English: Circuit of an early Oudin coil, a resonant transformer circuit very similar to a Tesla coil that produces high voltage, low current, radio frequency alternating current, used the in early 20th century medical treatment electrotherapy. It was invented around 1893 by French physician Paul Marie Oudin as a development of Jacques Arsene d'Arsonval's electrotherapy equipment. A handheld electrode would be attached by a wire to the high voltage terminal of the device, and the streamer arcs it produced would be applied to the patient's body to treat various medical conditions.

This was the later version of the circuit, in which the primary and secondary coils are wound on the same coil form as an autotransformer, to produce higher voltages. In the first version Oudin invented, shown in Oudin coil circuit - early type.svg, the secondary "resonator" coil (L2) was not inductively coupled to the primary (L1) and generated the high voltage solely by resonance.