Conventional automobiles burn gasoline in an internal combustion engine and convert that energy into motion. But first a spark is needed to ignite the fuel mixture. This animation shows how a 12-volt battery generates the high voltage required to create such a discharge.
source/image(PrtSc): National MagLab
An ignition coil consists of a laminated iron core surrounded by two coils of copper wire. Unlike a power transformer, an ignition coil has an open magnetic circuit — the iron core does not form a closed loop around the windings. The energy that is stored in the magnetic field of the core is the energy that is transferred to the spark plug. Watch the video from National MagLab for more info:
The primary winding has relatively few turns of heavy wire. The secondary winding consists of thousands of turns of smaller wire, insulated from the high voltage by enamel on the wires and layers of oiled paper insulation.
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The coil is usually inserted into a metal can or plastic case with insulated terminals for the high voltage and low voltage connections. When the contact breaker closes, it allows current from the battery to flow through the primary winding of the ignition coil./wikipedia
