Centenary of the Franck-Hertz experiments

James Franck and Gustav L. Hertz were awarded the Nobel Prize in Physics in 1925 for their discovery of the laws governing the impact of an electron upon an atom. Franck and Hertz were greatly influenced by the experimental techniques of Philipp Lenard, who had been awarded the Nobel prize in physics in 1905 for his work on cathode rays. Franck and Hertz adopted Lenard's technique because they intended to construct a kinetic theory of electrons in gases, and Lenard's triode-like apparatus was an appropriate tool for their experimental research. Franck and Hertz thought that the first inelastic impacts could produce ionization as well as emission of radiation. Indeed, their spectroscopic experiments gave a value for the smallest energy quantum which could be transferred that agreed within the limits of accuracy with the product h?, where h is Planck's constant and ? is the frequency corresponding to the spectral line.