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 first version Oudin invented; the secondary "resonator" coil (L2) was not inductively coupled to the primary (L1) and generated the high voltage solely by resonance. In the later version, shown in Oudin coil circuit - later type.svg the primary and secondary were inductively coupled, wound on the same axis to make an autotransformer