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An induction coil or "spark coil" (archaism known as an inductorium or Ruhmkorff coil after Heinrich Rühmkorff) is a type of transformer used to produce high-voltage pulses from a low-voltage direct current (DC) supply. p.98 To create the flux changes necessary to induce voltage in the secondary coil, the direct current in the primary coil is repeatedly interrupted by a vibrating mechanical Switch called an interrupter. Invented in 1836 by the Irish-Catholic priest Nicholas Callan, also independently by American inventor Charles Grafton Page, the induction coil was the first type of transformer. It was widely used in , Collins, 1908, p. iii spark-gap radio transmitters, arc lamp and quack medical electrotherapy devices from the 1880s to the 1920s. Today its only common use is as the in internal combustion engines and in physics education to demonstrate induction.
An electric current is passed through the primary, creating a magnetic field. Because of the common core, most of the primary's magnetic field couples with the secondary winding. The primary behaves as an inductor, storing energy in the associated magnetic field. When the primary current is suddenly interrupted, the magnetic field rapidly collapses. This causes a high voltage pulse to be developed across the secondary terminals through electromagnetic induction. Because of the large number of turns in the secondary coil, the secondary voltage pulse is typically many thousands of . This voltage is often sufficient to cause an electric spark, to jump across an air gap (G) separating the secondary's output terminals. For this reason, induction coils were called spark coils.
An induction coil is traditionally characterised by the length of spark it can produce; a '4 inch' (10 cm) induction coil could produce a 4 inch spark. Until the development of the cathode ray oscilloscope, this was the most reliable measurement of peak voltage of such asymmetric waveforms. The relationship between spark length and voltage is linear within a wide range:
Curves supplied by a 1984 reference agree closely with those values.E. Kuffel (1984). 008024212X, Pergamon Press. 008024212X
Opposite potentials are induced in the secondary when the interrupter 'breaks' the circuit and 'closes' the circuit. However, the current change in the primary is much more abrupt when the interrupter 'breaks'. When the contacts close, the current builds up slowly in the primary because the supply voltage has a limited ability to force current through the coil's inductance. In contrast, when the interrupter contacts open, the current falls to zero suddenly. So the pulse of voltage induced in the secondary at 'break' is much larger than the pulse induced at 'close', it is the 'break' that generates the coil's high voltage output.
The primary coil is first wound on the iron core and insulated from the secondary with a thick paper or rubber coating. Then each secondary subcoil is connected to the coil next to it and slid onto the iron core, insulated from adjoining coils with waxed cardboard disks. The voltage developed in each subcoil isn't large enough to jump between the wires in the subcoil. Large voltages are only developed across many subcoils in series, which are too widely separated to arc over. To give the entire coil a final insulating coating, it is immersed in melted paraffin wax or rosin; the air evacuated to ensure there are no air bubbles left inside and the paraffin allowed to solidify, so the entire coil is encased in wax.
To prevent , which cause energy losses, the iron core is made of a bundle of parallel iron wires, individually coated with shellac to insulate them electrically. The eddy currents, which flow in loops in the core perpendicular to the magnetic axis, are blocked by the layers of insulation. The ends of the insulated primary coil often protruded several inches from either end of the secondary coil, to prevent arcs from the secondary to the primary or the core.
Therefore much research went into improving interrupters and improved designs were used in high power coils, with the hammer interrupters only used on small coils under 8" sparks. Collins, 1908, p. 98 Léon Foucault and others developed interrupters consisting of an oscillating needle dipping into and out of a container of mercury. The mercury was covered with a layer of spirits which extinguished the arc quickly, causing faster switching. These were often driven by a separate electromagnet or motor, which allowed the interruption rate and "dwell" time to be adjusted separately from the primary current.
The largest coils used either electrolytic or mercury turbine interrupters. The electrolytic or Wehnelt interrupter, invented by Arthur Wehnelt in 1899, consisted of a short platinum needle anode immersed in an electrolyte of dilute sulfuric acid, with the other side of the circuit connected to a lead plate cathode. When the primary current passed through it, hydrogen gas bubbles formed on the needle which repeatedly broke the circuit. This resulted in a primary current broken randomly at rates up to 2000 breaks per second. They were preferred for powering X-ray tubes. They produced a lot of heat and due to this the hydrogen could explode. Mercury turbine interrupters had a centrifugal pump which sprayed a stream of liquid mercury onto rotating metal contacts. They could achieve interruption rates up to 10,000 breaks per second and were the most widely used type of interrupter in commercial wireless stations. Page 31 describes electrolytic interrupter, but does not identify as Wehnelt interrupter.
Michael Faraday discovered the principle of induction, Faraday's induction law, in 1831 and did the first experiments with induction between coils of wire. The induction coil was invented by the American physician Charles Grafton Page in 1836, archived and independently by Irish scientist and Catholic priest Nicholas Callan in the same year at the St. Patrick's College, MaynoothCallan, N. J. A Description of an Electromagnetic Repeater in and p.522 fig. 52 and improved by William Sturgeon. George Henry Bachhoffner and Sturgeon (1837) independently discovered that a "divided" iron core of iron wires reduced power losses. Fleming (1896) The Alternate Current Transformer in Theory and Practice, Vol. 2, p. 10-11 The early coils had hand cranked interrupters, invented by Callan and Antoine Philibert Masson (1837). On page 458, an interrupter consisting of a toothed wheel is described. On page 134, Masson describes the toothed wheels that functioned as an interrupter. The automatic 'hammer' interrupter was invented by Rev. Prof. James William MacGauley (1838) of Dublin, Ireland, presented at meeting of September 1837 in Liverpool, England Johann Philipp Wagner (1839), and Christian Ernst Neeff (1847). Description of Neeff and Wagner's earlier toothed wheel interrupter Hippolyte Fizeau (1853) introduced the use of the quenching capacitor. Heinrich Ruhmkorff generated higher voltages by greatly increasing the length of the secondary, in some coils using 5 or 6 miles (10 km) of wire and produced sparks up to 16 inches. In the early 1850s, American inventor Edward Samuel Ritchie introduced the divided secondary construction to improve insulation.American Academy of Arts and Sciences, Proceedings of the American Academy of Arts and Sciences, Vol. XXIII, May 1895 - May 1896, Boston: University Press, John Wilson and Son (1896), pp. 359-360Page, Charles G., History of Induction: The American Claim to the Induction Coil and Its Electrostatic Developments, Washington, D.C.: Intelligencer Printing House (1867), pp. 104-106 Jonathan Nash Hearder worked on induction coils. at page 360. Callan's induction coil was named an IEEE Milestone in 2006.
Induction coils were used to provide high voltage for early gas discharge and and other high voltage research. They were also used to provide entertainment (lighting , for example) and to drive small "shocking coils", and violet ray devices used in quack medicine. They were used by Hertz to demonstrate the existence of electromagnetic waves, as predicted by James Clerk Maxwell and by Oliver Lodge and Marconi in the first research into radio waves. Their largest industrial use was probably in early wireless telegraphy spark-gap radio transmitters and to power early cold cathode from the 1890s to the 1920s, after which they were supplanted in both these applications by AC and . However their largest use was as the ignition coil or spark coil in the ignition system of internal combustion engines, where they are still used, although the interrupter contacts are now replaced by solid state switches. A smaller version is used to trigger the flash tubes used in cameras and strobe lights.
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