File:Schall diathermy machine 1920.jpg

English: A longwave diathermy machine made by Schall and Sons, from 1920. Diathermy is the application of high frequency currents to the body to produce deep tissue heating. This machine consists of a spark excited Tesla coil circuit which produced radio frequency current of several thousand volts at 0.5 - 2 MHz. The current was applied to the patient's body with two metal plate electrodes. The current flowing through the body caused heating in the body tissue between the plates. This was not uncomfortable for the patient because alternating current over 10 kHz frequency does not cause the physiological sensation of electric shock. The two knobs on the top adjusted the series spark gap in the primary circuit, while the ammeter allowed the doctor to monitor the current through the patient's body. By 1930 these longwave diathermy machines were being replaced by shortwave diathermy machines, which used frequencies of 10 - 100 MHz produced by vacuum tube oscillators.