Oudin coil

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.

[4] Oudin and Tesla coils are spark-excited air-core double-tuned transformer circuits that use resonance to generate very high voltages at low currents.

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 secondary is directly connected to the primary circuit, which carries lethal low frequency 50/60 Hz currents at thousands of volts from the power transformer.

Since the Oudin coil was a medical device, with the secondary current applied directly to a person's body, for safety the Oudin circuit has two capacitors (C), one in each leg of the primary, to completely isolate the coil and output electrode from the supply transformer at the mains frequency.

[15] Because two identical capacitors in series have half the capacitance of a single capacitor, the resonant frequency of the Oudin circuit is The high voltage terminal of the coil was attached through a wire to various types of handheld electrode which the physician used to apply the high voltage to the patient's body.

To apply brush discharges (called "effluves") to the outside of the patient's body, electrodes consisting of one or more metal points on an insulating handle were used.

This consisted of a partially evacuated glass tube of various shapes, with an electrode sealed inside, attached to the high voltage wire.

The glass envelope of the tube formed a capacitor with the patient's body through which the current had to pass, limiting it to safe values.

Metal handrest electrodes at the sides, which the patient grasped during treatment, served as the "ground" return path and were attached to the bottom of the coil.

During the 1800s, experiments in applying electric currents to the human body grew into a Victorian era medical field, part legitimate experimental medicine and part quack medicine, called electrotherapy, in which currents were applied to treat many medical conditions.