thumb|upright=1.2|antique induction coil used in schools, from around 1900, bremerhaven, germany thumb|upright=1.2|induction coil showing construction, from 1920.  an induction coil or "spark coil" (archaically known as an inductorium or ruhmkorff coil after heinrich ruhmkorff) is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current (dc) supply.john archibald fleming  p.98 to create the flux changes necessary to induce voltage in the secondary, the direct current in the primary is repeatedly interrupted by a vibrating mechanical contact called an interrupter. developed beginning in 1836 by nicholas callan and others, the induction coil was the first type of transformer. they were widely used in x-ray machines,collins, 1908, p. iii spark-gap radio transmitters, arc lighting and quack medical electrotherapy devices from the 1880s to the 1920s. today their only common use is as the ignition coils in internal combustion engines, and in physics education to demonstrate induction. the term 'induction coil' is also used for a coil carrying high-frequency alternating current (ac), producing eddy currents to heat objects placed in the interior of the coil, in induction heating or zone melting equipment. ==construction and function== thumb|upright=1.3|schematic diagram see schematic diagram. an induction coil consists of two coils of insulated copper wire wound around a common iron core (m).collins, 1908, p. 16-19 one coil, called the primary winding (p), is made from relatively few (tens or hundreds) turns of coarse wire. the other coil, the secondary winding, (s) typically consists of many (thousands) 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 [[i
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