Franck-Hertz Experiment
An influential 1914 physics experiment that directly demonstrated the existence of quantized discrete energy states in mercury atoms.
Governing formula
E_loss = h ν = 4.9 eV (for Mercury vapor)
SI unit
eV (Electron-Volts)
In depth
James Franck and Gustav Hertz accelerated electrons through mercury gas. At electron kinetic energies below 4.9 eV, collisions were elastic; at 4.9 eV, inelastic collisions occurred as mercury atoms absorbed 4.9 eV to jump to an excited state, confirming Bohr's energy quantization.
Examples in the real world
Observing periodic drops in anode current at 4.9 V intervals during mercury vapor electron bombardment.