A magneto is a fairly reliable and compact electrical generator used in small gasoline engines that do not require a battery, such as those in lawn equipment, dirt bikes, mopeds, jet skis, outboard motors and RC model aeroplanes. Because they create a strong but brief electric pulse rather than a continuous current, magnetos are ideal for putting the spark in a sparkplug, which is what drives internal combustion and powers an engine.
Because of their reliability and size, magnetos are used in aeroplanes, and they were the power source behind the ringer in the early telephones.
The principle behind a magneto is the exact opposite of an electromagnet. Whereas an electromagnet uses electricity passing through a coil to produce a magnet, a magneto uses a magnetic field in the vicinity of a coil, called the armature, to produce an electric current.
A magneto thus consists of three essential parts. The armature, often in the shape of a U, has a primary coil of thick wire and a secondary coil of thin wire wrapped around it in layers. A flywheel with two strong magnets is used to create a magnetic field around the armature. Finally, an electric control unit, usually at least a breaker and a capacitor, disrupts the electromagnetic field and directs the resulting electric current away from the magneto to where it is needed.
To produce electricity, either the flywheel must rotate or the coil must move between the poles of the magnet, which explains why early telephones had a hand crank. On each rotation, an electromagnetic field is built in the coils on the armature. A cam on the electric unit creates contact with the armature, disrupting the field and creating electrical voltage in the primary coil.
The high tension of the secondary coil compared to the primary coil amplifies the voltage of the current as it is directed to a spark plug. The cam then breaks contact with the armature and the electromagnetic field regenerates for a new pulse of electricity. The entire process takes fractions of a second.
To function properly in an engine, a magneto must be installed so that its firing is timed appropriately to the compression stroke of the pistons. The sparkplug must ignite the fuel/air when it is compressed in the chamber to create combustion and drive the piston downward. In large engines, a distributor is traditionally used to time the electrical charges to each spark plug. A more recent advance is the use of small computers to produce more reliable timing.