An ignition coil (also called a spark coil) is a durable electrical transformer that contains the primary and secondary winding circuits (coils) of the ignition system. It transforms the battery's 12 volts (6 volts in some older vehicles) to the thousands of volts (20 to 30 thousand volts or more) needed to spark the spark plugs.

There are hundreds of rows of tiny copper wire inside of an ignition coil, all coiled up and wrapped in a circle. One coil of wire is called the primary coil with 100 to 150 turns of copper wire. Wrapped around the primary coil is the secondary coil, and it can have as many as 30,000 turns of much finer wire, and the secondary coil normally has hundreds of times more turns of wire than the primary coil.

Once the 12 volts travels through this series of wire, these two coils work together to create a large charge, then release it as the current quickly shuts off, the wires on both coils must be fully insulated. And the output is many times greater than the input--10,000 to 20,000 volts. Performance applications (racing ignitions, for example) might reach 40,000 or even 50,000 volts. The increase of output voltage is the reason that ignition coil can fire the spark plug.