of a high electric field of suitable polarity; or the temperature of the substance may be raised.
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