File:Large Oudin coil 1921.jpg

English: A large Oudin coil apparatus, a resonant transformer circuit similar to a Tesla coil invented by French physician Paul Marie Oudin around 1893, used in the obsolete medical field of electrotherapy during the first decades of the 20th century. It generated very high voltage, low current, high frequency AC current, at frequencies between 200 kHz and 2 MHz. The high voltage terminal at top was connected through a wire to a handheld pointed electrode, which produced luminous brush discharges which the physician applied to the patient's body to treat various medical conditions. This procedure was not painful for the patient because high frequency currents above 10 kHz do not cause the sensation of electric shock

This example is a powerful coil from 1921 that may have produced around a million volts. The resonant coil at top is an autotransformer, with a primary winding consisting of a few turns of coarse wire at bottom with an adjustable tap (right) connected in series with a secondary winding of many turns of fine wire at top, that generated the high voltage. The primary winding is connected to a large multiple glass plate capacitor (black box) to make a tuned circuit, and a spark gap (glass bulb above capacitor) The capacitor is charged to a high voltage ( perhaps 5 to 20 kilovolts) from a supply transformer (not shown); then a spark jumps the spark gap, creating oscillating high frequency currents in the primary winding. This induces a pulse of extremely high voltage in the secondary winding. The capacitor immediately charges up again and the cycle repeats, creating a rapid string of pulses of alternating current from the high voltage terminal. The primary tuned circuit was adjusted to resonance with the secondary winding using the tap on the coil to generate maximum voltage.