Back, Ernst E. A.

(b. Freiburg im Breisgau, Germany, 21 October 1881; d. Munich, Germany, 20 July 1959), physics.

Back, a scion of an old Alsatian family, entered the University of Strasbourg in 1902. There and at Berlin and Munich he studied law for four years. After passing his first state examination he served as a law clerk from 1906 to 1908. In 1908 Back abandoned the law and began to study experimental physics in Friedrich Paschen’s institute at the University of Tübingen, which specialized in atomic spectroscopy. After completing his doctoral dissertation, Zur Prestonschen Regel (accepted February 1913, published 1921), Back stayed on at Tübingen as Paschen’s assistant until the outbreak of World War I.

Preston’s rule—that the magnetic splittings (Zeeman effects) of the lines in a given series (e.g., triplet diffuse series, doublet sharp series, and so forth) are identical—suffered many apparent exceptions. Clarification required better—above all, sharper—spectrographs, and Back’s entire scientific career was devoted to meeting this need. His most important technical innovations were in the design and fabrication of the light source, because this was the component of the apparatus chiefly responsible for the failure of Zeeman-effect spectrographs to match the quality of those made in the absence of a magnetic field. By the late 1920’s Back’s unique vacuum arc, packed into the four millimeters between the pole pieces of Tübingen’s electromagnet, included extraordinarily elaborate features: the electrode containing the element to be investigated was advanced (through a vacuum seal) by clockwork, while the other electrode, oscillated (also through a vacuum seal) by a motor, intermittently struck and extinguished the are precisely on the axis of the field.

Although Back was able to rule out a number of apparent exceptions to Preston’s rule, others were confirmed. The latter were usually very narrow or unresolved doublets or triplets showing “normal” Lorentz triplets in a magnetic field rather than the anticipated superposition of the anomalous splittings of the individual lines. It was Paschen who saw the solution to the difficulty: in sufficiently strong magnetic fields the several splitting patterns characteristic of the different types of series are transformed into the normal triplet (the Paschen-Back effect, 1912). During the following years this effect was regarded by leading theorists as both one of the most important problems in atomic physics and one of the most promising sources of information on the subject of atomic structure.

A lieutenant in the reserve, Back was employed in the service corps (Train) on the Western Front from the outbreak of World War I until 1918. In 1918 he became head (Leiter) of the physical laboratory of Veifa-Werke, a manufacturer of electrical and X-ray equipment in Frankfurt am Main. Yet Back never abandoned his interest in the Zeeman effect, and in the spring of 1920 he returned to Tübngen as an assistant in Paschen’s institute. There Back accumulated spectrographs of unparalleled quality, including the Zeeman effects of elements in whose spectra Miguel Catalàn (working at the University of London) and Hilda Gieseler (working at Tübingen) were then discovering the existence of higher multiplicities (quartets, quintets, and so forth). When Alfred Landé arrived in tübingen in October 1922, Back made this data available to him. Six mnoths later, in consecutive articles in the Zeitschrift Für Physik, Landé published the well-known formula expressing the “g factors” for all multiplicities as a function of the quantum numbers of the stationary state of the atom, and Back published the data upon which it was based.

In the spring of 1923 Back became Privatdozent at Tübingen. An appointment as extraordinary professor at the Landwirtschaftliche Hochschule in Hohenheim in 1926 (ordinary professor in 1929) provided financial security, but his scientific work was still carried on in the Tübingen institute. There, in the fall of 1926, he showed Samuel Goudsmit photographs of the bismuth spectrum that he had made over the years. Together they effected an analysis of the hyperfine structure of several lines, interpreting it as the result of the interaction of the total electronic angular momentum with an angular momentum that they attributed to the bismuth nucleus. Back’s spectrographs (1927) fixed this nuclear angular momentum at 9/2 h/2π. They thus provided the first firm evidence of a counterexample to an almost inescapable inference from the contemporary assumption that nuclei consisted of protons and electrons-namely, that a nucleus of even atomic number must have integral angular momentum (in units of h/2π).

In 1936 Back became professor at Tübingen; he retired in 1948. After the acceptance of electron spin (1926) the Zeeman effect lost most of its intrinsic importance, and in the 1930’s other techniques vied with spectroscopy in determinations of angular momenta and magnetic moments. Moreover, after 1930 Back lacked the perceptive collaborators who had previously kept his very special talents directed toward significant problems in atomic physics.

BIBLIOGRAPHY

I. Original Works. A bibliography of Back’s works is in J. C., Poggendorff, Biographisch-literarisches Handwürterbuch zur Geschichte der exakten Naturwissenschaften (Leipzig, 1926–1962), V, 47–48; VI, 100–101; VIIA, pt. 1, 74. Some twenty-five letters to Sommerfeld, Landé, and Goudsmit are listed in T.S. Kuhn, et al., Sources for History of Quantum Physics. An Inventory and Report (Philadelphia, 1967), p. 13

II. Secondary Literature. For the Paschen-Back effect, see J. B. Spencer, “An Historical Investigation of the Zeeman Effect (1896–1913),” Ph.D. dissertation (University of Wisconsin, 1964). Back’s work in the early 1920’s is discussed in P. Forman, “Environment and Practice of Atomic Physics in Weimar Germany,” Ph.D. dissertation (University of California, Berkeley, 1967), ch. 3; the later 1920’s, in S. A. Goudsmit, “Pauli and Nuclear Spin,” in Physics Today (June 1961), pp. 18–21. The author of the present article is preparing a fuller account of Back’s work and career.

Paul Forman