Induction coil

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. 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 contact called an interrupter. Development began in 1836 by Nicholas Callan, Charles Page and others. The induction coil was the first type of transformer and they were widely used in x-ray machines,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.

See schematic diagram. An induction coil consists of two coils of insulated copper wire wound around a common iron core (M). 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 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 electric spark, to jump across an air gap (G) separating the secondary's output terminals. For this reason, induction coils were called spark coils.

...
Wikipedia