One hundred years of the Franck-Hertz experiment

The 1914 experiment of James Franck and Gustav Hertz provided a graphic demonstration of quantization properties of atoms, thereby laying the foundations of modern atomic physics. This article revisits the experiment on the occasion of its Centenary, compares traditional and modern interpretations, and focuses in particular on the link between microscopic processes, which are governed by the laws of quantum mechanics, and macroscopic phenomena as measured in the laboratory. A goal is to place the physics underlying the operation of the Franck-Hertz experiment within the context of contemporary gaseous electronics, and to that end we reach back even further in time to the 1872 kinetic equation of Ludwig Boltzmann. We also show how the experiment can be modelled using fluid equations and Monte Carlo simulation, and go further to show how non-local effects, resonances and striations in plasmas have much in common with the electron physics in the drift region of the Franck-Hertz experiment.