thumb|upright=0.5|Photograph of a vacuum tube used for the Franck–Hertz experiment in instructional laboratories. There is a droplet of mercury inside the tube, which is not visible in the photograph. C – cathode assembly; the cathode is hot, and glows orange. It emits electrons which pass through the metal mesh grid (G) and are collected as an electric current by the anode (A). |alt=Photograph of a sealed glass cylinder. Wires penetrate the cylinder at its top, bottom, and side. Three wires lead to a cathode assembly; the top and side wires lead to a disc and a mesh that are close and parallel to each other. The wires are attached to feedthroughs on an aluminum panel in the background.
The Franck–Hertz experiment was the first electrical measurement to clearly show the quantum nature of atoms. It was presented on 24 April 1914, to the German Physical Society in a paper by James Franck and Gustav Hertz. Therefore, Bohr had followed the instructions given in 1911 and copied the formula proposed by Lorentz and others into his 1913 atomic model. Lorentz had been correct. The quantisation of the atoms matched his formula incorporated into the Bohr model.
Experiment
thumb|left|Anode current (arbitrary units) versus grid voltage (relative to the cathode). This graph is based on the original 1914 paper by Franck and Hertz. A contemporary Franck–Hertz tube is shown in the photograph. It is fitted with three electrodes: an [[electron-emitting, hot cathode; a metal mesh grid; and an anode. The grid's voltage is positive relative to the cathode, so that electrons emitted from the hot cathode are drawn to it. The electric current measured in the experiment is due to electrons that pass through the grid and reach the anode. The anode's electric potential is slightly negative relative to the grid, so that electrons that reach the anode have at least a corresponding amount of kinetic energy after passing the grid.
The fundamental assumption of the Bohr model concerns the possible binding energies of an electron to the nucleus of an atom. The atom can be ionised if a collision with another particle supplies at least this binding energy. This frees the electron from the atom, and leaves a positively charged ion behind. There is an analogy with satellites orbiting the Earth. Every satellite has its own orbit, and practically any orbital distance, and any satellite binding energy, is possible. Since an electron is attracted to the positive charge of the atomic nucleus by a similar force, so-called "classical" calculations suggest that any binding energy should also be possible for electrons. However, Bohr assumed that only a specific series of binding energies occur, which correspond to the "quantum energy levels" for the electron. An electron is normally found in the lowest energy level, with the largest binding energy. Additional levels lie higher, with smaller binding energies. Intermediate binding energies lying between these levels are not permitted. This was a revolutionary assumption. In the Bohr model, the collision excited an internal electron within the atom from its lowest level to the first quantum level above it. The Bohr model also predicted that light would be emitted as the internal electron returned from its excited quantum level to the lowest one; its wavelength corresponded to the energy difference of the atom's internal levels, which has been called the Bohr relation.
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File:This file shows the Franck-Hertz experiment with Neon resulting in glowing regions appearing.webm|Franck-Hertz experiment with neon resulting in glowing regions appearing
File:Franck–Hertz Experiment.png|Franck-Hertz experiment with neon gas: three glowing regions
File:Franck-Hertz experiment with neon.png|Franck-Hertz experiment with neon: Anode current versus grid voltage (relative to the cathode).
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References
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Further reading
- Selection of images of a vacuum tube used for the Franck–Hertz experiment in instructional laboratories.
- Translation of Franck's Nobel lecture that he gave December 11, 1926.
- Translation of Hertz's Nobel lecture that he gave December 11, 1926.
- See also Nicoletopoulos, who died in 2013, had authored and co-authored several papers related to the Franck–Hertz experiment; these papers challenge the conventional interpretations of the experiment. See
- Franck and Hertz's original paper reported anode currents up to about 15 V, as illustrated in the figure above. Additional maxima and minima occur when current is measured to higher voltages. This paper notes that the spacing between the minima and maxima isn't exactly 4.9& V, but increases for higher voltages and varies with temperature, and provides a model for this effect.