Centenary of the Franck-Hertz experiments

In 1925 the Nobel Prize in Physics was awarded jointly to James Franck (1882-1964) and Gustav L. Hertz (1887-1975) "for their discovery of the laws governing the impact of an electron upon an atom." They conducted an ingenious set of experiments in 1913 and 1914 in which they measured the ionization potential of different gases; studied collisions between gas molecules and slow electrons; developed a theory of ionization by collision; determined the ionization potential of mercury vapor with the method of electron collision; and carried out spectroscopic experiments with mercury vapor which yielded the spectral line 253.6μμ (= 2536Å) by electron collisions.