DIATHERMY, a term first applied by Nagelschmidt in 1908 to the use of high frequency currents for heating locally the tissues of the body. The frequencies employed are 300,000 per sec. or above, which are too rapid to affect the nerve endings. Hence currents of an ampere or more can be used, the patient feeling only a sensation of warmth. The currents are generated in a coil of appreciable inductance and very low resistance by the oscillatory discharge of a condenser. The condenser is charged to a high voltage by a step-up transformer on an A.C. supply, and the discharge is produced by the passage of a spark through a multiple tungsten-faced spark gap. The oscillations are of the necessary high frequency and are maintained continuously by the spark discharge. The currents are led to the patient by pads or flexible metal sheets. (See ELECTROTHERAPY.)