THE FIELD EQUATIONS OF GRAVITATION 24514b. Interlude: Rotation by Magnetization
'I firmly believe that the road taken is in principle the correct one and that later
[people] will wonder about the great resistance the idea of general relativity is
presently encountering' [E27]. This prophesy was made by Einstein in the first
week of 1915. It would be fulfilled before the year was out, but not until Einstein
had passed through a crisis followed by an exhausting struggle. Toward the
autumn of 1915 he finally realized* that his theory up until then was seriously
wrong in several respects.
Meanwhile, early in 1915 he did not publish anything substantially new on
relativity.** He did write two review articles, one on relativity theory [Ell] and
one on the atomic theory of matter [E29], and a short paper on the statistical
properties of electromagnetic radiation in thermal equilibrium [E30]. Of more
interest are his activities in experimental physics. At that time Einstein made good
use of a temporary guest appointment at the Physikalisch Technische Reichsan-
stalt in Charlottenburg [K4]. 'In my old age, I am acquiring a passion for exper-
iment' [E31]. This passion led to the discovery of the Einstein-de Haas (EdH)
effect, the torque induced in a suspended cylinder (made of iron, for example) as
a consequence of its being abruptly magnetized. The present interlude is devoted
to a brief account of these activities.
Wander Johannes de Haas was a Dutch physicist of Einstein's age. He received
his PhD in Leiden, in 1912, with Kamerlingh Onnes. Later that same year, he
went to the University of Berlin to work in the laboratory of Henri du Bois.f In
August 1913, when Lorentz sent congratulations to Einstein on his forthcoming
appointment in Berlin, he must have added (the letter is lost) a query concerning
de Haas, as is seen from Einstein's reply: 'At present, I do not know what to do
in the matter of your son-in-law, since in Berlin I will have neither an institute
nor an assistant.' [E3]. Then came the visiting appointment at the Reichsanstalt.
Einstein was now in a position to do something for de Haas—and for Lorentz.^
I do not know when de Haas joined Einstein at the Reichsanstalt. However, their
gyromagnetic experiment was performed 'in a very brief period' [HIa]. De Haas
left the Reichsanstalt in April 1915.
Soon after the conclusion of their collaboration, Einstein wrote enthusiastically
about the results obtained. 'Scientifically, I have done a wonderful experimental
thing this semester, together with Lorentz's son-in-law. We have given firm proof
*See Section 14c.
**It is sometimes incorrectly stated that a brief abstract of a talk by Einstein before the Prussian
Academy [E28] contains the announcement of the final formulation of his theory as published in
November 1915 [Dl],
fin October 1912 the Ehrenfests visited de Haas and his wife in Berlin—at the suggestion of Lorentz
[K5].
HOn one occasion, Einstein referred in print to de Haas as Herr de Haas-Lorentz [E32].