Braunschweig, Friedrich Vierweg & Sohn; Berlin, J. Springer, 1924. 8vo (22.8 x 15.4 cm). 6 pp. Original printed wrappers.

On the theory of radiometer forces. "The Crookes radiometer (also known as a light mill) consists of an airtight glass bulb containing a partial vacuum, with a set of vanes which are mounted on a spindle inside. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. The reason for the rotation was a cause of much scientific debate in the ten years following the invention of the device" (Wikipedia). James Clerk Maxwell first gave a wrong, then a correct explanation. However, some subtleties still needed to be solved, and in 1924 Albert Einstein had a crack at it "A partial explanation is that gas molecules hitting the warmer side of the vane will pick up some of the heat, bouncing off the vane with increased speed. Giving the molecule this extra boost effectively means that a minute pressure is exerted on the vane. The imbalance of this effect between the warmer black side and the cooler silver side means the net pressure on the vane is equivalent to a push on the black side and as a result the vanes spin round with the black side trailing. The problem with this idea is that while the faster moving molecules produce more force, they also do a better job of stopping other molecules from reaching the vane, so the net force on the vane should be the same. The greater temperature causes a decrease in local density which results in the same force on both sides. Years after this explanation was dismissed, Albert Einstein showed that the two pressures do not cancel out exactly at the edges of the vanes because of the temperature difference there. The force predicted by Einstein would be enough to move the vanes, but not fast enough" (Wikipedia). Contained (first paper) in: Zeitschrift für Physik, volume 27(1). The complete issue, in its original wrappers. Front wrapper slightly spotted at the fore and bottom edges. Volume number written on spine. A very good, clean copy. Weil, 139.