Induction coil

An Induction coil or "spark coil" (archaically known as a Ruhmkorff coil) is a type of disruptive discharge coil. It is a passive electrical device used to produce high-voltage pulses from a low-voltage DC supply.

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Description

An induction coil consists of two coils of insulated copper wire wound around a common iron core. One coil, called the primary, is made from tens or hundreds of turns of coarse wire. The other coil, the secondary, typically consists of many thousands of turns of fine wire. 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 volts. This voltage is often sufficient to cause an electrical discharge, or spark, to jump across an air gap separating the secondary's output terminals. For this reason, induction coils were sometimes called spark coils. Most induction coils utilized a magnetically activated vibrating arm to rapidly connect and break current flowing into the primary coil.

The term "induction coil" is also used for a coil carrying high-frequency AC producing eddy currents to heat objects placed in the interior of the coil, as in induction heating or zone melting.

History

The induction coil was discovered during early experiments with electricity by Nicholas Callan in 1836 at the St. Patrick's College, Maynooth and further refined by Heinrich Ruhmkorff and others. Induction coils were used to provide high voltage for early gas discharge and Crookes tubes and for X-ray research. They were also used to provide entertainment (lighting Geissler tubes, for example) and to drive small "shocking coils" and violet ray devices used in quack medicine. A type of disruptive discharge coil remains in common use as the ignition coil or spark coil in the ignition system of internal combustion engines. A smaller version is used to trigger the flash tubes used in cameras and strobe lights. They were used by Hertz to demonstrate the existence of electromagnetic waves, as predicted by James Maxwell, and by Tesla and Marconi in early wireless telegraphy. By about 1920 they were supplanted in wireless or radio work by vacuum tubes.

Early patents

• U.S. Patent 52,054  The induction-coil, instead of being made movable upon the magnet
• U.S. Patent 72,616  This compound coil is made like any ordinary induction-coil
• U.S. Patent 74,905  The inner end of the induction-coil are surrounded by the prime coil
• U.S. Patent 76,654  The induction-coil consists of a metallic conductor, copper is generally preferred
• U.S. Patent 78,495  Energizing the primary wire of the induction-coil, the iron core becomes magnetized
• U.S. Patent 90,626  Making use of an induction-coil
• U.S. Patent 734,197  a split-coil improvement (1903).
• U.S. Patent 1,092,417  Induction coil comprising a soft iron core (Mar 5, 1913)

See also

• Spark gap transmitter
• Transformer
• Tesla coil
• Ignition coil

Further reading

• Norrie, H. S., "Induction Coils: How to Make, Use, and Repair Them". Norman H. Schneider, 1907, New York. 4th edition.
• Faraday M (1834): Experimental researches on electricity, 7th series. Phil. Trans. R. Soc. (Lond.) 124: 77-122.