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; the
parasitic 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.