thumb|250px|Oudin coil used for medical electrotherapy, 1907, and a [[schematic diagram of its circuit (left).]]
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 forms of electrotherapy around the turn of the 20th century. It is very similar to the 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 medical diathermy therapy as well as quack medicine until perhaps 1940. 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. They produce alternating current in the radio frequency (RF) range. 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.
- In earlier Oudin circuits the two coils were separate, not magnetically coupled, with a small horizontal primary "D'Arsonval" coil (L1) of 20-40 turns with a tap connected to a large vertical secondary "Oudin resonator" (L2) with many turns of fine wire (400 - 600 in large coils, 100 - 300 in small ones), Because two identical capacitors in series have half the capacitance of a single capacitor, the resonant frequency of the Oudin circuit is
:<math>f = {1 \over 2\pi\sqrt{L_1C/2\;</math>
Use
thumb|Electrodes used for electrotherapy. All attach to the black insulating handle (45, bottom) which is connected by wire to the Oudin terminal.
thumb|A vacuum electrode.
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. The treatment was not painful for the patient, because alternating current in the radio frequency (RF) range, above 10 kHz in frequency, does not generally cause the sensation of electric shock. The Oudin coil was a "unipolar" generator, with the lower end of the coil grounded, so sometimes only one electrode was applied to the patient and the return path for the currents was through the ground. However usually a ground wire from the bottom of the coil was used; attached to a ground electrode which the patient held. A drawback of the Oudin coil was that movement of the electrode and wire during use changed the capacitance of the top terminal of the secondary coil, and thus its resonant frequency. This threw the secondary coil out of resonance with the primary, causing a reduction in voltage. So the tap point on the primary coil had to be constantly adjusted during use to keep the primary and secondary in "tune" (seen in left image above).
thumb|left|upright=0.5|A vacuum electrode in operation showing violet glow
Many specialized types of electrodes were used to apply the current to various parts of the patient's body. These generally fell into two types. 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. Care had to be taken to keep these far enough from the body to prevent a continuous arc to the skin, which could cause painful RF burns. To apply current directly to the body surface, as well as to tissues inside the patient's body through the mouth, rectum, or vagina, a vacuum tube "condensing" electrode was used. This consisted of a partially evacuated glass tube of various shapes, with an electrode sealed inside, attached to the high voltage wire. This produced a dramatic violet glow when energized. 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.
To apply current to the whole body, a "condensing couch" was used. This was a bed or couch with a metal back under its mattress, connected by a wire to the high voltage terminal. 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. Thus the couch formed a capacitor, with the patient's body as one electrode.
History
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. The discovery of radio waves by Heinrich Hertz in 1886 and subsequent development of radio by Oliver Lodge, Guglielmo Marconi sparked interest in radio frequency currents and circuits for generating them. "High frequency" currents meant any frequency above the audio range, > 20 kHz, and the resonant coils which generated them were generically called "oscillation transformers". During the 1890s doctors began to experiment with applying these high voltage and high frequency currents to the human body (ethical standards in the medical profession were looser then and physicians could experiment on their patients). In 1890 French physician Jacques Arsene d'Arsonval founded the field of high frequency electrotherapy, performing the first experiments applying high frequency currents to the human body. He discovered that currents above 10 kHz do not cause muscle contraction or activate nerves to cause the sensation of electric shock, so that extremely high voltages could be applied to a patient without discomfort. In 1891 in America, engineer Nikola Tesla independently discovered the same thing. Three types of apparatus were used, developed by three pioneers in the field, D'Arsonval, Tesla, and Oudin, and separate bodies of clinical technique grew up around them:
See also
- Violet wand