I N T R O D U C T I O N T O V O L U M E 1 0 x l v i i
pist Hermann Anschütz-Kaempfe. He then went to Bad Nauheim to attend the
GDNÄ meeting, to Stuttgart for another popular lecture held on 28 September in
support of the local observatory, and to Benzingen in southern Germany, where he
spent a few days with his sons. Einstein arrived in Leyden on 21 October and finally
presented his inaugural lecture on 25 October 1920.
Einstein’s lecture took place in the midst of the week-long meeting that Ehren-
fest organized under the auspices of the Institut International du Froid, devoted to
discussions on paramagnetism at low temperatures. In addition to Einstein, Ehren-
fest, Lorentz, and Kamerlingh Onnes, the participants included Johannes P.
Kuenen, another Leyden physicist, as well as the French physicists Pierre Weiss
and Paul Langevin. Referred to as “Magnet-Woche” in Ehrenfest’s diaries, the
meeting presented an opportunity to discuss cutting-edge problems of both exper-
imental and theoretical condensed matter physics at low temperatures.
Notwithstanding the fact that Einstein delivered his “famous inaugural lecture”
(Doc. 183) on the topic of general relativity, the Leyden physicists hoped that his
appointment would bring Einstein closer to their own research on condensed matter
physics in the low temperature regime. Indeed, with Einstein’s professional ties to
Leyden, Onnes had cherished the “finest hopes for a flowering of the cryogenic
laboratory.”[40]
And, in fact, during his visits in the Netherlands, Einstein would
spend much time with Onnes discussing research (e.g., Doc. 9, 152a and Doc. 25)
and attend experimental demonstrations in his laboratory (Doc. 9). Lorentz, too,
had noticed Einstein’s interest in solid state physics, and invited him to give a talk
at the upcoming 1921 Solvay meeting on the current state of investigations on the
gyromagnetic effect as a follow-up on Einstein’s experiments performed with De
Haas several years earlier (Doc. 49).
Although little is known about the details of the discussions during the “Magnet-
Woche,” since no formal minutes or reports are extant, we do know that the phe-
nomenon of superconductivity was a topic of
debate.[41]
Ehrenfest and Einstein dis-
cussed, in particular, the question as to whether superconductors would exhibit
some kind of Hall effect. In a letter about which we only know through an excerpt
by Ehrenfest and from a manuscript with calculations (Doc. 227 and Appendix),
Einstein considered the Hall effect for a perfect conductor, and arrived at conse-
quences that, in principle, would be amenable to experimental verification, even if
not with the technological possibilities available to the Leyden laboratory at the
time.
Einstein’s considerations were based on a particular solution for Maxwell’s
equations, and were thus of a purely classical nature. The discussions of paramag-
netism during the “Magnet-Woche” and in the correspondence with Ehrenfest
often involved deliberations on the implications of the new quantum theory for