A pickup device acts as a detector and captures mechanical vibrations (usually from suitably equipped stringed instruments (such as the electric guitar, electric bass guitar and violin)) and converts them to an electronic signal which can be amplified and recorded.
Electromagnetic pickups use the principle of electromagnetic 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, close to the strings. When the instrument's strings vibrate in the magnetic field of the permanent magnet, they alter the magnetic flux linking the turns of wire. This changing flux induces a voltage in the winding. The signal created is then carried to amplification or recording equipment via a cable (or by radio transmission).
One problem with electromagnetic pickups is that - along with the musical signal - they also pick up mains hum. Mains hum consists of a fundamental signal at a nominal 50 or 60 Hz, depending on where you are in the world, and usually some harmonic content. The changing magnetic flux caused by the mains current links with the windings of the pickup, inducing a voltage by transformer action.
To overcome this effect, the "humbucking" pickup was developed, originally by the Gibson company. A humbucking pickup comprises 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 common mode extraneous noise that is picked up is cancelled out, while the musical signal is reinforced.
The turns of wire in close proximity to each other have an equivalent self-capacitance which, when added to any cable capacitance present, resonates with the inductance of the winding. This resonance can accentuate certain frequencies, giving the pickup a characteristic tonal quality. The more turns of wire in the winding, the higher the output voltage but the lower the resonant frequency. Thus a design compromise must be reached.
Electromagnetic pickups are usually designed to feed a high input impedance, typically a megohm or more. A low impedance load reduces the high-frequency response of the pickup because the inductance and input resistance form a low-pass filter.
More recently, semi-acoustic and acoustic guitars have been fitted with piezoelectric pickups instead of, or in addition to, magnetic pickups. These have a very different sound which some prefer, and also have the advantage of not picking up unwanted magnetic fields, such as mains hum and feedback from hearing-aid loops.