Automobile coil makes a spark.

Automobile coil makes a spark. This apparatus demonstrates how an automobile ignition coil generates a high voltage spark. Looking at the circuit diagram, a 6 volt battery is connected in series with the primary of the ignition coil and a capacitor. A switch is connected across the capacitor. When the switch is closed, a constant current flows through circuit 'A', producing a constant magnetic field in the primary coil. Because the field is constant, there is no voltage induced in the ignition coil secondary (circuit 'B'). However, when the switch is released, the energy stored in the primary coil magnetic field is quickly released, and a large voltage (about 5 kVolts) is induced in the secondary coil, producing a spark across the spark gap. (The capacitor is in the circuit mainly to prevent the 'points' of the switch from being damaged by large currents.)