I N T R O D U C T I O N T O V O L U M E 1 2 l i
investigation that we learn about in his letters produced a published outcome, at
least not by the end of the period covered by this volume. In fact, for most of these
experiments, we have very little evidence about the extent of their execution, out-
come, or interpretation.
A notable exception is an experiment aimed at deciding between the fundamen-
tally different accounts that the wave theory and the quantum theory of light gave
of the elementary process of light emission for light-radiating canal ray particles.
This experiment was suggested by Einstein in a short note published in the Prussian
Academy’s proceedings (Vol. 7, Doc. 68). It is the only experiment mentioned in
publications for 1921, yet it was only one among many in which Einstein took a
keen interest during the year.
Indeed, in his first letter of the year, on 1 January, to Hendrik A. Lorentz
(Doc. 3), Einstein mentioned two new experiments: one that he intended to pursue
in Berlin, and another that he was proposing to his colleagues in Leyden. The latter
concerned the existence and properties of the Hall effect in superconductors. Ein-
stein sketched an argument that discusses properties of Maxwell’s equations for the
case of infinite conductivity. He then specialized to an experimental setup that
would allow an investigation of the predicted
properties.[55]
Later in the year, he
proposed another experiment on superconductivity, also to be carried out in Heike
Kamerlingh Onnes’s cryogenic laboratory in Leyden. In a letter to Ehrenfest on 2
September, Einstein referred to their earlier discussions on the subject, and ex-
pounded an idea of superconductive electric charge transport that takes place with
electrons moving from atom to atom, in a snakelike motion, via multiple travels
around osculating Bohrian orbits (Doc. 225). This idea had been discussed with
Ehrenfest and other Leyden physicists during Einstein’s visit on the occasion of the
“Magnet-Woche” in November 1920. An immediate consequence of the idea, Ein-
stein now pointed out, is that superconductive currents have a minimum threshold
value determined by the Bohr quantum conditions for the allowed electron orbits.
He suggested that one should test the idea by checking whether a superconducting
loop would be responsive to inductive currents in a neighboring normal conductor
only above a certain threshold
value.[56]
The other experiment that Einstein mentioned to Lorentz was an investigation
that he himself wanted to carry out in Berlin in collaboration with the experimental
physicist Peter Pringsheim. Like the experiments on superconductivity, it, too, tar-
geted properties of the emerging quantum theory. Boldly referring to it three days
later as an “experimentum crucis,” Einstein designed it to decide whether the wave
theory of light correctly predicts the thermodynamic black-body spectrum
(Docs. 6, 24, and 37). The point was to look for the induced Stark effect for the