I see your logic Hank, but the blade wont generate the same magnetic field as a slug unless it was treated to localise the magnetic permeability. Otherwise the field lines from the magentic poles will travel horziontally through the steel and back round to the free pole. None of the field will escape the steel vertically and around the string other then a relatively small feild from the coil itself. The main purpose of the coil as I see it is to modulate the magnetic field from the slug/magnet to return energy to the string. But this can't happen if the field never leaves the blade. I'm no expert, but I did do some transformer design in the very distant past...