Physics 54 1-1 

THE FRANCK-HERTZ EXPERIMENT 


This experiment was performed by Franck and Hertz in 1914, following by one year Bohr's 
publication of the theory of the hydrogen spectrum. The Bohr theory, utilizing Rutherford's 
nuclear atom, is based upon a mechanical model--an electron circling about a proton in.a 
manner described by a new law of mechanics. The observations supporting the theory, and 
which necessitated a new description of atomic systems, were electromagnetic. Light is 
emitted and absorbed by atoms. The Bohr theory of hydrogen was a success because the 
energy difference between the various mechanical states of the electron-proton system corres


ponded, through the Einstein frequency condition E =hv, to observed frequencies of emitted 
and absorbed radiation. The Franck-Hertz experiment, on the other hand, was a direct 
mechanical confirmation of an essentially mechanical theory. 

The optical spectrum of mercury vapor shows distinct emission and absorption lines corresponding 
to transitions between discrete energy levels of the mercury atom. Franck and HertZ 
found thatdiscrete transitions ofthe mercury atomcould also beproducedbythe inelastic 
scattering of electrons from the atom. Consider the system of an electron with some initial 
kinetic energy incident upon a mercury atom at rest in the ground state. Ifthe electron energy 
is less than the energy required to excite the atom to its first excited state, the collision must be 
elastic. The kinetic energy of the electron-atom system cannot change. Due to the disparity of 
masses, the kinetic energy of the electron itself is essentially unchanged in the collision. If the 
electron energy equals or exceeds the energy for exciting the first level, however, the collision 
may in some cases be inelastic. The kinetic energy of the system is, in these cases, different 
after the collision than before. In an inelastic collision some of the initial kinetic energy is converted 
to potential or "excitation" energy of the atom. In due course, this energy is radiated 
from the excited atoms as light, but the primary interaction is one described in mechanical 
terms. Franck and Hertz observed such inelastic collisions by monitoring the current of 
electrons passing through a mercury vapor. 

Amaratus 

The apparatus consists of a special electron tube containing a small quantity of mercury. The 
vapor pressure of mercury in the tube is adjusted by placing the tube in a furnace whose temperature 
may be varied. Electrons emitted from the cathode must, then, traverse a controlled 
mercury atmosphere in reaching the anode ofthe tube (see Fig. 1). . 

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Fig. 1. Franck-Hertz Electron Tube and Circuit.