[1] He 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 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 his post in protest against the dismissal of fellow academics.

He assisted Frederick Lindemann in helping dismissed Jewish scientists find work overseas, before he left Germany in November 1933.

Franck participated in the Manhattan Project during World War II as Director of the Chemistry Division of the Metallurgical Laboratory.

[5] Franck 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 Frederick William University in Berlin.

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.

[14][16] 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.

[18][19] 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".

[20] 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.".

[23] 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.

He returned to Berlin, where he joined Hertz, Westphal, Hans Geiger, Otto Hahn and others at Haber's Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry, working on the development of gas masks.

Working with new, younger collaborators such as Walter Grotrian, Paul Knipping, Thea Krüger, Fritz Reiche and Hertha Sponer, his first papers at the Kaiser Wilhelm Institute examined atomic electrons in their excited state, results that would later prove important in the development of the laser.

[25] When Niels Bohr visited Berlin in 1920, Meitner and Franck arranged for him to come to the Kaiser Wilhelm Institute to talk with the younger staff without the presence of the bonzen ("bigwigs").

[26] In 1920, the University of Göttingen offered Max Born its chair of theoretical physics, which had recently been vacated by Peter Debye.

Göttingen was an important centre for mathematics, thanks to David Hilbert, Felix Klein, Hermann Minkowski and Carl Runge, but not so much for physics.

His doctoral students included Hans Kopfermann, Arthur R. von Hippel, Wilhelm Hanle, Fritz Houtermans, Heinrich Kuhn, Werner Kroebel [de], Walter Lochte-Holtgreven and Heinz Maier-Leibnitz.

He did not require his daughters to attend religious instruction classes at school,[38] and even let them have a decorated tree at Christmas;[39] but he was proud of his Jewish heritage all the same.

[41] Franck assisted Frederick Lindemann in helping dismissed Jewish scientists find work overseas, before he left Germany in November 1933.

[42] After a brief visit to the United States, where he measured the absorption of light in heavy water with Wood at Johns Hopkins University, he took up a position at the Niels Bohr Institute in Copenhagen.

[44] His original intention was to continue his research into the fluorescence of vapours and liquids, but under Bohr's influence they began to take an interest in biological aspects of these reactions, particularly photosynthesis, the process by which plants use light to convert carbon dioxide and water into more organic compounds.

[43][45][46] Franck found a position at the Polytekniske Læreanstalt in Copenhagen for Arthur von Hippel, who was now his son in law, having married his daughter Dagmar.

[47] When Germany invaded Denmark on 9 April 1940, the Hungarian chemist George de Hevesy dissolved the gold medal, along with that of Max von Laue in aqua regia to prevent the Germans from taking them.

His daughters still were, though, so they were restricted from travelling, and could not take care of their mother when she fell ill and died on 10 January 1942, although they were permitted to attend her funeral.

As part of the Manhattan Project, its mission was to build nuclear reactors to create plutonium that would be used in atomic bombs.

[57] The Metallurgical Laboratory's Chemistry Division was initially headed by Frank Spedding, but he preferred hands on work to administration.

[59] In addition to heading the Chemistry Division, Franck was also the chairman of the Metallurgical Laboratory's Committee on Political and Social Problems regarding the atomic bomb, which consisted of himself and Donald J. Hughes, J. J. Nickson, Eugene Rabinowitch, Glenn T. Seaborg, J. C. Stearns and Leó Szilárd.

[60] In 1945, Franck warned Henry A. Wallace of their fears that "mankind has learned to unleash atomic power without being ethically and politically prepared to use it wisely.

His research followed an almost straight line, from his early studies of ion mobilities to his last work on photosynthesis; it was always the energy exchange between atoms or molecules that fascinated him.