Gustav Ludwig Hertz and James Franck accelerated electrons through Mercury vapor and measured the collected electric current as the accelerating voltage changed. Repeated features in that curve indicate a preferred loss of kinetic energy in inelastic collisions. An electron must first acquire enough energy to excite the relevant atomic transition; further acceleration can enable another such collision. The Franck-Hertz experiment therefore connects an electrical current–voltage measurement with discrete atomic excitation energies.[ref]
Über Zusammenstöße zwischen Elektronen und den Molekülen des Quecksilberdampfes und die Ionisierungsspannung desselben by James Franck and Gustav Ludwig Hertz. Original collision experiment: accelerating voltage, collected current and characteristic energy loss
Physics.[ref]
https://mathshistory.st-andrews.ac.uk/Biographies/Hertz_Gustav/. Independent professional profile
Portrait of Gustav Ludwig Hertz
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Gustav Ludwig Hertz (1887-1975) was a German physicist known for his work in the fields of atomic and molecular physics. He is best recognized for his research on electron scattering and the development of the Hertz experiment, which involved the measurement of the ionization potentials of atoms. Hertz was awarded the Nobel Prize in Physics in 1925, along with James Franck, for their experiments on the impact of electrons on atoms and the resulting ionization of gases.