I N T R O D U C T I O N T O V O L U M E 1 3 x l i ductivity dated to 1912, especially in connection with the problem of zero-point en- ergy in the quantum domain.[3] Like many other physicists, Einstein tried, without success, to understand how this extraordinary effect comes about. During the years 1920 and 1921 he conceived possible experiments to clarify the problem.[4] For ex- ample, since electrons move on Bohr orbits without radiative energy loss, Einstein thought that the dissipationless superconducting current would be realized by elec- trons on Bohr orbits when they go from one atom to the next at places where the orbits osculate. This hypothesis gave rise to experimental tests. Around 11 March 1922, he completed a paper, “Theoretical Comments on the Superconductivity of Metals” (Einstein 1922k, Doc. 76), which he contributed to a Festschrift honoring Heike Kamerlingh Onnes. In this analysis, Einstein concluded that it was difficult to sustain a theory of superconductivity based on the free-electron theory. He then put forth the rather “vague idea” of the existence of “conduction chains,” according to which electrons are transferred from atom to atom (or molecule to molecule). Einstein knew that this idea still needed theoretical backing and referred to it as a form of “fantasiz- ing.” He understood that in order to have a proper theory, physicists had to be much better acquainted with the quantum mechanics of combined systems. Incidentally, this may well have been the first time the term “Quanten Mechanik” was used in print.[5] Nevertheless, Einstein suggested an experiment that might show whether his idea of conduction chains had any merit. He proposed attaching together two dif- ferent superconducting metals into a single circuit. Since it was unreasonable to think on the basis of his theory that conduction chains of different kinds of atoms would be able to connect into one chain, this combined circuit should cease to be superconducting. The experiment was carried out by Kamerlingh Onnes in Leyden in his laboratory, which was the only place, until 1923, capable of producing tem- peratures sufficiently low for superconductivity investigations. Kamerlingh Onnes and his collaborators patched together a ring of twenty-four alternating sectors of tin and lead and checked whether this ring, with its twenty-four interfaces between two different superconducting metals, could sustain a dissipationless supercon- ducting current. In January, Einstein asked Ehrenfest to find out what was happening and to “egg on” Kamerlingh Onnes (Doc. 31). Ehrenfest reported the latter’s results: the circuit consisting of lead and zinc showed no resistance (Docs. 54 and 77). Einstein react- ed immediately: “It is good that K. Onnes did the experiment.” But, he added, “Again a little hope for understanding is lost.” Superconductivity was linked to another topic of interest to Einstein: the ques- tion of zero-point energy. According to the kinetic theory of electrons, any zero- point energy would result in a residual resistance of the conduction. A few months