What does an electric guitar pickup actually do?
There is nothing mysterious about the basic principles of the electric guitar pickup. The phenomenon at work also makes many other devices possible, from the bicycle dynamo to the microphone. The principle involved is known as induction.
Any device based on induction exploits the relationship between two basic physical forces - electricity and magnetism. A guitar pickup is essentially an electromagnet consisting of a coil of copper wire wound around one or more magnetic cores usually one per string.
The most common applications of electromagnets involve sending an electric current through the coil in order to magnetise the core, which then attracts or repels some nearby metal object in order to do something useful such as moving a loudspeaker cone, or turning an electric motor. A magnetic pickup works exactly like this, but in reverse - the movement of a metal object near the core of the magnet generates or induces a current in the coil. The nearby object in question is of course the guitar string, which is why it has to contain a magnetic metal such as steel or nickel, and why strings made from other materials (for example plain nylon) do not work on electric guitars. You may have wondered why guitars dont pick up and amplify other sounds, even if you scream directly into the pickups the answer is that the air may be moving, but this does not disturb the magnetic field.
A slow-motion film of a vibrating guitar string would show it moving from one side of its rest position, all the way to the other side and back again, many times every second. This alternating motion induces a corresponding alternating electric current in the pickup. The frequency of vibration corresponds directly to the frequency of the induced current. For example, the note A (top E string, 5th fret) has a fundamental frequency of 440 cycles per second far too fast to observe with the naked eye and induces an alternating current with the same frequency - 440Hz. This current is electrically very weak if fed directly into a loudspeaker, the cone would not move enough to produce any sound at all it must therefore be boosted by an amplifier.