Glossary

The Meissner Effect is the defining feature of superconductivity, describing the phenomenon in which a material completely expels magnetic fields from its interior when it transitions into the superconducting state—that is, when it’s cooled below its critical temperature (Tc).

In ordinary conductive materials (metals like copper or aluminum), magnetic field lines can penetrate the material normally, and cooling them simply reduces electrical resistance without affecting the magnetic field. But in a superconductor, something far more dramatic happens. As the material cools below Tc, it not only loses all electrical resistance—it also actively repels magnetic fields from its interior. This occurs because the superconductor forms surface currents that generate magnetic fields exactly opposing the external field, causing the total internal magnetic field to drop to zero.

This phenomenon was discovered in 1933 by Walther Meissner and Robert Ochsenfeld, who found that magnetic flux lines were expelled from a sample of superconducting tin when it was cooled below its critical temperature in an existing magnetic field. Their observation proved that superconductivity is not merely perfect conductivity (which would allow magnetic fields to remain frozen inside) but rather a distinct thermodynamic phase with unique quantum mechanical properties.

The Meissner Effect is often illustrated through magnetic levitation. When a small magnet is placed above a superconducting material cooled below Tc, the magnetic field lines cannot penetrate the superconductor, and the resulting repulsion creates a stable lifting force that makes the magnet float or levitate. This visible effect is a direct consequence of the perfect diamagnetism of superconductors—that is, their ability to completely cancel internal magnetic fields.

Mathematically, the Meissner Effect is described by London’s equations, which show that magnetic fields decay exponentially from the surface of a superconductor over a characteristic distance called the London penetration depth (λ), typically just a few hundred nanometers. Inside the material beyond this depth, the magnetic field is effectively zero.