Superconductivity's a phenomenon occurring in certain materials generally at very low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect). It was discovered by Heike Kamerlingh Onnes in 1911. Like ferromagnetism and atomic spectral lines, superconductivity's a quantum mechanical phenomenon. It can't be understood simply as the idealization of "perfect conductivity" in classical physics. The electrical resistivi… (More on Superconductors)