An ignition coil is a coil in a car's ignition system that carries the 6 or 12 volts from the battery to the many volts used to spark the spark plugs in the ignition. An ignition coil is also called a spark coil, and is classified as an induction coil. All induction coils are electronic transformers that transform low-voltage pulses from the DC supply into higher voltage currents needed to spark machine engines.
An induction coil is made from two insulated copper wires, called the primary and secondary winding, wrapped around an iron core. A magnetic field occurs when the electric current passes through the primary winding, and the energy is transferred to the secondary by way of the iron core. This has to happen repeatedly so that the coil is able to power the output terminals, so an interrupter bar is placed between the coil and iron core to open and close the electronic current flow. In order to make sure the ignition coil is working, one can safely remove a spark plug and place the threaded end of the spark plug to an exposed metallic piece of engine. The car should be cranked while the plug is held to the engine. A large spark appearing signifies properly functioning ignition coils.