English: An Oudin coil used for 'electrotherapy' treatment of a patient's knee around 1907. The Oudin coil (left), invented in 1893 by physician Paul Marie Oudin, was a spark-excited resonant transformer circuit similar to a Tesla coil which generated very high voltage, low current radio frequency electricity, used until perhaps 1925 in the Victorian era field of electrotherapy. The doctor holds an insulated electrode with sharp metal points (B) connected to the top high voltage terminal of the coil, which produced a luminous "brush" discharge (called "effluves") which was directed toward the patient's knee. The patient holds a ground electrode (A) attached to the bottom of the coil which made a return path for the current. The apparatus which drives the coil, a spark gap (S.M.) and two Leyden jar capacitors (L.J.), can be seen below the coil. Note the doctor's other hand (D, left) adjusting the tap on the coil. The motion of the electrode changed the capacitance of Oudin coils, throwing them out of resonance, so during use they had to be constantly adjusted to keep them in "tune".

Photo caption: "Treatment by the effluvation method. The condenser electrode, B, is in connection with the top of the resonator. Patient is seated upon an insulated platform, holding the electrode, A, which is the other pole, from the outer side of the Leyden jar, L J. (By grounding the outer side of the Leyden jar, A, the patient's insulation is unnecessary and better effluavation is attained.) S.M. is the spark-gap."