A magneto is a device used in the ignition system of gasoline-powered internal combustion engines to provide pulses of high voltage electrical power to the spark plugs. Once found in every gasoline-powered automobile, it is now confined mainly to lawnmowers, chainsaws, and internal-combustion aviation engines. Magnetos and coils used in early internal-combustion engines generally used the term "tension" in lieu of the more modern term "voltage."
The magneto was introduced on the 1899 Daimler Phönix. This was followed by Benz, Mors, Turcat-Mery, and Nesseldorf, and soon were used on most cars up until about 1918 in both low voltage (voltage for secondary coils to fire the spark plugs) and high voltage magnetos (to fire the spark plug directly, similar to coil ignitions, introduced by Bosch in 1903).
On each revolution, a cam opens the contact breaker one or more times, interrupting the current, which causes the electromagnetic field in the primary coil to collapse. As the field collapses there is a voltage induced (as described by Faraday's Law) across the primary coil. As the points open, point spacing is such that the voltage across the primary coil will arc across the points. A capacitor is placed across the points to suppress the arc, set the amount of voltage across the primary coil, and to control the rate at which the electrical energy dissipates in the primary coil.
A second coil, with many more turns than the primary, is wound on the same iron core to form an electrical transformer. The ratio of turns in the secondary winding to the number of turns in the primary winding, is called the turns ratio. Voltage across the primary coil results in a proportional voltage being induced across the secondary winding of the coil. The turns ratio between the primary and secondary coil is selected so that the voltage across the secondary reaches a very high value, enough to arc across the gap of the spark plug.
Magneto-equipped aircraft engines are typically dual-plugged, that is, each cylinder has two spark plugs, with each plug having its own separate magneto system. This arrangement provides redundancy in the event of a failure of one of the magnetos. Two sparks provide two flame fronts within the cylinder. The two flame fronts decrease the time needed for the complete fuel charge to start burning and therefore most of the fuel is already burning at a lower temperature and pressure. As the combustion pressure rises within a single plug cylinder, lower octane portions of the fuel mixture far from the original flame front can explode lighting off another flame front in a different part of the cylinder at a different time. This leads to engine knock. Therefore two flame fronts can help to decrease the octane requirement for any given engine and situation. This was very important given the very large bore size of most aircraft engines developed around World War II.
Some luxury cars before that time were dual-plugged as well, but had only one set of plugs fired by magneto. The other set was wired to a coil, dynamo, and battery circuit because the magneto would generate high voltage at high engine speed, but not at low engine speed, whereas the other ignition system would perform well at low engine speed but not as well as a magneto at high speed.
Modern engines have much smaller combustion chambers and less opportunity for knock given their modern combustion chamber design which attempts to eliminate "hot" areas of the cylinder. These modern chambers decrease octane demand by their engineered shape and swirl/tumble of incoming charge. Therefore, they no longer require double plugs to decrease octane demand. Harley Davidson motorcycle engines, being single-pin, air-cooled, and having a large "antique" combustion chamber design, can still benefit from double plugging. The same holds true for most drag racing engines where the volume and type of fuel (nitromethane) and engine design (mostly based on the 1960's Chrysler Hemi engine) requires additional flame centers in order to use the massive amount of fuel injected during the intake cycle.