Ignition Coil Explained

The heart of the ignition system is the ignition coil. It is in this sealed unit that the high-voltage electrical pulse necessary to create an ignition spark is generated. 

The ignition coil contains two separate coils of wire arranged concentrically (one inside the other). The outer, primary coil is made up of around 200 turns of thick wire, whilst the inner, secondary coil comprises of around 20,000 turns of much thinner wire. Using the transformer effect, electro-magnetic charge can be transferred from the primary to the secondary coil, converting the 12v of electricity from the car's battery into the 10,000v necessary to create a spark.

Charge is only transferred to the secondary coil when a current ceases to pass through the primary coil: this is achieved by breaking the circuit with a set of contact breaker points. At this point the accumulated charge in the primary coil becomes unstable and begins to collapse. It then transfers to the secondary coil, with the voltage increasing during the transference.

Both coils are submerged in an oil bath which helps to prevent the unit overheating.

Coils are available of various different specifications to suit both 6v and 12v cars. Performance or 'sports' coils are a popular upgrade, having more turns of wire in the secondary coil to create a higher voltage spark than normal.

For more information on a car's ignition systems, see the Ignition section in Components and Systems Explained.
For how to test a coil's performance, see the Tutorial in the Step-by-Step section.