The Franck–Hertz experiment: Seeing cross sections

We re-examine the classic 1914 Franck–Hertz experiment, which was crucial both for completing the old quantum theory and for providing evidence for the quantized (Bohr) structure of atoms. Our motivation is twofold: first, to revive didactic interest in the Franck–Hertz experiment, and second, to reduce the difficulty of understanding electron–atom scattering cross sections. We perform the experiment using both neon (Ne) and mercury (Hg), accounting for the differences between measurements on the two gases using semiquantitative models. The present paper goes beyond the standard analysis of current–voltage characteristics in the Franck–Hertz experiment, which is typically used to extract the excitation energies of Hg (or Ne) atoms. The diversity and richness of electron-scattering phenomena accessible with readily available equipment underscore the enduring pedagogical value of the Franck–Hertz experiment at both undergraduate and advanced levels.