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valves - design - application
diode - cathode +• plate - rectifier
triode - . cathode + grid + plate - amplifier
tetrode - cathode + grid + screen-plate - multipurpose
The branch of electronics dealing with the emission of electrons from substances under the action of heat, particularly the study and design 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.
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