Oudin coil
![Oudin coil used for medical electrotherapy, 1907, and a schematic diagram of its circuit (left).](/Images/godic/202501/31/Oudin_coil_and_circuit_diagram1621.png")
![Early type Oudin coil, around 1904, with separate primary and secondary coils. In the drawing the primary (L1) is the small horizontal coil visible between the Leyden jars and the large vertical secondary resonator coil. The transformer (T) which powers the circuit is not shown.](/Images/godic/202501/31/Oudin_coil_circuit_-_early_type.svg1621.png")
![](/Images/godic/202501/31/Early_Oudin_coil1622.png")
![Later type with primary and secondary wound on the same form to make an autotransformer. The primary is the coarse winding at the bottom. In both pictures the spark gap (SG) is enclosed in a small box at right to reduce noise.](/Images/godic/202501/31/Oudin_coil_circuit_-_later_type.svg1622.png")
An Oudin coil, also called an Oudin oscillator or Oudin resonator, is a resonant transformer circuit that generates very high voltage, high frequency alternating current (AC) electricity at low current levels, used in the obsolete medical field of electrotherapy around the turn of the 20th century. It is very similar to a 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 quack medicine until perhaps 1930. 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.