A substance that reaches superconductivity, a state in which magnetic fields cannot penetrate, and there is no electrical resistance, is referred to as a superconductor. In 1911, Heike Kamerlingh Onnes of Leiden University, a Dutch physicist, observed a relation between the temperature and the resistance of the element mercury. The mercury sample was then cooled in liquid helium to study the linear dependence of resistance on temperature. It was observed that, as the temperature decreased, the resistance dropped. Kamerlingh Onnes continued to cool the mercury sample further, and as the temperature approached 4.2 K, the resistance abruptly went to zero. This is the critical temperature for mercury, where the mercury sample enters into a phase with absolute zero resistance, and this phenomenon is known as superconductivity. In the case of conductors, the resistance is not equal to zero but is less than 0.01 Ω. There are various methods to measure very small resistances, such as the four-point method, but an ohmmeter is not acceptable for testing superconductivity resistance.
Several other materials have been discovered in the superconducting phase when the temperature reaches near absolute zero. In 1941, an alloy of niobium-nitride was found to become superconducting at 16 kelvin; similarly, in 1953, vanadium-silicon was found to become superconductive at 17.5 kelvin. The temperatures for the transition into superconductivity were slowly creeping higher. Surprisingly, many good conductors, like copper, silver, and gold, do not exhibit superconductivity.