Franck , James

(1882–1964) German–American physicist

Franck, the son of a banker from Hamburg, was educated at Heidelberg and Berlin where he obtained his doctorate in 1906. After distinguished war service, in which he won two iron crosses, he was appointed to the chair of experimental physics at Göttingen. Although exempt from the 1933 Nazi law that excluded Jews from public office because of his military service, he insisted on publicly resigning. After spending a year in Copenhagen, he emigrated to America in 1935 where he served as professor of physical chemistry at the University of Chicago from 1938 to 1949.

In collaboration with Gustav Hertz he produced experimental evidence of the quantized nature of energy transfer, work that won them the 1925 Nobel Prize for physics. Their experiment, conducted in 1914, consisted of bombarding mercury atoms with electrons. Most of the electrons simply bounced off losing no energy in the process. When the velocity of electrons was increased it was found that on collision with mercury atoms they lost precisely 4.9 electronvolts (eV) of energy. If an electron possessed less energy than 4.9eV it lost none at all on collision; if it had more than 4.9eV it made no difference – only 4.9eV was absorbed by the mercury atoms. Franck and Hertz had thus succeeded in showing that energy can only be absorbed in quite definite and precise amounts. For mercury the minimum amount was 4.9eV. Their results were quickly confirmed and shown to hold for other atoms.

In America Franck worked mainly on the physical chemistry involved in photosynthesis although he is better known as the author of the Franck report published in 1946. This report, actually produced by a number of distinguished scientists of whom Leo Szilard was probably the most important, was sent to the Secretary of State for War in June 1945. It argued that it was not necessary to drop the recently produced atomic bomb on Japan as its explosion on a barren island would be sufficient to force the Japanese into submission.