When the ignition switch is ‘ON’ the current flows from the battery through the primary winding and produces a magnetic field in the coil. When the contact points open the magnetic field 12 volts from the battery to high tension voltage of about 20 to 30 thousand volts required to jump the spark at the spark plug gap (15000 volts are needed to jump 1mm gap). The distributor then directs this high voltage to the proper spark plug when it jumps the gap, producing a spark which ignites the combustible mixture in the cylinder.
