The typical electric guitar pickup is one of those things placed underneath the strings on the body of the guitar, usually close to the place where the player strums or picks the strings - usually there are two or three of them on a guitar. This Entry will focus on magnetic pickups, which represent the vast majority of guitar pickups. Piezoelectric pickups, resonance pickups or optical pickups will be briefly mentioned in the last part of this Entry1. As the name already says, the guitar pickups collect, gather or 'pick up' the oscillation of the guitar string. They are sometimes also called transducers because they transform mechanical oscillations into voltage oscillations, which can be sent through wires, processed, amplified, recorded etc.
To understand properly how a pickup works one will need to know a little about electromagnetism, more specifically about Magnetic Induction and its Relation to Magnetism. The essence of all that 19th Century physical mumbo-jumbo is the following: If you have a wire near a magnet, any change of the magnetic flux will induce a voltage in the wire (and vice-versa). This very principle can be used to build electric motors, brakes, generators, metal detectors and guitar pickups2.
A guitar pickup basically consists of a magnet with a wire wrapped around it, a coil. When a piece of metal, such as the guitar string right above it, moves, it changes the magnetic flux of the magnet, and this induces a voltage in the coiled wire around the magnet (this is also the reason why nylon strings cannot be used on electric guitars3). The induced voltage will oscillate exactly in the same way as the string above it oscillates mechanically - for this reason the pickup translates the mechanical oscillation, the sound, into a voltage oscillation - which is easier to transmit through wires.
The sensitivity of a pickup is mainly described by the number of coil windings around the magnet, the strength of the magnet and the distance to the guitar string. Usually a compromise between these parameters must be found. High magnetic fields caused by more powerful magnets, or by placing the magnet closer to the guitar string, dampen the guitar string's oscillation - the pickup starts acting as an electromagnetic brake. A high number of coil windings will make the pickup too sensitive, so that any other disturbing fields get captured by the pickup, also, the increased voltage could lead to a distorted sound.
The entire guitar pickup, that visible thing under the guitar strings, is actually a casing containing an array of magnetic poles - usually one per string - which are connected to a master magnet that has the coil wrapped around it. Sometimes the individual poles are visible as little circular bumps under each guitar string.
The author of this Entry has prepared a schematic graphic to illustrate all this:
Two Guitar Pickup Designs: Single Coil and Humbucker
There are basically two guitar pickup designs: The single-coil pickups4 and the double-coil pickups, better-known as humbuckers5. The difference, from the physical perspective, will be discussed in the following paragraphs.
The single-coil pickup is very sensitive to any change in the flux of the magnet. Such a change is induced primarily by the string swinging back and forth close to the magnet. However, there is a number of secondary sources for a change in the magnetic flux: Alternating currents in the electric wires of equipment nearby (60-50 Hz plus overtones), magnetic fields such as the ones caused by transformers in the housing of fluorescent light bulbs, or from the loudspeakers of an amplifier. Altogether these secondary sources generate a significant amount of electromagnetic noise, which is also sensed by the pickup and sent to the amplifier. The result is a 'humming' noise that is sometimes difficult to suppress.
The humbucker circumvents this problem, by mixing the fluctuations captured by two counterwound single-coil pickups in series: The noise fluctuations captured by the first and the second pickups are basically the same, and because of the inverted direction of the coil winding, the voltages have an opposite sign and will cancel out to a great extent. The price to pay for the noise-reduction is that a lot of the higher harmonics of the guitar string sound also gets cancelled out (some people say that the sound of the humbucker loses 'attack', that they are more 'deaf' or that the sound becomes 'warmer').
It is not possible to tell from a physical perspective which pickup performs better. Both pickup designs have advantages and disadvantages, and many guitarists use both types of pickup on their guitars.
Other types of pickup
One big limitation of the magnetic type of pickup described above is that it will only work with steel strings, which is a big bummer for all those folks trying to amplify classical guitars with nylon strings. Another limitation is the sensitivity towards other magnetic fields causing all kinds of (desired and undesired) distortions, feedbacks and noises.
Fortunately for some people, there are other ways to transform mechanical movement into oscillations of voltage. One of the ways is to use the piezoelectric effect, in which the compression of a crystal generates a voltage. Such a pickup is sometimes called contact pickup, it is attached directly to the body of the guitar and captures the oscillation of the body - note, not directly that of the string! There are presently a few very good guitar pickups that work according to this mechanism, the sound is said to be very clean, to have a high fidelity. Another new type of pickup works optically; the reflex or the shadow produced by a string that is illuminated by a strong LED is captured by an array of photodiodes and converted into a voltage (similar to those LED computer mice). Good results are allegedly being obtained with this kind of pickup for bass guitars (where the oscillations of the strings are slower).