M O M E N T O F M O M E N T U M 5 8 5
ON THE MOMENT OF MOMENTUM IN A
This appendix consists of two sections. Section 1 presents a transcription of the first part of
an autograph manuscript by Wander J. de Haas (1878–1960), Professor of Physics at the
Technische Hogeschool in Delft, for a lecture at the third Solvay Congress in Brussels on
2 April 1921 (“Distribué en Séance 2 Avril 1921” written in an unknown hand) that De
Haas attributes to Einstein as author (“§1. Theoretical, by Prof. Einstein.” BBU, Archives
du 3ème Conseil de Physique). It offers a generalization of the calculation given in Einstein
and De Haas 1915a (Vol. 6, Doc. 13) for the magnetic moment of orbiting electrons. Sec-
tion 2 consists of a page of calculations for this part in Einstein’s hand (NeLR, Archief H.
A. Lorentz) and is written on the verso of the formal letter of invitation to the Congress,
addressed to Einstein by Hendrik A. Lorentz on 6 January 1921.
When Lorentz initially invited Einstein to the Solvay meeting (Hendrik A. Lorentz to
Einstein, 9 June 1920), he enclosed a preliminary program containing a proposed lecture
by Einstein entitled “Die Elektronen und der Magnetismus. Gyroskopische Effekte.” Ein-
stein accepted the invitation (Einstein to Hendrik A. Lorentz, 15 June 1921). He apparently
turned to Wander de Haas and Geertruida de Haas-Lorentz with a request to check the per-
tinent literature, because in his reply (Wander de Haas to Einstein, December 1920) De
Haas sent a list of references used in the present document. In a preliminary program at-
tached to the formal letter of invitation of 6 January 1921, a lecture by Einstein and De Haas
was announced with the title “Les électrons et le magnétisme. Effets gyroscopiques” and
summarized as “Rotation d’un corps par suite de l’aimantation ou de la désaimantation.
Aimantation produite par une rotation. Relation théorique entre ces deux effets” [82 643].
Einstein had apparently suggested coauthorship.
Einstein had to withdraw from attending the meeting because of his forthcoming visit to
the United States. He asked De Haas to deliver the talk (Einstein to Hendrik A. Lorentz, 22
February 1921), and it was published as “Le moment de la quantité de mouvement dans un
corps magnétique” (Rapports 1923, pp. 206–227).
On the Einstein-De Haas experiment, see Vol. 6, the editorial note, “Einstein on
Ampère’s Molecular Currents,” pp. 145–149.
A P P E N D I X B