While technically not an EE question, until StackExchange adds a regular Engineering section, EE was the closest fit.
On small gas engines like those in weed wackers and lawn mowers, where do the spark plugs get the current necessary to create sparks large enough to set fire to the gas fumes?
There are other possibilities, but the usual low-tech approach is a magneto.
A permanent magnet is mounted in the flywheel, so that at approximately the right moment, it flies past a pickup coil at high speed, inducing a current in a circuit including that coil. Then, at exactly the right moment, a cam presses a microswitch to open that circuit, interrupting the current.
The resulting dI/dT creates a large V in the primary of a transformer, which is transformed in the secondary to sufficient voltage to make a spark.
As Dave says,there may not be a transformer, just the coil : or the coil itself may be the primary of the transformer, for economy.
Only in the vaguest sense is this the same thing as an alternator : the latter aims to deliver useful quantities of relatively steady power at an approximately constant voltage; the magneto only produces one short pulse per revolution, (sometimes every second revolution in a 4 stroke - the contact breaker can be driven off the camshaft) with no more power than the spark plug needs.
Small gas engines have a powerful permanent magnet built into the flywheel, which swings past a coil on each revolution of the crankshaft. This is timed to occur roughly about the time that a spark is needed, and a set of "points" (a switch driven by a cam on the crankshaft) provides the precise timing, opening the circuit and causing the coil to produce a high voltage pulse to the spark plug.
Essentially, the magnet "charges" the coil with energy directly, rather than using the 12V DC bus that you have on larger engines.
Re: the "wasted" spark of the magneto, mounting the magnet on the flywheel for four stroke engines does produce a wasted spark. This is of no real consequence unless a charge remains unignited from a previous compression stroke. It is just far simpler and more accurate to use the flywheel rather than a reduction gear (cam say). The velocity is greater, there is less lash, and the wiring harness is less cumbersome.
Even non-magneto four stroke engines as still fitted to some motorcycles will have a wasted spark, just because it is simpler and more reliable to run pickups from the flywheel.
