The primary winding of an ignition coil will typically contain 150 to 300 turns of wire; the secondary winding will typically contain 15,000 to 30,000 turns of wire, or around 100 times more than the primary winding.
The magnetic field is initially created when the vehicle’s electrical system applies approximately 12 volts to the ignition coil’s primary winding. When a spark is required at a spark plug, the ignition system will switch off the current flow to the primary winding, which will cause the magnetic field to collapse. The collapsing magnetic field will induce a voltage into the primary winding in the region of 200 volts; but the voltage induced into the secondary winding will be approximately 100 times greater, around 20,000 volts.