The ignition coil is a step-up transformer, which raises the nominal battery voltage of 12 volts up to the many thousands of volts necessary to provide a spark across the spark plug electrodes.
A standard ignition coil has a rod-shaped laminated iron core, which is located centrally by an insulator at its base. The secondary winding with 15,000 to 30,000 turns of very thin enameled copper wire is wound around the core and is insulated from the core by layers of treated insulated paper. The primary winding, with a few hundred turns of much heavier copper wire, is wound on the outside of the secondary.
A shield of soft iron surrounds the outer windings and the complete assembly is inserted into a one-piece steel or aluminum container. The container is then filled with a special transformer oil, which provides good electrical insulation and also permits rapid heat dissipation.
The cap has two terminals, positive and negative, for external connection to the primary circuit . The ends of the primary winding are connected internally to each. Provision is also made externally for a heavy insulated center terminal to connect the high tension coil lead to the distributor cap.
One end of the secondary winding is connected to this center terminal, and the other end is connected to one end of the primary winding. A rubber seal and a molded insulated cap seal the assembly in the container, and the container edges are swaged over to bind the coil into a compact vibration free unit.