English: Photo of an Oudin coil used for medical 'electrotherapy', and it's circuit diagram, from around 1907. The Oudin coil was a high voltage resonant transformer similar to a Tesla coil, invented by French physician Paul Marie Oudin around 1899. The coil was around 50 cm high. This particular apparatus also had a small handheld Tesla coil (CE, seen lying on the bottom shelf of the apparatus) attached to the top high voltage terminal, for applying to the patient.

The diagram's caption: "Diagram of the Oudin resonator. The Tesla coil is omitted. The current from the induction coil is connected with the inner tinfoil of the Leyden jars. The inner coat of one Leyden jar is in connection with the resonator and is also grounded. The outer coat of the other Leyden jar is connected with the handle, H, which, by a sliding movement in either the horizontal or vertical direction, decreases or increases the amount of winding of the resonator. M is the spark gap, for regulating the amount of current."

The picture's caption: "The Oudin resonator and Tesla coil, with electrode. (Biddle)