Cool certain materials below a critical temperature and their electrical resistance drops to zero. Heike Kamerlingh Onnes found this in mercury in 1911. For twenty-two years it was assumed to be the whole phenomenon.
The second effect
In 1933, Walther Meissner and Robert Ochsenfeld found that a superconductor also expels magnetic field from its interior. Not merely resists a change in field — expels the field that is already there, at the moment it is cooled through the transition.
This distinction matters. A hypothetical perfect conductor would trap whatever field was present when it became perfect, because changing the field would induce currents opposing the change. A superconductor pushes the field out regardless of its history. The final state does not depend on the order in which you cooled it and applied the field.
That makes superconductivity a distinct thermodynamic phase, not just an extreme limit of ordinary conduction.
Why magnets levitate
The expelled field is deflected around the material, and the resulting distortion produces a repulsive force. In type-II superconductors, which allow field to penetrate in discrete quantised tubes, those tubes are pinned in place by defects in the material. The magnet is then locked at a fixed distance and orientation — it can hang below the superconductor as readily as float above it.
Where it is used
Not levitating trains, mostly. The dominant application is high-field electromagnets: every MRI scanner in a hospital, the beam-steering magnets in particle accelerators, and the confinement coils of experimental fusion reactors all rely on superconducting windings to carry large currents without dissipating heat.
