4 5 4 D O C . 2 9 0 T H E O R Y O F R A D I O M E T E R F O R C E S Published in Zeitschrift für Physik 27 (1924): 1–6. Received 21 July 1924, published August–Sep- tember 1924. For a manuscript with a different §3 and an additional §4, see [1 042] and [2 098] (§§3 and 4 only). The alternative sections are presented in note 5. [1]Radiometer forces are the forces that are responsible for the rotation of a radiometer, a device consisting of vanes, blackened on one side and reflective on the other, mounted on a spike, and enclosed in a partially evacuated container. Because the rotational motion of the vanes starts when the device is illuminated, it was initially thought that it was caused by light pressure. It was later realized that tangential forces exerted by gas molecules in the container, as a result of temperature gradients, are responsible for the motion (see, e.g., Brush 1976, chap. 5.5, for a historical overview). Einstein had been interested in radiometer forces before: in 1919 he suggested the theory of the radiometer as a dissertation topic to his cousin Edith Einstein (see Edith Einstein to Einstein, 29 April 1919 [Vol. 9, Doc. 31]). For his continued interest in the topic, see also Einstein to Paul Epstein, 4 June 1920 (Vol. 10, Doc. 42), and Edith Einstein to Einstein, 4 December 1921 (Vol. 12, Doc. 310). Edith Ein- stein’s dissertation was published as Einstein, E. 1922. [2]See Knudsen 1910c. [3]In the equation, η should be n. [4]In eq. (9) as well as in (10a) below, should be . [5]At this point the manuscript continues with a different §3 and an additional §4. “§3. Wand mit Temperatursprung In dem Falle, dass durch Bestrahlung oder sonstwie eine Temperaturdifferenz der beiden Wand- flächen aufrecht erhalten wird, erhalten wir analoge Wirkungen, auch wenn kein Wärmestrom im Gase berücksichtigt wird. Wir betrachten wieder den Fall, dass die Kommunikation der Gasteile zu beiden Seiten der Wand eine solche ist, dass Druckausgleich praktisch vollständig erfolgt. Dem ent- spricht die Gleichung[6] . . . . (11) Setzt man , so erhält man[7] . . . . (11a) Ist eine kleine Oeffnung (σ) in der Wand, so bezeichnet –Δν die Intensität des Molekülstromes durch die Öffnung. Mit Rücksicht auf auf die Relationen erhält man hieraus . . . . . . (11b) 1 6 -- - 1 3 -- - p 2νnun 2νpup = = Δν νp νn –= ν 1 2 --( - νp νn) + = Δu up un –= u 1 2 --( - up un) + = Δν ν ------ - ΔT 2T ------ -–= –Δν nv = ν 1 6 --nu - = v 1 12 -----u------ - ΔT T -=
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