London equations

Summary

The London equations are two equations that relate the supercurrent in a superconductor to the electric and magnetic fields. Combined with Maxwell's equations, they show that a magnetic field decays exponentially inside a superconductor over the London penetration depth. This explains the Meissner effect, the expulsion of magnetic field from a superconductor. They are the simplest phenomenological description of superconductivity, and they appear as a limit of Ginzburg–Landau theory.