Magnetic "hum" is mostly created by transformers and power supplies inside various electrical equipment utilizing household AC current. As AC current is put though a coil, it induces a magnetic field around the coil that quickly becomes weaker with distance. When it reaches the guitar pickup, the strength is very slight, but once put through various pedals and amps it becomes much more evident. Using a guitar without humbuckers, a musician would hear a slight but annoying hum at the amp at silent sections of the music piece. Sources of magnetic hum generated in the studio and on stage include, but are not limited to, high power amps, processors, mixers, motors, etc., as well as criss-crossing power lines. Humbuckers dramatically reduce the hum effect compared to single coil pickups.
Humbuckers are great at canceling out low sine wave frequencies (such as those produced by big AC transformers) but poor at canceling out higher frequencies and various harmonics (such as those produced by guitar strings).
A humbucker has two coils with opposing windings and polarities. The string motion induces current in both coils in the same direction. Electromagnetic interference, on the other hand, induces current in opposing directions in each coil because of the reversed winding and polarity. When the signals from both pickups are summed together, the noise is cancelled due to destructive interference, while the actual signal is increased due to constructive interference, thus dramatically improving the signal-to-noise ratio. This technique is called common-mode rejection by electrical engineers, and is also used in balanced lines in audio recording.
Using two coils also changes the tone of the pickup. The humbucking pickup produces a "warm" and "fat" tone that has been popularly associated with Les Pauls and SGs, in contrast to the "bright" or "clear" tone of the single coil pickups that are typically used on Fender guitars such as the Stratocaster and Telecaster. However, there are humbucking pickups that have a bright tone, similar to that of single-coil pickups.
It is a common misconception that because the coils are at slightly different positions along the string some of the higher-frequency harmonics are diminished or cancelled out, thus producing this warmer sound. This is only true if the pickup's coils are in reversed phase, (i.e., a phase switch on Vol/tone). The main reason humbuckers sound different is that the two coils resonate at different frequencies causing a broad resonant peak in frequency response, a characteristic of the original Gibson humbuckers, and because any two pickups wired in series will attenuate some of the higher frequencies due to the summed impedance. In fact, not all humbuckers have two separate full-size coils with two separate rows of magnetic pole pieces facing the strings; see the paragraphs below about "stacked" and "rail" designs. The "stack" and "rail" pickups can still produce the "warm" and "fat" tone, in spite of sensing only a small section of the string, just like single-coil pickups. Humbucker pickups are also available in single coil size.
Usually, those who prefer the brighter sound of single-coil pickups have to simply live with the extra hum and buzz in order to get the tone they prefer, although technologies designed to preserve the tones exist.
Although the Fender Stratocaster-style single coil is by far the most frequently found pickup in a single coil size, humbuckers are available for most single-coil guitars. Fender produces several variations in their Telecaster, Jaguar, Jazzmaster, and Mustang guitars.
The same type of rails can also be found in a normal-size humbucker, however. Heavy metal guitarist Dimebag Darrell made heavy use of this type of pickup wired in the bridge position. These tend to also sound fuller and have a higher gain and attack than the single coil-size version.
While the original humbucker remains the most common noise-reducing pickup design, inventors have tried many other approaches to reducing noise in guitar pickups.
Many instruments will use a combination of separate single coil pickups in a hum reducing configuration, where the magnetic polarity is different and the coils are electrically reversed. This arrangement is similar to that of a humbucking pickup and noise is effectively reduced. Some examples of this are the Fender Jazz Bass, introduced in 1960, which has used a pair of single coil pickups, one near the bridge and another one about half way between the bridge and the neck, and many Stratocaster style guitars, which often have 3 pickups with the middle one reversed electrically and magnetically. The (usually) 5-way selector switch allows 2 humbucking settings, where the reversed middle pickup is used in parallel with either the bridge or neck pickup.
In 1957, Fender introduced a split pickup to its Precision Bass, which was wired in humbucking fashion, with one coil serving the E and A strings, the other the D and G strings. Both coils pick up the same noise, but since each string is only served by one coil, a single-coil sound is provided. The concept of this later expanded to G&L's Z-coil pickup, which is used for standard guitars.
In 1985, Lace Music Products introduced the Lace Sensor pickups, which utilize a proprietary hum-screening technique to eliminate noise while preserving single-coil tone.
In 1996, Kinman Guitar Electrix introduced pickups based on a differential coil technology, essentially a stacked humbucker where the lower pickup coil functions solely as a noise sensing coil, while only the upper pickup coil is able to sense the string vibrations.