The purpose of the pickup is to create an electric current from the mechanical vibration of the steel guitar strings.

A coil is wound around a permanent magnet, and the string passes through the magnetic field that is generated. The disturbance from the oscillation of the string changes the flux, or strength, of the magnetic field. This change in flux then creates an electric current that moves through the coil of wire and finally to the amplifier.

diagram of the bobbin under each string (string shown running into screen)

The small magnet and coil of wire are called a Bobbin. There is a bobbin under each string. This way the magnetic fields are seperately concentrated under each string. This eliminates extra unwanted noise and distortion that occurs when simply amplifying and acoustic guitar.

diagram of all 6 bobbins underneath the strings

The actual working of the bobbin is quite intricate. The movement of the steel ferromagnetic string inside the magnetic field changes the flux in the bobbin. This change in flux created by the AC current running through the coil of wire is opposite that of the change in flux created by the movement of the string in the magnetic field. The actual movement of the string is what creates the Alternating current rather than a Direct current. The string moves towards and away from the magnet, decreasing and increasing the flux of the magnetic field.

The actual movement of the guitar string isnt simply in the vertical direction (towards and away from the bobbin). The string actually rotates in an elliptical pattern. The string therefore moves both horizontally and vertically. The vertical nature is that the string moves away and towards the bobbin, increasing and decreasing the flux of the magnet. The horizontal nature is that it moves closer to the center of the magnet and further away, also increasing and decreasing the flux of the magnetic field. This results in increasing and decreasing the current two times, which when amplified is twice the frequency of a normal sound wave. The produced signal therefore is much more complex than that of a simple amplified acoustic guitar.