type-I superconductor
/ type-WUN SOO-per-kun-DUK-ter /
Picture a stubborn doorman who keeps a room exactly empty of one unwanted guest — magnetic field — turning away every bit of it. He holds the line firmly as the pressure outside grows, refusing entry completely. Then, the moment the crowd outside exceeds what he can manage, he gives up all at once and the room is suddenly flooded. There is no halfway.
A type-I superconductor behaves like that doorman. Below its critical field it expels magnetic field entirely through the Meissner effect, keeping its interior perfectly field-free. But the expulsion holds only up to one sharp threshold; cross that single critical field and superconductivity collapses abruptly across the whole sample, which reverts to an ordinary metal in one clean step. There is no gradual letting-in of field.
This matters because it represents the simplest, cleanest form of the superconducting state, and most pure elemental metals — lead, tin, aluminium, mercury — behave this way. The practical drawback is that their critical fields are small, so even a fairly modest magnet destroys their superconductivity. That weakness is exactly why type-I materials are almost useless for building strong magnets, and why type-II superconductors are needed instead.
Lead stays superconducting only until the field reaches about 0.08 tesla — weaker than the field between the poles of many ordinary horseshoe magnets — at which point its superconductivity vanishes all at once.
Type-I metals like lead surrender to even a fairly weak magnetic field, all at once.
The clean either-or behaviour comes from the type-I material's geometry of pairing: its coherence length is long compared with how deep field can penetrate, so it is energetically cheaper to keep field entirely out than to admit it in small pieces.