A German diathermy machine from 1933. This induced local deep tissue heating of the body by radio waves. Earlier "long wave" diathermy machines using Tesla coils had operated at around 0.5-2 MHz, which caused only general heating. "Short wave" vacuum tube diathermy machines like this which produced frequencies of 10-100 MHz were capable of more localized heating, and replaced them around 1930. Two capacitor plates are applied to the body, attached to the vacuum tube oscillator, and the oscillating voltage is applied to them, with the intervening body acting as the capacitor's dielectric.