There exists another critical parameter which obstructs the occurrence of superconductivity. This is the critical current or, as the critical value of a current depends on the sample size, it is better to speak of a critical current density, i.e. the current conducted through unit cross section of a superconductor. This value is denoted by j _c and is measured in A/m ², as well as A/cm ² and other units.

We have just discussed the manner in which a magnetic field destroys superconductivity (look at the phase diagram in Fig. 12 once again the greater the external magnetic field, the lower will be the temperature at which superconductivity occurs, and whenever the field strength H exceeds H _c, superconductivity ceases). Now suppose that an external magnetic field is absent, but a current conducted by a superconductor will also create its own magnetic field, and this field has the same destructive effect upon superconductivity. So, the current that creates a critical magnetic field must also become critical. This actually proves to be the case for many superconductors.

We have frequently mentioned two types of current which can run in superconductors. First, there is an eddy screening current running along the sample surface and providing the Meissner effect. A screening current runs, of course, only if there exists an external magnetic field which should be kept out of the superconductor.

Second, a transport current which does not depend on the external magnetic field can run...