A coil tap is a wiring feature found on some electrical transformers, inductors and coil pickups, all of which are sets of wire coils. The coil tap(s) are points in a wire coil where a conductive patch has been exposed (usually on a loop of wire that extends out of the main coil body).
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When the coil taps are disconnected, the coil operates as normal (see transformer). When a coil tap is connected to one end of the coil (or the end disconnected and reconnected to the tap), the section of coil between the tap and its connected end is bypassed - effectively reducing the number of turns in the coil.
Musical Instruments
Single coil magnetic pickups found in electric guitars can be coil tapped to reduce the number of windings around the magnet. A tapped single coil pickup typically contains three wires: a ground, an output, and a tapped output - with the two outputs generally wired to a switch on the guitar. The guitarist can then choose between the loud, punchy, midrange-heavy sound of the entire coil, or 'tap' into the inner coil for a quieter, yet bright vintage tone with a more clear and detailed high end.
Many guitarists mistakenly refer to humbucker coil splits as a coil taps, however, this is incorrect: a coil split is a humbucker with one coil removed from the wiring, leaving a single coil. Because of the ubiquity of this error, and the rareness of coil taps in general, it is difficult to find tappable single coil pickups. However, pickup manufacturer Seymour Duncan offers tapped versions of many of their Telecaster and Stratocaster pickups on their website at a slightly greater cost than a standard version.
A Few Tapped Coil Pickup Manufacturers:
Transformers
In a transformer, coil taps are often used on both the input and output coils.
Inductors
Coil taps on inductors are quite rare, but are sometimes used for band switching in tuning circuits.
Coil Pickups
Coil pickups used with measuring instruments often feature coil taps to compensate for band rejection or equipment input impedance.
Telephone tapping
Coil taps can be used as a rudimentary method for recording telephone conversations. See Telephone tapping.