It turns out that an intermediate state does not occur in all superconducting materials. There exists a whole class of superconductors in which a magnetic field penetrates in another way. These are mainly alloys, but among the pure elements, niobium is one of them. They are called type II superconductors. Superconductors such as mercury, lead and aluminium, which we have studied above, are called type I superconductors.

What is the difference between these two types of superconductivity? Type I superconductors force out the magnetic field and are capable of fighting against it until its strength reaches the critical value H _c. Above this limit, the substance goes over into a normal state. In an intermediate state, the sample lets the magnetic field inside, but from the point of view of physics, it would be more precise to say that the sample simply splits into large neighboring pieces which are normal and superconducting. Through the normal pieces, there flows a magnetic field of strength H _c, while in the superconducting pieces, the magnetic field is, just as it should be, equal to zero. If we scrutinise any boundary between such regions, we shall see what is shown in Fig. 13, i.e. field screening.

Type II superconductors also force out the magnetic field, but only a very weak one. As magnetic field strength increases, a type II superconductor finds it possible to let the field inside, simultaneously preserving superconductivity. This happens when the field strength...