Electric Guitar Pickups
Electric guitars depend upon the electrical pickup of the string vibration, in contrast to acoustic guitars. The original pickups consisted of magnets wrapped with fine wire pickup coils which interacted with strings made of steel or nickel which are ferromagnetic. The motion of the steel string close to the magnetic core of the coil changed the magnetic flux in the coil, generating a varying voltage by Faraday's law. This voltage could be amplified to produce a sound from loudspeakers. Such pickups were used on the early Gibson guitars. Arrays of these single-coil pickups were used on the Fender Telecaster and Stratocaster solid body electric guitars.
Since single-coil pickups were prone to pick up stray 60Hz "hum" voltages from 60-Hz electrical circuits around them, two counter-wound coils were soon used by the Gibson company and dubbed "humbucking" pickups.
The piezoelectric effect can also be used for guitar pickups; such pickups are often used for acoustic guitars. They typically have preamps associated with them to boost and equalize the signal.
Optical pickups came into use in the 1990s for bass guitars. Consisting of an infrared LED and detector pair, these pickups can be thought of as imaging the shadow of the string as it passed over the detector. These detectors have been built onto the bridge of the guitar.