Ein Weg zur experimentellen Prufung der Richtungquantelung im Magnetfeld [and] Der experimentelle Nachweis. .

Stern, Otto (1888-1969). et al. (1) Ein Weg zur experimentellen Prüfung der Richtungsquantelung im Magnetfeld. In Zeitschrift fur Physik 7 (1921): 249-253. (2) (with Walter Gerlach [1889-1979]). Der experimentelle Nachweis des magnetischen Moments des Silberatoms. In ibid. 8 (1921): 110-111. Together 2 whole volumes. [iii]-vi, 414; iv, 419pp. Braunschweig: Fried. Vieweg & Sohn; Berlin: Julius Springer, 1921-22. 224 x 149 mm. Vol. 7 in half morocco, cloth boards, a little rubbed; Vol. 8 in half cloth, marbled boards. Library stamps on titles.

First Editions. Stern, whom Emilio Segrè considered "one of the major physicists of the century" (p. 138), developed Dunoyer's molecular beam method and used it to devise what is now known as the Stern-Gerlach experiment, which demonstrates the reality of space-quantization of atoms. This experiment had an enormous impact on modern physics, and has become a paradigm of quantum measurement.

Spatial quantization had been introduced as a theoretical concept by Sommerfeld in 1916, but no one before Stern had ever demonstrated its existence, and some quantum physicists even considered it to be nothing more than a mathematical tool. In his 1921 paper, Stern proposed an empirical test: "If an electron in an atom carried a magnetic moment of about 1 Bohr magneton (= eh/4 me), an atomic beam of silver atoms should split in passing through a magnetic field of strong inhomogeneity" (Twentieth Century Physics, p. 164). In the 1922 paper, written jointly with Walther Gerlach, Stern described the initial results of their experimental work. In three later papers (not included here) Stern and Gerlach continued their investigations, which provided direct experimental proof of directional [space-] quantization in a magnetic field.