The document discusses the history and theory of superconductivity. It was discovered in 1911 by Kamerlingh Onnes who observed mercury's electrical resistance dropping to zero at 4.2K. BCS theory from 1957 explained superconductivity as electrons forming Cooper pairs mediated by phonons. Superconductors expel magnetic fields below their critical temperature, known as the Meissner effect. Applications include magnetic levitation trains that are propelled and levitated using superconducting magnets and copper coils.