# Sauer, Tilman: *Probing Concepts, Old and New: Einstein's Interpretation of Superconductivity*

In 1920, Einstein was appointed visiting professor at the University of Leiden. A major motivation for this appointment was his perception as a leading quantum theorist and the expectation that he would provide theoretical guidance to the research at Kamerlingh Onnes' cryogenic laboratory. In the paper, I will discuss Einstein's interpretation of the phenomenon of superconductivity in the early twenties. On a phenomenological level, he explored consequences of Maxwell's equations for the case of perfect conductivity. On a microscopic level, he suggested a model of superconductive charge transport which made explicit use of the Bohr-Sommerfeld quantum theory of electron orbits in the atom. Along both these lines of investigations, he suggested to do certain experiments. At least one of them was indeed carried out in Leiden.