Junkyard Planet: Travels in the Billion-Dollar Trash Trade

by Adam Minter

At the end of those conveyors something curious happens. The metal fragments don’t fall so much as they seem to leap into the air and deposit themselves into chutes and bins they wouldn’t reach if they just relied on the momentum of the conveyors. Meanwhile, the remaining trash—the rubber and the plastic—falls away harmlessly. The device that makes this curious separation possible is called an eddy current. Thomas Edison developed and patented the first one in the 1880s, and it’s most assuredly the case that he didn’t foresee its application to shredded automobiles. Still, he’d immediately recognize what’s going on. The spinning rotor that drives the conveyor is packed with magnets. As the magnets spin, they create a kind of magnetic field around the nonmagnetic metal fragments (such as aluminum and copper, which are not ordinarily magnetic) that approach the rotor. When those fragments reach the rotor, the field repels them just as two magnets repel each other when similar poles are placed end to end. The practical result is this: pieces of metal are ejected from the remaining mix of conveyed trash.