A magnetic guitar pickup turns the movement of a suitable metal string into an electrical signal. Its magnets establish a magnetic field around the string. As the string vibrates, the changing magnetic flux through the pickup’s coil induces a voltage. That signal goes through the guitar’s circuit and cable to the next device.
This describes magnetic pickups. Acoustic guitars may instead use piezoelectric pickups, microphones or other systems. A nylon string does not work with an ordinary magnetic pickup in the same way as a ferromagnetic electric-guitar string.
Why pickup position changes the sound
A plucked string vibrates in a mixture of modes, heard as a fundamental and overtones. Those modes have different patterns of motion along the string. A pickup senses a region of that motion, so moving the sensing position changes the mixture in its output.
Near the bridge there is less string movement, and the sound is commonly brighter. A neck pickup commonly sounds rounder or less bright. Pickup construction, height, the note being played and the rest of the signal chain also affect the result. A particular harmonic is not always strongest at one pickup: fretting changes the vibrating length and its pattern of motion.
Manufacturers often design bridge and neck versions differently to balance their outputs. Position alone therefore does not tell you which pickup will be loudest. See pickup-height adjustment before changing the height to compensate.
Single coils and humbuckers
A conventional single-coil pickup uses one sensing coil. It can also pick up electrical interference, including mains hum. A humbucker uses two coils arranged so that some common interference cancels while the string signal combines.
Single coils are often described as bright and clear; many humbuckers sound fuller and have higher output. These are tendencies. There are low-output humbuckers, powerful single coils and hum-cancelling pickups in narrow housings. Neither type restricts you to a particular genre, and a humbucker does not remove every possible source of noise.
Some humbuckers offer coil splitting, which turns off one coil. That is different from a coil tap, which accesses a different point in a coil’s winding. A switch or push-pull control may offer these options, but only if the pickup and circuit support them.
Passive and active electronics
A passive magnetic pickup produces a signal without a powered onboard preamp. An active pickup system includes powered electronics and commonly needs a battery. A guitar can also combine passive pickups with a powered preamp, so check the system rather than identifying it by appearance.
The amplifier’s input and preamp shape or amplify the incoming signal; a power amplifier drives the speaker. Their exact circuit and order of processing vary. There is no requirement that every guitar preamp use a particular transistor type.
Compare your own pickups
Use a clean sound at a comfortable level and play the same phrase through each selector position. Keep your picking position and attack similar. Then try a small change to the guitar’s tone control. The controls lesson explains a common selector arrangement.
For more detail, see Seymour Duncan’s explanations of magnetic sensing, pickup types and neck and bridge output. The harmonics lesson introduces another way to hear a string’s overtones.