The magneto is an alternating-current generator that contains a permanent magnet. A magneto requires no source of direct current for its operation. The most common use of magnetos is in the ignition system of some internal-combustion engines. Magneto ignitions are used in lawn mowers, snow blowers and other 2-stroke engines; they may be used in vehicles—including certain tractors, snowmobiles, motorboats, motorcycles, and small airplanes—that do not require storage batteries to operate auxiliary equipment. Like the storage battery in an automobile, the magneto provides the electric voltage that causes the spark plugs in the engine to spark.
The two main parts of a magneto are a permanent magnet and an armature. The armature is a piece of metal around which is wound the primary, a coil of a few turns of heavy wire.
In most magnetos today, the armature is stationary and the permanent magnet rotates; in others, the permanent magnet is stationary and the armature rotates within the magnetic field produced by the magnet. The relative motion of the magnet and armature generates an alternating current in the primary.
In the typical magneto used in an internal-combustion engine, the magnet is turned by the crankshaft of the engine. The voltage generated by the magnet in the primary coil is not sufficient to cause a spark in the spark plugs. The secondary, which consists of many thousands of turns of fine wire, is wound around the primary. The two coils act like a step-up transformer. The breaker points are electrical contacts in the circuit containing the primary. They are used to interrupt the current in the primary, causing an extremely short, high-voltage burst of current in the secondary. This current is directed to the spark plugs.
Magneto ignition was developed in 1878 by Nikolaus August Otto. It was widely used on early automobiles.
http://science.howstuffworks.com/magneto-info.htm
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