Other attributes
Most electric guitars and electric basses use magnetic pickups. Acoustic guitars, upright basses and fiddles often use a piezoelectric pickup.
A typical magnetic pickup is a transducer (specifically a variable reluctance sensor) that consists of one or more permanent magnets (usually alnico or ferrite) wrapped with a coil of several thousand turns of fine enameled copper wire. The magnet creates a magnetic field which is focused by the pickup's pole piece or pieces. The permanent magnet in the pickup magnetizes the guitar string above it. This causes the string to generate a magnetic field which is in alignment with that of the permanent magnet. When the string is plucked, the magnetic field around it moves up and down with the string. This moving magnetic field induces a current in the coil of the pickup as described by Faraday's law of induction. Typical output might be 100–300 millivolts.
The pickup is connected with a patch cable to an amplifier, which amplifies the signal to a sufficient magnitude of power to drive a loudspeaker (which might require tens of volts). A pickup can also be connected to recording equipment via a patch cable.
The pickup is most often mounted on the body of the instrument, but can be attached to the bridge, neck or pickguard. The pickups vary in power, and they vary in style. Some pickups can be single coil, while other pickups can be double coil humbuckers. The pickup is one of the most important aspects to distinguishing an electric guitar's sound. Most guitar models have a distinction in pickups, which act as a new selling point for guitar companies.
Pickups have magnetic polepieces (one or two for each string, with the notable exceptions of rail and lipstick tube pickups), approximately centered on each string. (The standard pickups on the Fender Jazz Bass and Precision Bass have two polepieces per string, to either side of each string.)
On most guitars, the strings are not fully parallel: they converge at the nut and diverge at the bridge. Thus, bridge, neck and middle pickups usually have different polepiece spacings on the same guitar.
There are several standards on pickup sizes and string spacing between the poles. Spacing is measured either as a distance between 1st to 6th polepieces' centers (this is also called "E-to-E" spacing), or as a distance between adjacent polepieces' centers.
Some high-output pickups employ very strong magnets, thus creating more flux and thereby more output. This can be detrimental to the final sound because the magnet's pull on the strings (called string capture) can cause problems with intonation as well as damp the strings and reduce sustain.
Other high-output pickups have more turns of wire to increase the voltage generated by the string's movement. However, this also increases the pickup's output resistance/impedance, which can affect high frequencies if the pickup is not isolated by a buffer amplifier or a DI unit.
