“If you try to change the magnetic field through a metal, a current will be induced in that metal and will try to oppose the change in that magnetic field,” Greene says. “When the material goes superconducting and the magnetic field is expelled, that’s a change in the magnetic field. Normally, that current would die out because the metal has resistance … but if you have a superconductor, then there’s no resistance anymore, so the current stays there forever.”
This induced diamagnetism—magnetic field repulsion—is why levitation is a tell-tale sign of a superconductor, but if you induce a strong enough magnetic field (a specific threshold known as “critical field”), the material will stop being superconductive. However, it’s worth noting that other effects can also cause levitation (which is why you can buy a variety of levitating desk toys on Amazon).
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