Oudin and Tesla coils are spark-excited air-core double-tuned transformer circuits that use resonance to generate very high voltages at low currents. They produce alternating current in the radio frequency (RF) range. The medical coils of the early 20th century produced potentials of 50,000 to nearly a million volts, at frequencies in the range 200 kHz to 5 MHz. The primary circuit of the coil has Leyden jar capacitors (C) which in combination with the primary winding of the coil (L1) make a resonant circuit (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 (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 (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. 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 (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 resonance in the high Q 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, so the primary induces an EMF in the secondary by electromagnetic induction. 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 is also a resonant circuit (electrical resonator); the parasitic capacitance between the ends of the secondary coil resonates with the large inductance of the secondary at a particular resonant frequency. 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 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.
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D'Arsonval apparatus - D'Arsonval first used a high frequency alternator, but this device could not produce frequencies over 10 kHz. To reach higher frequencies he developed a circuit consisting of a single tuned circuit, made of two Leyden jar capacitors and a small coil of relatively few, 10 to 30 turns of heavy wire, with an induction coil and spark gap 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 (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 pioneered the field of high-voltage RF resonant transformers. He and Elihu Thomson 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, 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.
Oudin coil
https://en.wikipedia.org/wiki/Oudin_coil