James Franck (German: [ˈdʒɛɪ̯ms ˈfʁaŋk] ; 26 August 1882 – 21 May 1964) was a German–American physicist who shared the 1925 Nobel Prize in Physics with Gustav Hertz "for their discovery of the laws governing the impact of an electron upon an atom."
Franck completed his doctorate in 1906 and his habilitation in 1911 at the Frederick William University in Berlin, where he lectured and taught until 1918, having reached the position of professor extraordinarius. He served as a volunteer in the German Army during World War I. He was seriously injured in 1917 in a gas attack and was awarded the Iron Cross 1st Class.
Franck became the head of the physics division of the Kaiser Wilhelm Gesellschaft for Physical Chemistry. In 1920, Franck became professor ordinarius of experimental physics and Director of the Second Institute for Experimental Physics at the University of Göttingen. While there he worked on quantum physics with Max Born, who was Director of the Institute of Theoretical Physics. His work included the Franck–Hertz experiment, an important confirmation of the Bohr model of the atom. He promoted the careers of women in physics, notably Lise Meitner, Hertha Sponer, and Hilde Levi.
After the Nazi Party came to power in Germany in 1933, Franck resigned from his position at the University of Göttingen in protest against the dismissal of fellow colleagues. He assisted Frederick Lindemann in helping dismissed Jewish scientists find work overseas, before he left Germany in November. After a year at the Niels Bohr Institute in Denmark, he moved to the United States, where he worked at Johns Hopkins University in Baltimore and then the University of Chicago. During this period he became interested in photosynthesis.
Franck participated in the Manhattan Project during World War II as director of the chemistry division of the Metallurgical Laboratory. He was also the Chairman of the Committee on Political and Social Problems regarding the atomic bomb, which is best known for the compilation of the Franck Report, which recommended that the atomic bombs not be used on the Japanese cities without warning.
James Franck was born on 26 August 1882 in Hamburg, Germany, into a Jewish family, the second child and first son of Jacob Franck, a banker, and Rebecca Nachum Drucker. He had an older sister, Paula, and a younger brother, Robert. His father was a devout and religious man, while his mother came from a family of rabbis.
In 1891, Franck enrolled at the Wilhelm-Gymnasium in Hamburg, which was then a boys-only school. Hamburg had no university then, so prospective students had to attend one of the 22 universities elsewhere in Germany. Intending to study law and economics, In 1901, Franck entered the University of Heidelberg, as it had a renowned law school. He attended lectures on law, but was far more interested in those on science. While there, he met Max Born, who would become a lifelong friend. With Born's help, he was able to persuade his parents to allow him to switch to studying physics and chemistry. He attended mathematics lectures by Leo Königsberger and Georg Cantor, but Heidelberg was not strong on the physical sciences, so he decided to go to the University of Berlin.
At Berlin, Franck attended lectures by Max Planck and Emil Warburg. For his Ph.D., Warburg suggested that he study corona discharges. He found this topic too complex, so he changed the focus of his thesis. Titled Über die Beweglichkeit der Ladungsträger der Spitzenentladung (On the mobility of ions), it would subsequently be published in Annalen der Physik.
With his thesis completed, Franck had to perform his deferred military service. He was called up on 1 October 1906 and joined the 1st Telegraph Battalion. He suffered a minor horse riding accident in December and was discharged as unfit for duty. He took up an assistantship at the Physikalische Verein in Frankfurt in 1907, but did not enjoy it, and soon returned to the University of Berlin. At a concert, he met Ingrid Josefson, a Swedish pianist. They were married in a Swedish ceremony in Gothenburg on 23 December 1907. They had two daughters: Dagmar "Daggie", who was born in 1909; and Elisabeth "Lisa", who was born in 1912.
To pursue an academic career in Germany, having a doctorate was not enough; one needed a venia legendi, or habilitation. This could be achieved with either another major thesis or by producing a substantial body of published work. Franck chose the latter route. There were many unsolved problems in physics at the time, and by 1914 he had published 34 articles. He was the sole author of some, but generally preferred working in collaboration with Eva von Bahr, Lise Meitner, Robert Pohl, Peter Pringsheim, Robert W. Wood, Arthur Wehnelt, and Wilhelm Westphal. His most fruitful collaboration was with Gustav Hertz, with whom he wrote 19 articles. He received his habilitation on 20 May 1911.
In 1914, Franck teamed up with Hertz to perform an experiment to investigate fluorescence. They designed a vacuum tube for studying energetic electrons that flew through a thin vapour of mercury atoms. They discovered that when an electron collided with a mercury atom it could lose only a specific quantity (4.9 electronvolts) of its kinetic energy before flying away. A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons just bounce off mercury atoms without losing any significant speed or kinetic energy.
These experimental results provided confirmation of Albert Einstein's photoelectric effect and Planck's relation (E = fh) linking energy (E) and frequency (f) arising from quantisation of energy with the Planck constant (h). But they also provided evidence supporting the model of the atom that had been proposed the previous year by Niels Bohr. Its key feature was that an electron inside an atom occupies one of the atom's "quantum energy levels". Before a collision, an electron inside the mercury atom occupies its lowest available energy level. After the collision, the electron inside occupies a higher energy level with 4.9 eV more energy. This means that the electron is more loosely bound to the mercury atom. There were no intermediate levels or possibilities.
In a second paper presented in May 1914, Franck and Hertz reported on the light emission by the mercury atoms that had absorbed energy from collisions. They showed that the wavelength of this ultraviolet light corresponded exactly to the 4.9 eV of energy that the flying electron had lost. The relationship of energy and wavelength had also been predicted by Bohr. Franck and Hertz completed their last paper together in December 1918. In it, they reconciled the discrepancies between their results and Bohr's theory, which they now acknowledged. In his Nobel lecture, Franck admitted that it was "completely incomprehensible that we had failed to recognise the fundamental significance of Bohr's theory, so much so, that we never even mentioned it once". On 10 December 1926, Franck and Hertz were awarded the 1925 Nobel Prize in Physics "for their discovery of the laws governing the impact of an electron upon an atom".
Franck enlisted in the German Army soon after the outbreak of the First World War in August 1914. In December, he was sent to the Picardy sector of the Western Front. He became a deputy officer (offizierstellvertreter), and then a lieutenant (leutnant) in 1915. In early 1915, he was transferred to Fritz Haber's new unit that would introduce clouds of chlorine gas as a weapon. With Otto Hahn, he was responsible for locating sites for the attacks. He was awarded the Iron Cross, Second Class, on 30 March 1915, and the city of Hamburg awarded him the Hanseatic Cross on 11 January 1916. While in hospital with pleurisy, he co-wrote yet another scientific paper with Hertz, and he was appointed an assistant professor in his absence by Frederick William University on 19 September 1916. Sent to the Russian front, he came down with dysentery. He returned to Berlin, where he joined Hertz, Westphal, Hans Geiger, Hahn, and others at Haber's Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry, working on the development of gas masks. He was awarded the Iron Cross, First Class, on 23 February 1918. He was discharged from the army on 25 November 1918, soon after the war ended.