The coil The igition coil provides high voltage to the spark plug, to provide the spark to ignite the fuel in the combustion process inside the engine. Likely, you know that m
…ost cars only have 12 volts that run all the functions in a car. But 12 volts is not enough to provide a spark across the electrode of a spark plug, so more voltage is needed to jump the gap of the spark plug electrode. The coil provides this by magnetic induction inside the coil. It converts 12 volts to tens of thousands of volts, which is what is needed by the spark plug. If you are shocked by a car's ignition coil output voltage, you will definitely feel it! But you will not be permentately harmed or killed, because this voltage is at a very low amperage. ======= The ignition coil is basically a transformer. It converts a low voltage into a high voltage to create a spark at the spark plug. The way it works may seem backwards, but basically, a low voltage (about 12 volt) electrical current is sent through a coil of wire (the "primary" winding), which causes an electromagnetic field to build up. When this low-voltage electrical is inturrupted (by the points in a conventional distributor, or by the engine's computer), it causes this electromagnetic field to collapse suddenly. As it collapses, it cuts across the windings of a "secondary winding" which is wound around the primary winding. Since the secondary has many more turns of wire than the primary, the magnetic field causes a much higher voltage (anywhere from 5,00 to 20,000 volts) to flow in it. This is the voltage that is sent out through the ignition leads to the spark plugs. Why do they have it trigger the high voltage when the current is shut off rather than when it's turned on? Because, back when they first started making ignition systems for engines, they used mechanical "points" to switch the voltage on and off. When the points "made" to allow the current to flow, they had a tendency to bounce, causing the voltage to build up erratically. When they were opened, though, the current was shut off very rapidly, giving the sudden change in current flow necessary to create the higher voltage to be induced. Nowadays, with electronic ignition systems, it could be done either way, but they usually stick with the "old" way just because it's what they're used to. (MORE) 
