The fact that the resistance is zero has been demonstrated by sustaining currents in superconducting lead rings for many years with no measurable reduction. One of the properties of a superconductor is that it will exclude magnetic fields, a phenomenon called the Meissner effect . The disappearance of electrical resistivity was modeled in terms of electron pairing in the crystal lattice by John Bardeen, Leon Cooper, and Robert Schrieffer in what is commonly called the BCS theory . A new era in the study of superconductivity began in 1986 with the discovery of high critical temperature superconductors . The critical temperature for superconductors is the temperature at which the electrical resistivity of a metal drops to zero.
Several materials exhibit superconducting phase transitions at low temperatures.
The highest critical temperature was about 23 K until the discovery in 1986 of some high temperature superconductors . They are called Type I superconductors. The superconductivity exists only below their critical temperatures and below a critical magnetic field strength. I superconductors are well described by the BCS theory . They were found to have much higher critical fields and therefore could carry much higher current densities while remaining in the superconducting state.