The Induction Coil
The induction coil is the system in petrol driven cars that produces sparks to ignite the air fuel mixture in the combustion chamber. It consists of two coils, a primary and secondary, both wound on an iron core. The primary coil consists of a few turns of thick wire, able to carry large currents, and the secondary coil consists of many turn of thin wire.
In the circuit above, a current flows in the primary coil and magnetises the iron core, which attracts the armature and breaks the circuit. The magnetic field inside the core falls and the armature is released. The circuit is complete again, current flows, the iron core is magnetised and the process repeats. The rapidly changing magnetic field produces a changing voltage in the primary coil. A large voltage is induced in the secondary coil because of the larger number of turns, and because a capacitor connected between the contacts causes the magnetic field to die away very rapidly when the contacts are broken.
For this reason the EMF induced in the secondary is much greater when the circuit is broken than when it is made. The voltage in the secondary coil is large enough to break down the air in the spark gap, and a spark is produced to ignite the fuel.