Generally the pickup has a permanent magnet and pole pieces that become magnetized and continue to magnetize the string depending on several factors. As the string moves, the movement of the magnetic field through the coil generates and alternating current which can be sent through various circuits and finally the amplifier. The string area that senses the magnetic field can be narrow in a single coil pickup or wider in a humbucking pickup. There are so many variables to the subject such as pole material, diameter, distance from the strings, distance from the bridge or neck, coil shape, width, length and height of a pickup, number of turns, gauge of wire, magnet material shape and strength, string support, string length and string diameter are some of the variances that determine what are a string will sense or pick up.