Power grid tubes
Even though their use has declined considerably, studying the properties of vacuum diodes is important for a better understanding of the principles of operation of power grid tubes. A vacuum diode consists of two electrodes in a vacuum: a thermoelectronic cathode and an anode. When the cathode temperature is high enough, it can emit electrons from the surface. The anode is brought to a positive voltage V p with respect to the cathode, and thus attracts electrons emitted by the cathode, giving rise to a current I p between the cathode and anode. The curve I p versus V p is called the diode characteristic. For a given cathode temperature T, I p increases with V p up to a maximum current I m. For a higher cathode temperature T′ > T, the characteristic curve is the same up to I m, but continues to increase to a higher maximum current I m′ > I m (Fig. 3.1).
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