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Franck and Hertz experiment

This experiment, which was a direct verification of Bohr's energy level model of the atom, was first performed in 1914. Franck and Hertz used a three-electrode tube containing mercury vapour at a pressure of about 1 mm of mercury (about 102 Pa). This is shown in the accompanying diagram.

The distance between the filament F and the grid G was considerably greater than the mean free path of the electrons in the gas at this pressure, so that many collisions were made in this region. The distance from the grid to the anode was made relatively small, however. The anode A was slightly negative compared with the grid G, so that electrons were retarded between G and A. The accelerating potential between the filament and the grid was slowly increased from zero and the current in the electrometer E rose. When a potential Vc was reached, however, the current fell a little before rising again, and this also happened for other values of the potential (2Vc, 3Vc and so on).

This can be explained as follows. As the voltage across the tube is increased the current increases, as the electrons collide elastically with the atoms of the gas, but when it reaches Vc inelastic collisions occur, and the electrons are brought practically to rest. The electric field in most of the tube is small since most of the potential drop occurs near the very thin filament. Therefore when the electrons reach the grid they have insufficient kinetic energy to overcome the retarding field between the gird and anode, and cannot reach the anode, so the anode current falls.

This shows that no increase in the energy of the atom can occur if the energy of the electrons is less than eVc. The existence of a definite series of energies with no intermediate values suggests that the atom must have a set of well-defined energy levels. A further drop at 2eVc shows electrons losing energy to two atoms in successive collisions.

The transition of electrons from one energy level to another gives the characteristic spectrum of the material. No two elements have identical energy level structures and therefore the spectrum of an element is unique. Notice that energy must be put in to raise the electrons within the atom to higher energy states.


© Keith Gibbs 2020