A magnetic field will induce an electric field of opposing direction, and same magnitude in any conductor placed near it - this is the principle mentioned above, also known as Lenz's law. Well, it goes something like that anyway
. That's the reason for using a nice non magnetic material that is a good conductor (of electricity), then grounding it. A magnetic metal (ferrous, like iron) will merely be magnetised relative to the magnet, and extend the field.
I think, anyway...