As of today, we know of over 500 pure elements and alloys which exhibit the property of superconductivity. According to their behaviour in strong magnetic fields, they are classified into type I and type II superconductors. In type I superconductors (cylindrical samples where the cylinder axis coincides with direction of the external magnetic field), when the external magnetic field reaches the critical value H_c the field very rapidly penetrates the superconductor which transits, throughout its entire volume, to the normal (nonsuperconducting) state (in samples of other forms or at other orientations of the external magnetic field the transition is stretched over a certain range of values within which the sample remains in the so-called intermediate state). Type I superconductors include all superconducting elements except Nb and V and some alloys.

In type II superconductors, when the external magnetic field reaches the critical value H_c1 (called the first critical field), the superconductor transits into the so-called mixed state: inside the superconductor is a magnetic field, which increases as long as the external magnetic field grows up to the second critical field H_c2 due to the intromission into the superconductor of the so-called Abrikosov vortices. Once the second critical field is reached the material transits to the normal state throughout the entire sample except for a thin surface layer in which superconductivity disappears at the third critical field H_c3 = 1.69 of H_c2. Type II superconductors include, basically, alloys and composite materials (NbTi, Nb₃Ge, etc.) as well as Nb and V.

The temperature interval of transition to the superconducting state (and vice versa, as this transition is invertible) for pure samples does not exceed a thousandth of a degree, so it makes sense to use T_c as an exact value. The currently known Tc temperatures vary from 0.0005 K (Mg) to 135 K in mercury-containing high-temperature superconductors (under external pressure of 350 thousand atmospheres this temperature increases to 164 K).

The phenomenon of superconductivity was discovered by Dutch physicist Kamerlingh Onnes in 1911 (Nobel Prize 1913). The phenomenon of superconductivity is used to create strong magnetic fields and small-size electric motors, in maglev trains, to build highly sensitive electronic devices, in computer logic and memory cells as well as in elements of a quantum computer.