Oudin coil
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An Oudin coil, also called an Oudin oscillator or Oudin resonator, is a resonant transformer circuit that generates very
high tension ''High Tension'' (French: ''Haute Tension'', ; released in the United Kingdom as ''Switchblade Romance'') is a 2003 French slasher film directed by Alexandre Aja, co-written with Grégory Levasseur, and starring Cécile de France and Maïwenn, ...
, high frequency
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
(AC) electricity at low
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
levels, used in the obsolete forms of
electrotherapy Electrotherapy is the use of electrical energy as a medical treatment. In medicine, the term ''electrotherapy'' can apply to a variety of treatments, including the use of electrical devices such as deep brain stimulators for neurological dise ...
around the turn of the 20th century. It is very similar to the
Tesla coil A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low-current, high-frequency alternating-current electricity. Tesla experimented with a number of different ...
, with the difference being that the Oudin coil was connected as an
autotransformer An autotransformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting alone, not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as ...
. It was invented in 1893 by French physician Paul Marie Oudin as a modification of physician
Jacques Arsene d'Arsonval Ancient and noble French family names, Jacques, Jacq, or James are believed to originate from the Middle Ages in the historic northwest Brittany region in France, and have since spread around the world over the centuries. To date, there are over ...
's electrotherapy equipment and used in medical
diathermy Diathermy is electrically induced heat or the use of high-frequency electromagnetic currents as a form of physical therapy and in surgical procedures. The earliest observations on the reactions of high-frequency electromagnetic currents upon the ...
therapy as well as
quack medicine Quackery, often synonymous with health fraud, is the promotion of fraudulent or ignorant medical practices. A quack is a "fraudulent or ignorant pretender to medical skill" or "a person who pretends, professionally or publicly, to have skill, k ...
until perhaps 1940. The high voltage output terminal of the coil was connected to an insulated handheld
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
which produced luminous
brush discharge A brush discharge is an electrical disruptive discharge similar to a corona discharge that takes place at an electrode with a high voltage applied to it, embedded in a nonconducting fluid, usually air. It is characterized by multiple luminous wr ...
s, which were applied to the patient's body to treat various medical conditions in
electrotherapy Electrotherapy is the use of electrical energy as a medical treatment. In medicine, the term ''electrotherapy'' can apply to a variety of treatments, including the use of electrical devices such as deep brain stimulators for neurological dise ...
.


How it works

Oudin and Tesla coils are spark-excited air-core double-tuned
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
circuits that use
resonance Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
to generate very
high voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and conductors that carry high voltage warrant spec ...
s at low
currents Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
. They produce alternating current in the
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
(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. The
primary Primary or primaries may refer to: Arts, entertainment, and media Music Groups and labels * Primary (band), from Australia * Primary (musician), hip hop musician and record producer from South Korea * Primary Music, Israeli record label Works * ...
circuit of the coil has
Leyden jar A Leyden jar (or Leiden jar, or archaically, sometimes Kleistian jar) is an electrical component that stores a high-voltage electric charge (from an external source) between electrical conductors on the inside and outside of a glass jar. It typi ...
capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...
s ''(C)'' which in combination with the
primary winding A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer's c ...
of the coil ''(L1)'' make a
resonant circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
(tuned circuit). In medical coils usually two capacitors were used for safety, one in each side of the primary circuit, to isolate the patient completely from the potentially lethal low frequency primary current. The primary circuit also has a
spark gap A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductor ...
''(SG)'' that acts as a switch to excite oscillations in the primary. The primary circuit is powered by a high voltage transformer or
induction coil An induction coil or "spark coil" (archaically known as an inductorium or Ruhmkorff coil after Heinrich Rühmkorff) is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current (DC) supply. p.98 To ...
''(T)'' at a potential of 2 - 15 kV. The transformer repeatedly charges the capacitors, which then discharge through the spark gap and the primary winding (a detailed description of the operation cycle in the
Tesla coil A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low-current, high-frequency alternating-current electricity. Tesla experimented with a number of different ...
article also applies to the Oudin coil). This cycle is repeated many times per second. During each spark, the charge moves rapidly back and forth between the capacitor plates through the primary coil, creating a damped RF oscillating current in the primary tuned circuit which induced the high voltage in the secondary. The
secondary winding A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces a varying magnetic flux in ...
''(L2)'' is open-circuited, and connected to the output electrode of the device. In the Oudin coil, one side of the primary winding ''(L1)'' is grounded and the other side is connected to the secondary, so the primary and secondary are in series. There were two versions of the Oudin coil: * 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), connected to the high voltage terminal on top. In this circuit the high voltage was generated entirely by self-resonance in the
high Q ''High Q'' is the name of various local television student quiz shows broadcast throughout the United States. While the formats vary, all featured two or three teams representing high schools from the station's coverage area, which would compet ...
secondary coil. The addition of the "resonator" coil to the "D'Arsonval" coil was Oudin's contribution; the rest of the circuit was invented by Jacques D'Arsonval. * In later Oudin circuits the coils were magnetically coupled, forming an
autotransformer An autotransformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting alone, not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as ...
, so the primary induces an EMF in the secondary by
electromagnetic induction Electromagnetic or magnetic induction is the production of an electromotive force (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1831, and James Clerk ...
. Both coils were usually wound on the same coil form, the primary consisting of relatively few turns of heavy wire at the bottom with an adjustable tap, connected to the secondary winding, made of many turns of fine wire. Oudin found this circuit produced higher voltages due to the large turns ratio of the transformer. Although it doesn't include a capacitor, the
secondary winding A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces a varying magnetic flux in ...
is also a
resonant circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
(electrical
resonator A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonator ...
); the
parasitic capacitance Parasitic capacitance is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. When two electrical conductors at different voltages a ...
between the ends of the secondary coil
resonate Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied Periodic function, periodic force (or a Fourier analysis, Fourier component of it) is equal or close to a natural frequency of the system ...
s with the large
inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of the ...
of the secondary at a particular
resonant frequency Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
. When it is excited at this frequency by the primary, large oscillating voltages are induced in the secondary. The number of turns in the primary winding, and thus the
resonant frequency Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
of the primary, could be adjusted with a tap on the coil. When the two tuned circuits are adjusted to resonate at the same frequency, the large turns ratio of the coil, aided by the high Q of the tuned circuits, steps up the primary voltage to hundreds of thousands to millions of volts at the secondary. 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. Because two identical capacitors in series have half the capacitance of a single capacitor, the resonant frequency of the Oudin circuit is :f = \;


Use

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 Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
(RF) range, above 10 kHz in
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
, does not generally cause the sensation of
electric shock Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce ...
. 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 Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
.Manders, 1902
Some phenomena of high frequency currents
p. 223
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)''. 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 discharge A brush discharge is an electrical disruptive discharge similar to a corona discharge that takes place at an electrode with a high voltage applied to it, embedded in a nonconducting fluid, usually air. It is characterized by multiple luminous wr ...
s (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 In the history of the United Kingdom and the British Empire, the Victorian era was the period of Queen Victoria's reign, from 20 June 1837 until her death on 22 January 1901. The era followed the Georgian period and preceded the Edwardia ...
medical field, part legitimate experimental medicine and part
quack medicine Quackery, often synonymous with health fraud, is the promotion of fraudulent or ignorant medical practices. A quack is a "fraudulent or ignorant pretender to medical skill" or "a person who pretends, professionally or publicly, to have skill, k ...
, called
electrotherapy Electrotherapy is the use of electrical energy as a medical treatment. In medicine, the term ''electrotherapy'' can apply to a variety of treatments, including the use of electrical devices such as deep brain stimulators for neurological dise ...
, in which currents were applied to treat many medical conditions. The discovery of
radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (short ...
s by
Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's Maxwell's equations, equations of electrom ...
in 1886 and subsequent development of
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
by
Oliver Lodge Sir Oliver Joseph Lodge, (12 June 1851 – 22 August 1940) was a British physicist and writer involved in the development of, and holder of key patents for, radio. He identified electromagnetic radiation independent of Heinrich Rudolf Hertz, H ...
,
Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquis of Marconi (; 25 April 187420 July 1937) was an Italians, Italian inventor and electrical engineering, electrical engineer, known for his creation of a practical radio wave-based Wireless telegrap ...
sparked interest in
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
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 Ancient and noble French family names, Jacques, Jacq, or James are believed to originate from the Middle Ages in the historic northwest Brittany region in France, and have since spread around the world over the centuries. To date, there are over ...
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 Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce ...
, so that extremely high voltages could be applied to a patient without discomfort. In 1891 in America, engineer
Nikola Tesla Nikola Tesla ( ; ,"Tesla"
''
* ''D'Arsonval apparatus'' - D'Arsonval first used a high frequency
alternator An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature.Go ...
, but this device could not produce frequencies over 10 kHz. To reach higher frequencies he developed a circuit consisting of a single
tuned circuit An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can ac ...
, made of two
Leyden jar A Leyden jar (or Leiden jar, or archaically, sometimes Kleistian jar) is an electrical component that stores a high-voltage electric charge (from an external source) between electrical conductors on the inside and outside of a glass jar. It typi ...
capacitors and a small coil of relatively few, 10 to 30 turns of heavy wire, with an induction coil and
spark gap A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductor ...
to excite oscillations. It resembled a Tesla circuit without the secondary winding. The circuit produced relatively low voltage (perhaps 30 kV), high current electricity, called "D'Arsonval currents". The circuit was "bipolar" with two wires from opposite ends of the coil connected to electrodes at different places on the patient's body. The high frequency currents could produce heat in the patient's body, similar to the operation of a microwave oven; this was called ''
Diathermy Diathermy is electrically induced heat or the use of high-frequency electromagnetic currents as a form of physical therapy and in surgical procedures. The earliest observations on the reactions of high-frequency electromagnetic currents upon the ...
'' (Behary). Other names were also used such as ''Thermo-Faradism'' and ''Thermo-Penetration'' depending on whether it was used for therapy or for destructive effects in surgery (Behary) * ''Oudin resonator'' - French physician Paul Marie Oudin in 1893 modified the D'Arsonval circuit by adding a second large "resonator" coil made of many turns of fine wire to a tap on the small D'Arsonval coil. By adjusting the tap point he could bring the second coil to resonance with the tuned circuit, generating higher voltages at lower currents, similar to the Tesla circuit. In early "Oudin resonator" apparatus the two coils are separate and not magnetically coupled. Later Oudin found that the efficiency could be increased by magnetically coupling the coils, making the small coil the primary winding for the big coil. The Oudin resonator generated very high voltage, low current electricity. It was a "unipolar" circuit, with one end grounded and a single output terminal. * ''Tesla-Thomson apparatus'' - Serbian-American engineer
Nikola Tesla Nikola Tesla ( ; ,"Tesla"
''
Tesla coil A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low-current, high-frequency alternating-current electricity. Tesla experimented with a number of different ...
circuit in 1891. He and
Elihu Thomson Elihu Thomson (March 29, 1853 – March 13, 1937) was an English-born American engineer and inventor who was instrumental in the founding of major electrical companies in the United States, the United Kingdom and France. Early life He was born ...
experimented with applying high voltage to their bodies. Although Tesla invented many different resonant transformer circuits, the electrotherapy circuit that bears his name was a "bipolar" circuit consisting of a spark-excited tuned primary coil of few turns encircling the center of a symmetrical bipolar secondary coil of many turns of fine wire, with output terminals on each end. The D'Arsonval and Oudin apparatus became popular in Europe, while the Tesla-Thompson apparatus was mostly used in America. During the first decades of the 20th century there was a rivalry between these camps, and debate in the medical literature as to whether "Tesla currents" or "Oudin currents" were better for various conditions. By 1920 it was realized that the currents were very similar. Since the circuits were so similar, medical suppliers sold combination "high frequency" units that could be set up for Tesla, D'Arsonval, or Oudin therapy, often also combined with Rontgen ray (X-ray) After Oudin combined the primary and "resonator" coil together on the same form, making them an air-core
autotransformer An autotransformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting alone, not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as ...
, the only significant difference between the Tesla and Oudin apparatus was that the medical Tesla coil was "bipolar" while the Oudin coil was "unipolar", with one end grounded. As time went on the meaning of the terms changed, until (by perhaps 1920) the term ''Tesla coil'' meant a "bipolar" coil; any high voltage coil with an ungrounded balanced secondary with two output terminals, while the term ''Oudin coil'' meant a "unipolar" coil; any coil with a grounded secondary and a single output terminal. Around the 1930s
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
oscillators Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
replaced spark-excited circuits in high frequency medical equipment. The field of electrotherapy was replaced by the modern field of
diathermy Diathermy is electrically induced heat or the use of high-frequency electromagnetic currents as a form of physical therapy and in surgical procedures. The earliest observations on the reactions of high-frequency electromagnetic currents upon the ...
, and the Oudin coil became obsolete. Ironically modern day Tesla coil designs are unipolar, with a single high voltage terminal, and so are sometimes called Oudin coils.


See also

* Violet wand


References

{{Electric transformers Electric transformers Electromagnetic coils