The Meissner Effect

Cool a superconductor below $T_c$ in a magnetic field and it does not merely become a perfect conductor: it actively expels the field. A superconducting sphere is a perfect diamagnet, screening currents flowing so that $B=0$ inside and the normal field vanishes at the surface, which forces the field lines to bend around it and crowd to $\tfrac32 B_0$ at the equator. The state survives only while $B_0\lt B_c(T)=B_{c0}(1-(T/T_c)^2)$; raise the temperature or the field past that parabola and the flux floods back in. A type-II superconductor instead admits the field as a triangular Abrikosov lattice of vortices, each carrying one flux quantum $\Phi_0=h/2e$.