Photo from the Nobel Foundation archive.
Gustav Ludwig Hertz
The Nobel Prize in Physics 1925
Born: 22 July 1887, Hamburg, Germany
Died: 30 October 1975, Berlin, East Germany
Affiliation at the time of the award: Halle University, Halle, 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, Gustav Hertz and James Franck 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.