### Question

How electric guitar pick-ups work?

Guitar pickups work by the principles of magnetic induction. The quarter inch plug which runs from the amplifier to the guitar is electrically wired to the pickups. The pickups themselves (each circular metal disc) are composed of small electromagnets, which are small magnets with a coil of wire wrapped around it. These small magnets produce a field in their immediate vicinity, and thus induce a north and south pole on the actual guitar string. When the string is struck by the guitarist, it begins to oscillate. These oscillations affect the field in the vicinity of the pickup magnet, it induces a change in the flux of the magnetic field. Thus, by Faraday's law, which states that a changing magnetic flux induces an electrical field, hence a current is established in the wires which wrap the pickup magnet. As the string continues to oscillate over the pickup magnet, the flux continually changes (increases and decreases depending upon the location of the string and the intensity in which it was struck).

The components of a guitar pickup consist of a coil of wire known as a bobbin and a permanent magnet situated underneath each guitar string.

There is a single pole (or pole piece) which extends upwards through the centre of the bobbin. The guitar string is positioned directly over the pole piece. There is a bobbin and a pole piece under each of these strings however it is quite often that all of the pickups share one magnet and have separate pole pieces.

These guitar strings are primarily steel, and the heavier strings would be nickel wound. On a normal 6 string electric guitar, the heaviest three are wound and the lightest three are single strands. As these ferromagnetic substances (the guitar strings) move within the magnetic field of the permanent magnet, it causes the flux through the bobbin to change. As this bobbin is such an excellent conductor, the change in the magnetic flux is opposed in the bobbin by the induction of an alternating current. The change in magnetic field that is created from the alternating current that is opposite to that of the change in the magnetic field in the bobbin. This is created because of a principle known as Lenzï¿½s law. There is a alternating current, instead of a direct current within the bobbin because of the motion of the string. It moves towards and away from the pole piece of the pickup like the way the voltage of an alternating current increases and decreases. This movement by the string is the most significant fact. The string moves towards the centre, and then away, and back toward the centre repeatedly. Within these cycles, the flux switches back and forth twice (causing an increase and decrease). The musical importance of this, is that the signal which is produced has a frequency twice as high as in the case of the vertical motion of the string. So it can be noted that the sound produced by a guitar is not a simple, first harmonic wave formation, it is much more of a complicated interaction.
Answered by: Robert Hooks, Sixth Form Student, Bay House Sixth Form, Gosport

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