DOCUMENT 180 OCTOBER 1909 211 Einstein assumed a position as Extraordinary Professor at the University of Zurich on 15 October (see Doc. 154, note 2). At the Salzburg meeting Max Born had presented a paper on rigid motion in special rel- ativity (Born 1909b), on which Sommerfeld had commented in the discussion following the paper. Einstein and Born had discussed the subject and had discovered that setting a rigid disk into rotation would give rise to a paradox: the rim becomes Lorentz-contracted, whereas the radius remains invariant (see Born 1910, p. 233). The existence of this paradox was first point- ed out in print by Paul Ehrenfest (1880-1933) in a paper that was received on the date of this letter (see Ehrenfest 1909). See also Stachel 1980 for a historical discussion of Einstein's treat- ment of the rigidly rotating disk. See Einstein 1907j (Vol. 2, Doc. 47), §§17-20, where Einstein introduces the equivalence principle and explores some of its consequences. See Einstein 1905i (Vol. 2, Doc. 14), in which Einstein shows that the entropy of mono- chromatic radiation of small density depends on its volume, as if the radiation consists of in- dependent energy quanta. 180. To Lucien Chavan [Zurich, 19 October 1909] Lieber Herr Chavan! Ich sende Ihnen mein Dienstbüchlein & die Niederlassungs-Bewilligung mit der Bitte, mich bei Polizei und Kreiskommando und Polizei abzumel- den. Es ist hier schon losgegangen. Meine neue Situation gefällt mir sehr gut. Aber es ist anstrengend. Mit herzlichen Grüssen Ihr A. Einstein. [. . .] AKS (Sz, Archiv der Einstein-Gesellschaft). [72 277]. The verso is addressed "Herrn L. Cha- van Beundenfeld-str. 5 Bern.," and postmarked "Zürich 8 (Fluntern) 19.X.09.-5." On moving from Bern to Zurich-on 14 October (see Mileva Einstein-Maric to Helene Savic, 3 September 1909, Milan Popovic, Belgrade)-Einstein, a Swiss national, was required to notify the Bern military command of his new address and to register his change of address with the police. Einstein's departure is noted in his military service book but no date is given (see Dienstbüchlein, 13 March 1901). The police registration is dated 22 October 1909 (see Einwohnerregister Berner und Schweizer (Einsassen), SzBe-Ar, E 220.127.116.11. 054). Winter semester 1909/1910 at the University of Zurich began 18 October (set Zürich Ver- zeichnis 1909b, title page). Einstein lectured on introductory mechanics (four hours a week to eighteen students) and on thermodynamics (two hours a week to fifteen students and three auditors), as well as con- ducting a physics seminar one hour a week for twelve students (see Kontrollbücher über die Honorargebühren, W.S. 1909/10, SzZU, Kassa-Archiv, and Zürich Verzeichnis 1909b, p. 22). The lecture notes on mechanics are presented as Vol. 3, Doc. 1. A postscript by Mileva Einstein-Maric to Jeanne Chavan-Perrin is omitted.