  Einstein's early thoughts about superconductivity are discussed as a case
study of how theoretical physics reacts to experimental findings that are
incompatible with established theoretical notions. One such notion that is
discussed is the model of electric conductivity implied by Drude's electron
theory of metals, and the derivation of the Wiedemann-Franz law within this
framework. After summarizing the experimental knowledge on superconductivity
around 1920, the topic is then discussed both on a phenomenological level in
terms of implications of Maxwell's equations for the case of infinite
conductivity, and on a microscopic level in terms of suggested models for
superconductive charge transport. Analyzing Einstein's manuscripts and
correspondence as well as his own 1922 paper on the subject, it is shown that
Einstein had a sustained interest in superconductivity and was well informed
about the phenomenon. It is argued that his appointment as special professor in
Leiden in 1920 was motivated to a considerable extent by his perception as a
leading theoretician of quantum theory and condensed matter physics and the
hope that he would contribute to the theoretical direction of the experiments
done at Kamerlingh Onnes' cryogenic laboratory. Einstein tried to lived up to
these expectations by proposing at least three experiments on the phenomenon,
one of which was carried out twice in Leiden. Compared to other theoretical
proposals at the time, the prominent role of quantum concepts was
characteristic of Einstein's understanding of the phenomenon. The paper
concludes with comments on Einstein's epistemological reflections on the
problem.
