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Iron which is brought near a spiral of copper wire, traversed by an electrical current, becomes magnetic, and then attracts other pieces of iron, or a suitably placed steel magnet.

把铁放在绕着铜线的螺旋线附近，通过电流，它就会变成磁性的，然后吸引其他的铁片，或者适当放置的磁铁。

— Hermann von Helmholtz 亥姆霍兹