Robert Pohl

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.". 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 Göttingen, City Cemetery: The gravestone of Prof. Robert Wichard Pohl and his father-in-law, Prof. Otto Wilhelm Madelung, as well as their wives, Tussa Madelung Pohl and Ottilie Franziska Madelung, and the Pohls' daughter, Ottilie Pohl. 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. His first publication dates from this period, 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.

Monorail: RWP demonstrates the stabilization of a monorail vehicle using a gyroscope (R. W. Pohl, Mechanics. all editions).

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 , on, among other things, the technically important problem of the fabrication of metal mirrors. In 1910, his monograph on the remote transmission of images appeared, and in 1912 he completed the Habilitation. In an addendum, his Habilitation thesis 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. By the outbreak of World War I, he had published 54 scientific articles and three books. (first part).) They discovered that diamond crystals become electrically conducting after irradiation with light. 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 led to the discovery of color centers, which were investigated in detail in the following years. 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.

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; for the chemist Adolf Windaus, he applied optical spectroscopy to the separation of Ergosterol from Cholesterol. He helped the archeologist to photograph antique vases without disturbing highlights. 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.

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". An intense spatial optical-interference field, projected onto the wall of the lecture hall (R.W. Pohl, "Optik", 1941. Since the 22nd edition in 2006 titled "Elektrizitätslehre und Optik") Pohl torsion pendulum used to demonstrate harmonic motion, damping and chaotic motion. 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, Following the war, he was active in re-establishing the University of Göttingen, until 1948 as a member of the Denazification Commission.

The work of Pohl's Institute became internationally known only shortly before the outbreak of World War II, when Pohl and his research assistant Hilsch were invited to a conference on The conduction of electricity in solids at Bristol in 1937. This resulted in a 1951 invitation to the University of Illinois in Urbana, including visits to Bell Telephone Laboratories, the Naval Research Laboratory and other research institutes. On this occasion, Pohl once again encountered James Franck and they renewed their friendship which had been interrupted in 1933 by Franck's forced emigration. In 1956, the first International Color Center Conference took place at Argonne National Laboratory; it was repeated at three-year intervals in the following years until 1977.

Following his retirement in 1952, Pohl dedicated his time to improving his textbooks. In an interview with his former student Heinz Pick in 1974, he described some of his experiences at Göttingen in detail.

In 1980 Nobel Prize in Physics laureate Nevill Mott summarized the significance of Pohl's research at Göttingen: "R.W. Pohl of Göttingen is in my view the real father of solid state physics."

Textbooks Einführung in die Physik, Springer, Berlin (three-volume editions) Volume 1 Mechanik und Akustik, (1st ed., 1930, 2nd ed., 1931) Mechanik, Akustik und Wärmelehre, (3rd/ 4th ed., 1941 – 18th ed. (R.O. Pohl, editor), 1983) Volume 2 Elektrizitätslehre, (1st ed., 1927 – 21st ed., 1975) Volume 3 Optik, (1st ed. 1940 – 8th ed., 1948) Optik und Atomphysik, (9th ed., 1954 – 13th ed., 1976) Einführung in die Physik, Springer, Berlin (two-volume editions) Volume 1 Mechanik, Akustik und Wärmelehre, (19th ed. (K. Lüders und R.O. Pohl, eds.), 2004; 20th ed., 2009) Mechanik, Akustik und Wärmelehre, (21st ed. 2017) Volume 2 Elektrizitätslehre und Optik, (22nd ed. (K. Lüders und R.O. Pohl, eds., 2006); 23rd ed., 2010; 24th ed. 2017) Both volumes appeared as English translations in 2017 and 2018.