An Oudin coil, also called an Oudin oscillator or Oudin resonator, is a resonant transformer circuit that generates very high tension, high frequency alternating current (AC) electricity at low current levels, used in the obsolete forms of electrotherapy around the turn of the 20th century. It is very similar to the Tesla coil, with the difference being that the Oudin coil was connected as an autotransformer. It was invented in 1893 by French physician Paul Marie Oudin as a modification of physician Jacques Arsene d'Arsonval's electrotherapy equipment and used in medical diathermy therapy as well as quack medicine until perhaps 1940. The high voltage output terminal of the coil was connected to an insulated handheld electrode which produced luminous brush discharges, which were applied to the patient's body to treat various medical conditions in electrotherapy. How it works Oudin and Tesla coils are spark-excited air-core double-tuned transformer circuits that use resonance to generate very high voltages at low currents. They produce alternating current in the radio frequency (RF) range. The medical coils of the early 20th century produced potentials of 50,000 up to a million volts, at frequencies in the range 200 kHz to 5 MHz. The primary circuit of the coil has Leyden jar capacitors (C) which in combination with the primary winding of the coil (L1) make a resonant circuit (tuned circuit). In medical coils usually two capacitors were used for safety, one in each side of the primary circuit, to isolate the patient completely from the potentially lethal low frequency primary current. The primary circuit also has a spark gap (SG) that acts as a switch to excite oscillations in the primary. The primary circuit is powered by a high voltage transformer or induction coil (T) at a potential of 2 - 15 kV. The transformer repeatedly charges the capacitors, which then discharge through the spark gap and the primary winding (a detailed description of the operation cycle in the Tesla coil article also applies to the Oudin coil). This cycle is repeated many times per second. During each spark, the charge moves rapidly back and forth between the capacitor plates through the primary coil, creating a damped RF oscillating current in the primary tuned circuit which induced the high voltage in the secondary. The secondary winding (L2) is open-circuited, and connected to the output electrode of the device. In the Oudin coil, one side of the