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Induction heating at radio frequencies is rapidly taking its place in many industrial processes. The high-power vacuum tube, in the past used principally in radio applications, is now generating radio-frequency energy for industrial use. To apply this energy properly to industrial heating problems, it becomes necessary that engineers active in all phases of industry understand the characteristics and limitations of the vacuum-tube radio-frequency generator. The fundamentals of the vacuum-tube self-excited oscillator and design considerations which determine the characteristics of the radio-frequency generator are reviewed and illustrated. In general, the characteristics show a high-impedance, constant-current, variable-voltage generator which requires manipulation of load circuits to load the generator properly. Methods are illustrated for accomplishing proper loading, and numerical examples are given illustrating the formulas and procedures necessary to any induction-heating problems.