Magnets in Guitar

While acoustic guitars use resonance to amplify their sound, electric guitars rely on their electromagnetism and magnetic flux.  This is a picture of (low-quality) magnetic pickups from a Squire Stratocaster:

The pickups are made of a permanent magnet of some iron alloy wound a few thousand times with copper wire.  When the metal strings above the magnets are strummed, they produce a constantly changing magnetic flux to induce an alternating current in the copper wire.  The magnitude of the current depends on how much the strings are vibrating (how hard they are strummed).  Stronger magnets could also induce a higher voltage for more sound, but the copper wire feeds into an amplifier anyway, where the alternating current is converted into sound energy by another set of magnets.

Unfortunately, the pickups also are affected by the interference from surrounding electric and magnetic fields. Such interference is generated by almost any electric current in the area and can produce a loud and annoying humming sound.  This is why the humbucker was invented. This is a picture of a humbucker (sorry, I had to use Google Images for this):

The humbucker consists of two coils wrapped in reverse around the magnets.  The magnets are also positioned to alternate polarities for each winding between their neighbors.  When ambient electric and magnetic fields would normally induce an interfering current, the reversal of the polarities causes the sinusoidal waves to be produced in equal magnitude but 180 degrees out of phase, effectively canceling each other out (similar to the degenerative interference found in noise-cancelling headphones).