What would happen if you take a magnet to a pickups sound through an amp? Would it kill the pickup or have any affect at all?
A simplified picture of how an electric pickup works is that: the magnet in the pickup induces the string (steel/nickel alloy) to be a magnet itself. When you pluck the, string, the magnetized string moves relative to the coils in the pickup. Moving a magnet near a circuit causes a voltage across the circuit (or equivalently a current for a closed circuit). This voltage is then amplified by the amplifier. The complications involve the fact that the magnitization of the string is higher when the the string is closer to the pickup, and less when further away, so the idea of the string just "being a magnet that moves around" doesn't capture the whole picture. The key point is that moving the string around changes the magnetic field that the coils experience, and thus produce a voltage.
Moving any kind of magnet near the coils will induce a voltage across them.
For a hand held magnet, it would probably be hard to move the magnet fast enough to produce a sound (you'd have to shake it back and forth 10's of times per second), but a strong enough magnet moving fast enough might produce enough voltage to overload an amplifier if it is connected.
Holding the magnet near the pickup while moving string will either enhance (if the magnet is aligned such that it is attracted to the pickup) or degrade the gain in the pickup since the external magnet's field will add to the pickup's field either "in phase" or "out of phase". Horse-shoe style pickups have magnetic elements both above and below the strings to affect the "shape" of the magnetic field near the strings.
if you have look and listen to Mick Taylor live in Japan on youtube you will notice that he is able to carry the field up and down the strings and to the last fret by using his steel which rarely touches the strings unless intentional and he dampens trhe field by lightly touching the strings to change sound/notes just like bob Dylan does whether he plays acoustic or electric..someting to think about.And isn,t mick very very good .Have a listen to Train of 1964 0n a 1968 album..diary of a band when he was 14 with John mayall