In 1911 H. Kamerlingh Onnes was investigating the electrical properties of metals such as Pt and Au in the range of a few K (Kammerlingh Onnes 1911). He observed that the resistance is determined by the number of impurities in this regime. (This is now called Mattheissen's rule, Eq. (7.24).) In order to eliminate impurities he turned to mercury which was available in a very pure form. Instead of a lower resistance, which he expected, he found a sudden jump to a vanishingly small resistance at about 4.2 K. This temperature is now called the transition temperature, Tc. His statement was: “Mercury has passed into a new state which on account of its extraordinary electrical properties may be called the superconductive state.” He also found transitions to zero resistance for lead and tin with different Tc's, also in the range of a few K. (We should note that later work showed that superconductivity is, in fact, not affected very much by impurities.)

The superconductive state is now known to occur for around forty elements and hundreds of compounds. The transition temperature is less than about 100 K for all known cases. The superconductivity of copper oxides discovered by Bednorz & Muller (1986) is of particular current interest, and gives the highest known Tc's.

Supercurrents flow without friction and thus are persistent: they have been observed to flow around a ring without decay for the better part of a year.