Pickup
From GuitarWiki
The pickup is mainly found on electric guitars, while some acoustic, hollowbody, and other string instruments contain them as well. There are two kinds of pickups, passive and active, passive pickups "pick up" the vibration of the strings, via 6 (or however many strings are present) pole-pieces, that are wrapped thousands of time with hair-thin wire, usually between 6- and 9-thousand times per coil, and contain a magnet underneath this apparatus, which gives the power to the whole thing. These magnets are usually AlNiCo (aluminum-nickel-cobalt alloy) I II III IV or V, ceramic, or neodymium on rare occasions. Besides pole-pieces for each string, there are also pickups that utilize a rail pole-piece, so that the sound isn't limited by the small radius of standard pole-pieces. As well as have f-spaced (for floyd rose equipped guitars) because of the wider strings spread, but really there is not much difference tone-wise. Active pickups use a 9 volt battery to increase their range, and generally have much less coil windings, and much higher output than their passive counterparts.
Single-coil pickups will pick up "noise" from other electric devices, commonly heard as a faint droning 50 or 60 Hz tone, but have a more desireable, cleaner tone, and sound somewhat glassy, giving them a good sound for blues and jazz and country-esque music, but are used in almost every genre of music that uses guitar.
In 1955, Seth Lover developed the humbucker, which is essentially 2 single coils wired together, in series. what this does is not only cancel out the 60-cycle hum of single coils, but it adds a more fuller, grisly tone to the sound. Humbuckers are used in any genre, as with single coils, but are mainly found in rock through jazz, and more commonly in all heavy metal subgenres.
Two single-coils may also be wired in parallel, which gives the same hum-canceling properties of a humbucker, but retains the single coil tone, and has less output. this was discovered by early guitarists such as Jimi Hendrix, by making the early 3-way blade-toggle of a strat, stick in between the first & second, and second & third positions, which allowed both the middle and bridge/neck pickups, respectively, to be on at the same time, in parallel.
As far as tone goes, generally, the more coil windings you have, the more output (gain) you will receive. The higher-output humbuckers led to heavier sounding music, and eventually allowed clipping (distortion) and fuzz/overdrive into music, which was used by many artists throughout the 60's.
