POHL, ROBERT WICHARD

(b. Hamburg, Germany, 10 August 1884; d. Göttingen, Germany, 5 June 1976)

physics.

Pohl was the son of a shipbuilding engineer, Robert Pohl, and his wife, Martha Lange. His father’s career had a major influence on his early interest in engineering and natural science. In Hamburg he attended the prestigious Johanneum, a classical gymnasium, from which he graduated in 1903. Even in his later years as a scientist, Pohl maintained a close relationship with this school, where the teaching of physics and mathematics was excellent. As a young student he was especially impressed by the worldwide excitement created by the discovery of X rays in 1895. He continued to have this enthusiasm for important but spectacular phenomena all his life.

Pohl studied physics at Heidelberg for one semester under Georg Hermann Quincke, to whom, later, he often attributed his guiding principle: “Theories come and go, facts are here to stay”. Pohl was almost notorious for his contempt for theoretical physics, though a closer look at his life reveals evidence of his high regard for it. At Heidelberg he also made the acquaintance of James Franck, studied with him at Berlin, later became his colleague at Göttingen, and eventually lost contact with him in 1933 after he resigned and left Germany because he opposed National Socialist policy against Jewish citizens. An important friendship developed between the two.

It was in Berlin (1904–1914) that Pohl’s scientific personality took its definitive form. There he came to know Paul Drude, Heinrich Rubens, Emil Warburg, Walter Nernst, and their scientific work. While in the scientific colloquium, he reviewed Philipp Lenard’s publications on luminescence. During his vacations he went home to Hamburg, where he conducted experiments on X-ray diffraction that were published in 1908. He had not forgotten Röntgen’s discovery, the object of his childhood fascination.

But Pohl was also intrigued by the physics of gas discharge, the prerequisite of Röntgen’s discovery. His first three publications, before his dissertation under Warburg in 1906, were on radiation of light caused by gas ionization. They soon gave his essential ideas for his first theses on the movement of electrons in solids. He qualified to lecture in 1911 with a research thesis, on X rays, that demonstrated that all diffraction experiments were ultimately inconclusive.

However, as early as 1904, Pohl had been working in a new field, solids. He had started with the photoelectric effect on metal surfaces, which had just gained great importance for science and metrology. With his colleague Peter Pringsheim he studied the effects of the state of polarization and the angle of incidence of light on the number of electrons emitted from surfaces of, for example, solid platinum, and also the quantum yield (electrons per light quantum). They utilized the new method of cathode sputtering to form metal surfaces. Moreover, in 1912 they developed their own experimentation technique, the evaporation of reflecting metal surfaces at pressures of 10⁻³ mm to 10⁻⁴ mm mercury. Their mastery of experimentation as well as their feel for important developments (such as quantum theory) brought the first major recognition for their work until 1914, just before World War I. During the war Pohl was a captain assigned to the development of radio technology.

In 1916 Pohl was offered an associate professorship of experimental physics at Göttingen University. However, he was not able to assume the post until after the war. In 1920 he became full professor and was named director of the First Physics Institute. In 1922 he married Auguste Madelung, sister of the physicist Erwin Madelung; they had two daughters and one son. With the newly appointed Franck and Max Born. Pohl started what came to be known as the splendid era of Göttingen as one of the world’s centers for atomic physics; it was, however, brutally terminated by the National Socialists’ accession to power in 1933 and the emigration of Franck, Born, and other Göttingen physicists.

The students at Göttingen had the choice among Pohl, the master of experimentation and demonstration; Franck, the genius at reconciling experiments with theory; and Born, the absolute theorist. The relationship among these three had its tensions; their personalities, fields of work, and opinions were too different. Franck’s and Born’s research met with great recognition from the beginning. Pohl’s work remained virtually unknown until the 1930’s.

In 1920 Pohl’s interest had turned from the external to the internal photoelectric effect. From then on, he was especially interested in all phenomena related to electric processes within solids. The internal aspects of solids, however, were to many physicists, such as Wolfgang Pauli as early as 1931, only an “order-of-magnitude-physics” or simply “dirt effects.” How could these lead to any major insights into nature? Indeed, one big problem was the low grade of purity of the crystals available in nature. The production of very pure alkali halides (first by Spiro Kyropoulos) in 1925 was the first major achievement of Pohl’s Institute. Alkali halides as the ideal prototype then became an important object of study at Pohl’s institute within the framework of a comprehensive research program pursued until World War II.

Analogous to the successful case of atomic physics, the existence of simple physical laws would be shown particularly by means of the optical and electrical characteristics of crystals. For instance, “multicolor” phenomena of light absorption would have a simple explanation, just like “multicolor “phenomena of gas discharges. The results achieved in these studies had a significance that went far beyond alkali halides as prototype, as Frederick Seitz put it in his review in 1946, which greatly influenced the future course of solid-;state physics; “The properties of the alkali halides. . . have gradually provided us with a better and better understanding of some of the most interesting properties of all solids.”

This was especially true of the optical and electrical explanation of a specific part of the visual spectrum, the absorption band of which was finally attributed to atomic defects joined with separable electrons, known after 1930 on as Farbzentren (color centers). The development of semiconductor technology and physics from 1926 on—especially with the rectifying substance copper dioxide (Cu₂O) used by the company Siemens and Halske in Berlin—was instrumental in the long discussions on the exact structure of these point defects. Based on the rejection of interstitial lattice ions being the primary carriers of current in Pohl’s alkali halides, Walter Schottky developed his famous thesis of defects, named for him, stating that the number of anion and cation vacancies in a crystal lattice were equal. At an international conference in Bristol, England, in 1937, the most important feature of which was a comprehensive review lecture by Pohl on alkali halide research, the color center was definitively identified as an electron in an anion vacancy. This conference was as significant to the physics of the nearly perfect crystal as the first Solvay Conference was for quantum theory. Defects in crystals and their significant effects on many qualities of solids were becoming increasingly important as objects of research in physics and engineering.

Other studies conducted by Pohl’s school were on ion conductivity, photochemical processes (in connection with silver saltes), and the luminescence of alkali halides and related substances. Pohl’s most important colleagues were Bernhard Gudden (even before 1925), Rudolf Hilsch, and Erich Mollwo. Pohl was the one who determined how and in which direction research was to be conducted, but the results depended largely on the others’ skill. What made these results greatly significant for the subsequent development in the theoretical field were the meticulousness and creative imagination with which the institute utilized equipment and conducted experiments. Pohl’s feel for broad interconnections as well as for the limitations of big programs, and the ability of his colleagues to find questions answerable through experimentation and to answer them. These results were no doubt achieved also through Pohl’s persistent adherence, as much as possible, to irrefutable quantitative facts obtainable through experiments, as well as through his excellent—even though authoritarian—leadership and his knack for wise management (especially in his contacts with the university, the institutions promoting research, and industry). His institute also conducted research of an international order but of an entirely different nature, such as the first important tests leading to the discovery of vitamin D (absorption spectroscopy of cholesterol, 1926) and research on the eyes of spiders.

Many of the results also led to technologically important developments. One was already mentioned: the production of artificial crystals. The discovery of crystal luminescence, for example, led to the design of the scintillation counter; photo-chemistry led to a better understanding of the processes in photographic layers; the production of thin layers and the observation of the reduction in reflection led to the optical coating of lenses. The first working amplifier crystal (analogous to the three-;electrode amplifier tube) was built at Pohl’s institute by R. Hilsch in 1938. But it can hardly be considered the direct precursor of the transistor, since it was basically unusable for steady operation and common frequencies. Pohl was highly interested in the technological significance of scientific results, but he regarded basic research as the exclusive duty of his university institute.

Pohl’s lectures, speeches, and textbooks played an instrumental role in the way the teaching of physics developed. His experimental lectures—called “Zirkus Pohl” with a mixture of respect and mockery—have been shaping university teaching for over thirty years, adding important innovations in method, such as the elimination of fixed lecture desks and the introduction of slide presentation of instruments and experiments. His university textbooks, which were even more influential, had appeared in fifty-one editions in Germany by the time of his death. They have been translated into several languages.

BIBLIOGRAPHY

I. Original Works. There is no scientific bibliography or edition of Pohl’s publications. There are, however, lists in Poggendorff, V, 989–990; VI, 2037–2038; and VIIa, pt. 3, 601–602.

Between 1905 and 1955 Pohl’s name appeared as author or coauthor of about 170 articles. His family, in Göttingen, owns a collection of special editions in bound single volumes. The Deutsche Museum in Munich owns a copy of their catalog. There were further publications after 1955. For the period between 1905 and 1938 there is a printed list of Pohl’s publications in Jahrbuch der Deutschen Akademie der Luftfahrtforschung, 2 (1939), 118–124. Besides his textbooks Einführung in die Elektrizitätslehre (Berlin, 1927); Einführung in die Mechanik und Akustik (Berlin, 1930); Einführung in die Optik (Berlin, 1940; after 1954, . . . Optik und Atomphysik), he published two major monographs: Die Physik der Röntgenstrahlen (Brunswick, 1912) and Dielichtelektrischen Erscheinungen (Brunswick, 1914), with Peter Pringsheim.

There are archival documents, such as letters from and to Pohl, manuscripts, and tape recordings of interviews with him, at various locations; some copies are at the Deutsches Museum, Munich. Unfortunately, there is no longer a record of almost the entire correspondence of Pohl before 1945. For a more detailed listing, see Jürgen Teichmann, “Für Geschichte der Festkörperphysik-;Farbzentrenforschung bis 1940” (Stuttgart, 1988). A tape recording of an interview with Pohl has been published: “Statement from R. Pohl to C. A. Hempstead, 25 July 1974, in Krefeld,” Rosemarie Teare, trans., in Nevill F. Mott, The Beginnings of Solid State Physics (London, 1980), 112–115. See also Joan Warnow-;Blewelt and Jürgen Teichmann, comps., Guide to Sources for History of Solid State Physics (New York, 1989).

II. Secondary Literature. Ernst Caspari, “The Lectures of Professor Robert Pohl in Göttingen,” in American Journal of Physics, 19 (1951), 61–63; Walter Gerlach, “Robert Wichard Pohl 10.8.1884–5.6.1976,” in Jahrbuch der Bayerischen Akademie der Wissenschaften, 1978, 214–219; and Bernhard Gudden, “R. W. Pohl zum 60, Geburtstag,” in Die Naturwissenschaften, 32 (1944), 166–169. See also Frederick Seitz, “Color Centers in Alkali Halide Crystals,” in Reviews of Modern Physics, 18 (1946), 384–408; R. W. Pohl, Gedächtniskolloquium (Göttingen, 1978).

JÜrgen Teichmann