English: Early spark diathermy machine by Watson and Sons from 1921, with the cover removed to show the parts. Diathermy is the application of radio frequency currents to the body to produce deep heating. This device is an early spark excited machine that produced voltages of 2000-5000 volts at frequency of 0.5 - 2 MHz. Its circuit is essentially a compact spark gap radio transmitter or Tesla coil. A high voltage transformer steps up the mains voltage to several thousand volts to charge a capacitor. The capacitor repeatedly discharges through the quenched spark gap (top center) and the primary of the variometer resonant transformer (bottom left) to create damped sinusoidal waves. The secondary of the variometer is connected the output terminals of the machine, which are attached through wires to metal plates applied to the patient's body. The high frequency alternating current flowing through the patient's body heats the tissues between the plates. This is not uncomfortable for the patient, because frequencies above 10 kHz do not cause the sensation of elecric shock. The hot-wire ammeter (top left) monitors the RF current through the body. Alternately the electromagnetic field may be applied to the patient by capacitive plates or a coil of wire.