Meissner Effect

A bar magnet above a superconductor. Below the critical temperature the superconductor expels the magnetic field from its interior (the Meissner effect), modelled exactly by the image dipole that makes the normal field vanish at the surface, and the resulting repulsion levitates the magnet. The field lines, integrated through that real field, curve cleanly around the cold sample and thread straight through a warm one; the thin glowing skin is the London penetration layer, over which any leaked field dies as exp(-d/lambda_L). Warm it past Tc, or push the applied field past the critical field, and superconductivity quenches: the screening vanishes and the magnet drops.