Robert Wichard Pohl (10 August 1884 – 5 June 1976) was a German physicist and professor of the University of Göttingen. The physical institute in Göttingen led by Pohl was one of the first schools in solid state physics and Nevill Francis Mott described Pohl as the "father of solid state physics.". See also: "Components of the solid state", Nevill Mott, New Scientist, Vol. 69, No. 993, p. 663-666 (1976) He is known for relating color in alkali metal halides with the presence of vacancies and F-centers (also called color centers), a type of crystallographic defect. He also demonstrated the first transistor based on color centers. The Gudden–Pohl effect and the Pohl torsion pendulum () are named after him.
Early years and education
Robert Wichard Pohl was born in Hamburg as the son of the naval engineer Eugen Robert Pohl and his wife Martha. She was the daughter of , founder of the private 'Dr. Wichard Lange School', and granddaughter of , who founded the first German kindergarten, together with Friedrich Fröbel.
After completing the Dr. Wichard Lange School, Pohl entered the 'Gelehrtenschule des Johanneums' in 1895 and obtained his Abitur. In the summer semester of 1903, he enrolled for studies of natural science at the University of Heidelberg. There, he met James Franck, who up until Franck's death in 1964 remained a close friend. In the winter semester of 1903, Pohl transferred to the University of Berlin, where he majored in physics. Beginning in the summer semester of 1904, he had already begun scientific work in the Physics Institute with Emil Warburg on the topic which became his doctoral thesis."Über die Einwirkung stiller elektrischer Entladung auf Ammoniak und Sauerstoff", R. Pohl, Verhandlungen der Deutschen Physikalischen Gesellschaft, VIII. Jahrgang Nr. 2 (1906), p. 11, and dissertation with the same title (1906) His first publication dates from this period,"Über das Leuchten bei Ionisation von Gasen. Zur Deutung der Versuche von Herrn B. Walter", R. Pohl, Annalen der Physik 4. Folge (1905), Vol. 17, p. 375 motivated by Bernhard Walter of the Hamburg State Physical Laboratory, where Pohl worked during his vacations, in particular attempting to observe the diffraction of X-ray radiation."Weitere Versuche über die Beugung der Röntgenstrahlen", B. Walter und R. Pohl, Annalen der Physik 3. Vol. 29 (1909), p. 331
Career up to World War I In the summer of 1906, Pohl completed his doctorate (Dr. Phil.) and took an assistantship in Berlin, working as instructor in the physics teaching labs under Heinrich Rubens, the Institute's director. He published joint articles with James Franck on ionic mobility in gases and on the propagation velocity of X-rays. From 1909 onward, he carried out research on the normal and the selective photoelectric effect in metals, and from 1910 he worked with ,"Die lichtelektrischen Erscheinungen", R. Pohl und P. Pringsheim, Verlag Friedr. Vieweg & Sohn, Braunschweig 1914 on, among other things, the technically important problem of the fabrication of metal mirrors."Über die Herstellung von Metallspiegeln durch Destillation im Vakuum", Verhandlungen der Deutschen Physikalischen Gesellschaft Band 14 (1912), S. 506 In 1910, his monograph on the remote transmission of images "Die elektrische Fernübertragung von Bildern", R. Pohl, Verlag Friedr. Vieweg & Sohn, Braunschweig 1910 appeared, and in 1912 he completed the Habilitation. In an addendum, his Habilitation thesis "Die Physik der Röntgenstrahlen", R. Pohl, Verlag Friedr. Vieweg & Sohn, Braunschweig 1912 contains a discussion of Max von Laue's discovery of X-ray diffraction.
Following his Habilitation, Pohl began giving lecture courses on experimental physics, which also motivated him to start acquiring a private collection of lecture-demonstration apparatus. He also performed demonstration experiments at the meetings of the German Physical Society.Sitzung vom 20. Nov. 1914: "… Ferner demonstriert Hr. R. Pohl einige Vorlesungsversuche". Verhandlungen der Deutschen Physikalischen Gesellschaft, 30. Nov. 1914 By the outbreak of World War I, he had published 54 scientific articles and three books.
When war broke out, Pohl attempted unsuccessfully to volunteer for military service, but was refused for health reasons. His suggestion, together with Erich Regener, to set up and operate privately-funded diagnostic X-ray apparatus in two military reserve hospitals was gratefully accepted. In November 1914, he began cooperating with military radio operators on locating enemy transmission stations; this led to his appointment to a position as chief engineer with the rank of captain on the Board of Transport Examiners (VPK), which he held until the end of the war.
Professorship in Göttingen In February 1916, Pohl received the offer of an associate professorship at the University of Göttingen (as successor to Eduard Riecke), but due to the war, he was unable to accept the position until early 1919. In his baggage for the move to Göttingen were more than 40 boxes with apparatus for his lectures. Because of the offer of a professorship at the Technical University of Stuttgart in September 1919, Pohl was promoted to Full Professor in Göttingen in December 1920 and became director of the 1st Physical Institute there. In June, 1922, he was offered a position in Würzburg, which he also refused. Thus, he was a participant in the golden age of physics in Göttingen during the 1920s as one of the three full professors there, along with James Franck (director of the 2nd Physical Institute) and the theoretician Max Born.
On Christmas 1922, Pohl married Tussa Madelung, the sister of Erwin Madelung, who was a research assistant in Göttingen when Tussa moved there from Strasbourg with her family in May, 1920. Robert and Tussa had three children: Ottilie, Eleonore, and Robert Otto, later physics professor at Cornell University.
Color centers
Photoelectric observations – however not from surfaces, as in Berlin, but rather in bulk insulators – were started by Pohl together with his assistant in 1919. (The research work from this period was described in detail in Biographische Notizen von Robert Wichard Pohl: "Erinnerungen an die Anfänge der Festkörperphysik in Göttingen und Lebenslauf und politische Haltung von R. W. Pohl", R. Pohl und H. Pick(first part).) They discovered that diamond crystals become electrically conducting after irradiation with light."Über lichtelektrische Leitfähigkeit von Diamanten", B. Gudden und R. Pohl, im September 1919 Zeitschrift für Physik Band 3 (1920), p. 123 Later, they observed the same effect in the alkali halide sodium chloride; however only after it had been colored as a result of X-ray irradiation. Systematic observations of this coloration effect using artificially-prepared crystals "Electron conductivity and photochemical processes in Alkali-Halide crystals. R.W. Pohl, Proc. Phys. Soc. vol. 49, extra part, p. 3, 1937" and "Zusammenfassender Bericht über Elektronenleitung und photochemische Vorgänge in Alkalihalogenidkristallen", R. Pohl, Physikalische Zeitschrift Vol. 39 (1938), pp. 36-54 led to the discovery of color centers, which were investigated in detail in the following years."Ein Verfahren zur Herstellung großer Kristalle", S. Kyropoulos, Zeitschrift für anorganische und allgemeine Chemie Vol. 154 (1926), p. 308 By inserting three electrodes into a potassium bromide crystal, Pohl and Rudolf Hilsch were able to demonstrate the first model of a transistor based on color centers in 1938."Steuerung von Elektronenströmen mit einem Dreielektrodenkristall und ein Modell einer Sperrschicht", R. Hilsch und R. Pohl, Zeitschrift für Physik Vol. 111 (1938), p. 399
Alongside this research work in his own institute, Pohl collaborated with his scientific colleagues in a variety of ways. With the zoologist Alfred Kühn, he investigated the color perception of bees;"Dressurfähigkeit der Bienen auf Spektrallinien", A. Kühn und R. Pohl, Die Naturwissenschaften Vol. 9 (1921), p. 1 for the chemist Adolf Windaus, he applied optical spectroscopy to the separation of Ergosterol from Cholesterol."Zum optischen Nachweis eines Vitamins", R. Pohl, Die Naturwissenschaften, Vol. 15 (1927), p. 433 He helped the archeologist to photograph antique vases without disturbing highlights."Über die Vermeidung störender Reflexe beim Fotografieren griechischer Vasen", Kurt Müller, Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Altertumswissenschaften, Vol. II, Nr. 5 (1937), p. 103 He gave active support to his student Hans Joachim Pabst von Ohain when the latter began the first experiments on jet propulsion following completion of his doctoral thesis, in the Physics Institute but privately financed."Meine Liebe war die reine Physik" (PDF; 2,1 MB), Gespräch mit H. J. Pabst von Ohain, Georgia Augusta Universität Göttingen, Spektrum, Vol. 2 (1995). p. 10
Introductory lecture courses on physics For Pohl, the introductory lecture courses on physics were important from the very beginning; he frequently contributed new ideas for demonstration experiments, which he had developed and used in his lectures and textbooks, to the scientific literature. The first edition of his famous introductory texts in physics, his "Electromagnetism", was published in 1927. In 1930, the companion volume on "Mechanics and Acoustics" appeared, and it was extended from the third edition on to include "Thermodynamics". The third volume of the series, "Optics", was first published in 1941, and from the 9th edition in 1954 it was extended to include "Atomic Physics".
A new chapter on "Quantum Optics of Solids" summarized the research work at Pohl's Göttingen Institute. The electrical properties of solids were treated in the volume on "Electromagnetism" as well, from the 15th edition (1955) on, including the results of Pohl's work in Göttingen. After his death in 1976, the three volumes were reduced to two and these chapters were removed. In their modern form, both volumes contain videos with all together 110 experiments, many carried out using Pohl's original apparatus. Volume two also includes a video with a biography of Pohl by Ekkehard Sieker (Video 1). It also includes a video on current amplification by a three-electrode crystal, and also an audio recording of the conferral of an honorary doctorate on Ernest Rutherford in Göttingen by the Dean Max Born (1931). The later editions of the textbooks were edited by Klaus Lüders and Robert Wichard Pohl's son Robert Otto Pohl.
Post–World War II reception Pohl's attitude towards the Nazi regime was described in his autobiographical résumé at the request of the occupying forces (second part). According to this source, he never joined a political party, and his attitude towards the National Socialists ranged from reserved to hostile; he was convinced from the beginning of the eventual defeat of Germany in World War II (he had contacts to the Goerdeler Group; his contact, the teacher and lecturer Hermann Kaiser, was sentenced to death and was executed on the 23 January 1945 in Berlin-Ploetzensee). Following the war, he was active in re-establishing the University of Göttingen, until 1948 as a member of the Denazification Commission.