How Humbuckers Work
In any magnetic pickup, a vibrating guitar string, magnetized by a fixed magnet within the pickup induces an alternating voltage across its coil(s). However, wire coils also make excellent antennae and are therefore sensitive to electromagnetic interference caused by alternating magnetic fields from mains wiring (mains hum) and electrical appliances like transformers, motors, and computer screens. Guitar pickups reproduce this noise, which can be quite audible, sounding like a constant hum or buzz.
The direction of a voltage induced across a coil by the moving string depends on both the coil winding direction and the direction of the fixed magnets. A humbucker has two coils wound in opposite directions, one clockwise and the other counterclockwise. The magnets in the two coils are arranged in opposite directions so that the string motion induces voltages across both coils in the same direction. Electromagnetic interference, on the other hand, induces voltages in opposing directions across the coils because it is only sensitive to the winding direction. When the signals from both coils are summed together, usually by connecting the coils in series, the two noise voltages cancel, while the signal voltages add thus dramatically improving the signal-to-noise ratio. The technique has something in common with what electrical engineers call common-mode rejection and is also found in balanced lines in audio recording.
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