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a vacuum diode designed for rectification of industrial-frequency alternating current. Kenotrons are used in rectifiers of radio-receiving and amplifying and measuring apparatus, as well as X-ray equipment. Low-voltage kenotrons (rated inverse voltage at the anode up to 2 kilovolts [kV]; rated direct current up to several amperes) have directly or indirectly heated oxide cathodes and blackened or etched ribbed anodes (usually two). High-voltage kenotrons (voltage up to 100 kV, current up to 500 milliamperes) have an oxide or carbonized cathode and also one blackened ribbed anode. With the development of semiconductor technology, kenotrons are gradually being replaced by semiconductor diodes.