We used mag brooms or magentic rakes a lot around aircraft parking areas. They are permamnent magnets with a simple release system. The magnet is covered by an aluminum shield so when th magnet is raised up the debris falls away from the shield. Super simple, no wires, coils, headaches. We had them on hand held units up to truck mounted units.
Check out these links:
http://www.magnetsinc.com/behind_magnets/behind.htm
http://www.thompsonmagnetics.com/ma...1-general/24-deluxe-magnetic-push-brooms
http://www.shieldscompany.com/brooms-rakes.html

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P