What is a magneto?
A magneto was an early form of electrical generator. Almost all early aviation engines had a magneto, (often 2) to spark the plugs.
A magneto is the earliest form of generator. When a magnet passes a coil, electrical current is created. A magneto creates a low voltage that gets sent to the secondary coil ,which is a large capacitor, this turns the low voltage to a very high voltage that then goes directly to the spark plug. Only electrically insensitive auxiliaries can be run with the electricity provided by a magneto (eg lights, some horns). With some circuitry, many more things can be run from a magneto.
The above answer is mixing two separate devices and describes the function of a 'magdyno' such as were commonly used on small engines.
A magneto is a device specifically used to generate the high voltage needed to fire a spark plug.
They were common on larger motorcycle engines of the past (pre-1960) and are still used on some aircraft engines.
Some people still prefer to use them on racing engines and souped up Harley-Davidson motors.
The magneto does use two sets of coils, the first producing the low energy current which is raised by the secondary high energy output coils, this pair of coils works as a transformer.
The magneto also uses mechanical switching through a set of points that open and close to fire the spark and the points or contact breakers also include a capacitor which is a device that stores electrical energy whose purpose is to prevent sparking at the points which would quickly cause them to become burnt and start to fail.
the magneto and alternator work in the same principle. the main difference is that in an altrnator, the the amrmature spins in a statinonary set of magnetic fields.,but, in a magneto, the amrature portion is stationary, and the fields are sets of PERMANENT magnents that spin around the core(armature), to produce electrical current...
On small or light aircraft such as a Cessna, there are TWO (2) magneto's and two sets of spark plugs in each cylinder. Each Magneto creates a spark for different plug in each cylinder therefore if one mag quits or failes the other one would still be firing one plug in all cylinders. One other note about magneto's, the left one is usually set to retard the timing for start. This is done through the use of an impulse coupling once the engine is at idle speed the impulse coupling will change the spark timing back to normal, usually around 25 deg before top dead center. Note usually only happens on one mag and is usually the left mag.