Meissner effect
/ MICE-ner ih-FEKT /
Hold a small magnet over an ordinary metal and nothing dramatic happens; the field threads straight through the metal as if it were not there. But cool a piece of superconductor below its critical temperature near a magnet, and the magnet can hover in mid-air, held up by the material refusing to let the field inside. This levitation is the most photographed trick in all of superconductivity.
The Meissner effect is the active expulsion of magnetic field from the interior of a superconductor. The moment the material turns superconducting, tiny screening currents spring up on its surface and circulate without resistance; these currents create their own magnetic field that exactly cancels the applied one inside the body of the material. Crucially, the field is pushed out even if it was already there before cooling — the superconductor does not merely refuse new fields, it ejects existing ones.
This matters because it proves superconductivity is a true thermodynamic state of matter, reached the same way no matter what order you cool and apply the field — not just a leftover of zero resistance. The common misconception is that the magnet floats because 'like poles repel'; in fact it floats because the superconductor reshapes the entire field to keep its insides field-free. A strong enough field overwhelms the screening currents and destroys superconductivity altogether.
Place a small magnet on a chip of superconductor sitting in liquid nitrogen vapour, and the magnet lifts off and floats steadily above the surface — a tabletop demonstration seen in classrooms worldwide.
A magnet levitating above a chilled superconductor — the field is locked out of the material's interior.
In type-II superconductors the expulsion is incomplete: above a first critical field, the material lets the field in as thin quantized tubes called vortices while staying superconducting around them, so the levitation can also pin the magnet firmly in place.