Meissner effect

The Meissner effect is the expulsion of the magnetic field from a material as it passes into the superconducting state. Discovered in 1933 by Walther Meissner and Robert Ochsenfeld, it involves the superconductor generating a counter-field with persistent surface currents that cancels the external magnetic field, so that it behaves as a perfect diamagnet. This property defines superconductivity as a distinct state and not merely a matter of zero resistance.