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High-Vacuum Thermionic DiodesThis high-vacuum diode consists essentially of an electron-emitting hot cathode and an electron-collecting anode or plate. For small tubes the cathodes are of two types: first, the filament type consisting of nickel wire or some alloy that is coated with oxides of barium and strontium and suitably treated to make an excellent emitter; and second, the indirectly heated or separate-heater type. This type consists of a small cylinder of nickel or other metal on which the oxide coating is placed and which is heated by an insulated tungsten filament placed at the center. For large high-vacuum tubes the filaments are often tungsten. The emitted electrons accumulate as a negative space charge about the cathode. If the plate is positive, it pulls negative electrons from the space-charge region. Because the negative space charge repels electrons, it is an important factor in determining the magnitude of the voltage drop across the tube. The plate is heated by the electrons that strike it, and the average anode current, which is the direct current flowing, must not exceed the rated value. The peak (or crest) inverse anode voltage, defined1 as "the maximum instantaneous anode voltage in the direction opposite to that in which the tube is designed to pass current," also must be kept within rated values. Otherwise, the tube may arc back and may be damaged.
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Last Update: 2011-05-30