"The difference between Type I and Type II superconductivity is of substantial practical importance. To understand why, let us return to the Meissner effect ... What happens when the external magnetic field is increased to the critical value Bcrit? Magnetic flux begins to penetrate the material, initially in the form of flux lines. In the Type I case, the flux lines attract, forming a large region of normal metal, and superconductivity is soon lost altogether. In the case of a Type II superconductor, when one reaches the critical magnetic field, flux lines appear inside the superconductor. But since they repel each other, they can form a stable arrangement, a “lattice” of parallel flux lines ... As a result, a Type II superconductor for B > Bcrit can reach a stable arrangement in which part of an externally applied magnetic field is expelled to the outside world, while part penetrates the superconductor in the form of the flux lattice. The material remains superconducting in such a state, and as a result, Type II superconductors can support considerably higher magnetic fields and currents, making them more useful for many applications. Superconductivity is finally lost at a higher value of the magnetic field called the upper critical field."