Franck-Hertz experiment

Franck-Hertz experiment, in physics, first experimental verification of the existence of discrete energy states in atoms, performed (1914) by the German-born physicists James Franck and Gustav Hertz. Franck and Hertz directed low-energy electrons through a gas enclosed in an electron tube. As the energy of the electrons was slowly increased, a certain critical electron energy was reached at which the electron stream made a change from almost undisturbed passage through the gas to nearly complete...