This is my 25 cent pickup lecture.
Magnetic pickups consist of a coil of fine wire in a magnetic field.
As the string moves it tends to drag the magnetic field along with it, since the field would rather go through the string than the air. The change in the field produces a current in the coil, and this is fed out to the amplifier. In order to use them you must therefore have steel strings, and the pickup must be far enough from the string termination to allow for some motion. I think the response from this list would be firmly against the use of steel strings. They neither sound nor feel the way a harp is supposed to, and they are very hard on fingernails.