Many different pickup designs were tried during the early history of the electric guitar, the most important being the simple 'bar' pickup found on pre-war Gibson ES-series guitars.
This used a magnetic core consisting of a single blade spanning all six strings. The design was very largely superseded by the major pickup innovations of the Fifties, though a few bar models are still produced today, and original Gibson ES pickups are highly prized among jazz players seeking to recreate the sound of Charlie Christian, with whom they are generally associated.
Single Coil vs Humbucker
Almost all pickups produced today can be classified as either 'single coil' or 'humbucker'. Many companies contributed to the evolution of pickup design in the immediate post-war era, most notably Fender and Gibson, and it was soon discovered that a design with one magnetic pole per string, as opposed to a single bar, gave a much more musical response, and the modern single coil pickup was born. The name reflects the fact that, though there are six pole pieces, they are all placed within a single copper coil.
About half of all modern electric guitars still use this design. The most famous designs built around single coil pickups, the Fender Telecaster and Stratocaster, sound the way they do primarily because of their single coil pickups.
One problem inherent in the standard single coil design is its susceptibility to electromagnetic interference. Electrical appliances run on AC (alternating current) and often emit enough electromagnetic radiation to induce a current in a guitar pickup. Single coil pickups are noticeably prone to hum caused by electric lighting, as well as many other devices including guitar amplifiers themselves!
In 1955, Gibson introduced the humbucker or hum-cancelling pickup, which is designed to reject electromagnetic interference by placing two coils in opposite phase. Any noise induced in one coil will also be induced in the other, but the phase relationship of the coils will cancel out the two noise signals. As each coil is placed at a different point along the same vibrating string, the two string signals are not identical, and therefore do not cancel out.
Just as single coils are generally associated with Fender, humbuckers tend to be associated with Gibson guitars, particularly the Les Paul and ES-335, though early Les Pauls actually used Gibson's own P-90 single coil design. In addition to their cleaner signal, humbuckers have a louder output than single coils, and so in the early days of rock, they were much better at producing distortion, there being little in the way of effects pedals to boost the signal before the amp. Most of today's rock and metal players use humbucker-equipped guitars.
The Best of Both - Single Coil/Humbucker Combinations
Though the standard Les Paul, Strat and Tele designs use either one pickup type or the other, many modern guitars combine at least one of each. A favourite of recent years is the so-called 'superstrat' combination, in which the bridge pickup on a Strat-type guitar is replaced with a humbucker ('S-S-H'). This gives Strat players access to the screaming lead sound of a Les Paul bridge pickup, while still maintaining some of their favourite single coil sounds for lighter musical moments. Another popular variant ('H-S-H') adds the sound of a humbucker rhythm pickup at the neck - either of the humbuckers can be mixed with the middle single coil pickup, again retaining access to much of the Strat's traditional sonic palette.