Expulsieur

Expulsieur is the maître d’ of a restaurant made of lead, and magnetic fields do not get a table.

A superconductor loses all electrical resistance when it is cold enough. A merely perfect conductor would freeze in whatever magnetic field it held at that moment. In 1933 Walther Meissner and Robert Ochsenfeld cooled cylinders of tin and lead in a weak magnetic field and measured the field around them. As the metal turned superconducting, the field inside was pushed out, including field that was already there. That is the Meissner effect: a superconductor is not just a perfect conductor but a perfect diamagnet, the first line on this page.

Lead turns superconducting below about 7.2 K. Above that, field lines wander through the dining room like any customer. Below it, Expulsieur shows them out. They bend round the building and crowd at the edges, continuous, never ending inside.

Two years later the brothers Fritz and Heinz London wrote equations for it. The field does not stop dead at the surface; it fades exponentially over a short distance, the London penetration depth, the second and third lines. Lines may lean in at the doorway, but not far.

He has limits. Lead is a type-I superconductor, and the expulsion works only for applied fields below its critical field, about 80 mT near absolute zero; above that, superconductivity collapses. And in real samples, cooling in a field often expels the flux only partly, a caveat he would rather you didn’t mention.

Still, once the room is cold, the rule at his door is simple.

Your field waits outside.