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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.

Contents 1 Description 2 History 3 See also 4 Further reading 5 External links

An induction coil is an early version of an electrical transformer. It consists of two coils of insulated copper wire wound around a common iron core. One coil, called the primary, is made using tens or hundreds of turns of coarse wire. The other coil, called the secondary, typically consists of many thousands of turns of fine wire. In operation, an electric current is passed through the primary, creating a magnetic field. Because of the common core, most of the primary's magnetic field also couples to 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 switch (called the interrupter) 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 and intended to induce eddy currents to heat objects placed in the interior of the coil, such as in induction heating or zone melting.

The induction coil was discovered during early experiments with electricity, probably by Nicholas Callan in 1836, and further refined by Heinrich Ruhmkorff. Induction coils were used to provide high voltage used for early gas discharge, Crookes tube, and X-ray research. They were also used to provide entertainment (such as lighting Geissler tubes) and to drive small "shocking coils" and Violet ray devices used in quack medicine.

This 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 electromagnetic waves existed, as predicted by James Maxwell, and by Tesla and Marconi in early wireless telegraphy. They were supplanted in wireless or radio work by vacuum tubes by 1920. Since the mid-1960's these coils have also been used for Induction Sealing .

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

• 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.

• Battery powered Driver circuit for Induction Coils
• The Cathode Ray Tube site