Stringed instruments such as the electric guitar, electric bass guitar and others, are often equipped with electromagnetic pickups to capture the vibrations from the strings and convert them to an electronic signal which can be amplified or recorded.
Electromagnetic pickups use the principal of induction. The pickup consists of a permanent magnet wrapped with many turns of fine wire. The pickup is mounted on the body of the instrument, in close proximity to the strings. When the instrument's strings vibrate in the magnetic field of the permanent magnet, a current is induced in the wire wrapped around the magnet. The signal created is then carried to amplification or recording equipment via a cable.
One problem with electromagnetic pickups is that--along with the musical signal--they also pick up a fair amount of 60 Hz hum. A variant of the pickup is the "humbucking" pickup, first developed by the Gibson Company. A humbucking pickup is nothing more than two standard pickups wired together in series. However, the magnets of the two pickups are reversed in polarity, and the windings are also reversed. Thus, any hum or other extraneous noise that is picked up is immediately cancelled out, while the musical signal is reinforced.
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