Nergaard, Leon S. Microwave Research Laboratory, RCA Laboratories, Princeton, New Jersey.
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An electron tube that relies upon thermally emitted electrons from a heated cathode for tube current. Thermionic emission of electrons means emission by heat. In practical form an electrode, called the cathode because it forms the negative electrode of the tube, is heated until it emits electrons. The cathode may be either a directly heated filament or an indirectly heated surface. With a filamentary cathode, heating current is passed through the wire, which either emits electrons directly or is covered with a material that readily emits electrons. Some typical filament structures are shown in Fig. 1. Filaments of tungsten or thoriated tungsten are commonly used in high-power transmitting tubes where their ruggedness and ability to withstand high voltages are essential. Oxide-coated filaments are used in a few small high-voltage rectifier tubes.
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