Photo from the Nobel Foundation archive.
The Nobel Prize in Physics 1925
Born: 26 August 1882, Hamburg, Germany
Died: 21 May 1964, Göttingen, West Germany (now Germany)
Affiliation at the time of the award: Goettingen University, Göttingen, Germany
Prize motivation: "for their discovery of the laws governing the impact of an electron upon an atom."
James Franck and Gustav Hertz received their Nobel Prize one year later, in 1926.
Prize share: 1/2
After the publication of Niels Bohr's theory on the structure of the atom, James Franck and Gustav Hertz conducted an experiment in 1913 to verify it. A potential difference was applied to a tube containing a low-pressure gas. When the potential difference was increased, the current flowing through the tube also increased until it reached a certain voltage, when it suddenly declined. The result supported Niels Bohr's theory, in which electrons can only have specific, discrete energies. The potential difference increased the free electrons' mobility until, at a certain energy level, they jumped to a higher-energy orbit instead.