English: An early 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, 200 - 1000 kV at frequency between 200 kHz and 2 MHz. The high voltage terminal at top was connected through a wire to a handheld electrode, which produced streamer arcs and brush discharges which were applied to the patient's body to treat various medical conditions.

It consists of a small solenoid coil of 20-40 turns (center) connected to two Leyden jar capacitors to make a tuned circuit with a spark gap (right) enclosed in a small box to muffle the sound. A potential of 3 - 15 kV from an induction coil (separate, not shown) repeatedly charges the capacitors, which then discharge through the spark gap and solenoid, creating damped radio frequency oscillations. The large resonator coil (left) is connected to a tap on the solenoid coil. The resonator acted as a second tuned circuit; theparasitic capacitance between its ends resonates with its large inductance. When the tap on the solenoid was adjusted so the primary tuned circuit and the resonator coil had the same resonant frequency, large oscillating voltages were induced in the resonator due to its high Q. This is an early circuit, in later Oudin circuits the two coils were wound on the same axis to make a transformer.