Editing Franck–Hertz experiment (section)

== Experiment ==
[[File:Franck-Hertz en.svg|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.<ref name=FH1 /> |alt=Graph. The vertical axis is labelled "current", and ranges from 0 to 300 in arbitrary units. The horizontal axis is labeled "voltage", and ranges from 0 to 15 volts. The curve is described in the article's text.]]

Franck and Hertz's original experiment used a heated vacuum tube containing a drop of [[mercury (element)|mercury]]; they reported a tube temperature of 115&nbsp;°C, at which the vapour pressure of mercury is about 100 [[Pascal (unit)|pascals]] (about a thousandth of the atmospheric pressure).<ref name=FH1 /><ref>{{cite web |title=The vapor pressure of mercury |first1=Marcia L. |last1=Huber |first2=Arno |last2=Laesecke |first3=Daniel G. |last3=Friend |date=April 2006 |publisher=National Institute of Standards |page=5 |url=http://www.boulder.nist.gov/div838/SelectedPubs/NISTIR.6643.pdf |access-date=2014-04-08 |archive-date=2016-12-24 |archive-url=https://web.archive.org/web/20161224075259/http://www.boulder.nist.gov/div838/SelectedPubs/NISTIR.6643.pdf |url-status=dead }} NISTIR 6643.</ref> A contemporary Franck–Hertz tube is shown in the photograph. It is fitted with three electrodes: an [[electron]]-emitting, hot [[cathode]]; a metal mesh [[Control grid|grid]]; and an [[anode]].  The grid's [[electric potential|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.<ref name=Brandt />

[[File:FHlines.svg|thumb|right|upright=0.5|Wavelengths of light emitted by a mercury vapour [[mercury vapor lamp|discharge]] and by a Franck–Hertz tube in operation at 10&nbsp;V. The Franck–Hertz tube primarily emits light with a wavelength near 254 nanometres; the discharge emits light at many wavelengths. Based on the original 1914 figure.<ref name=FH2 />]]

The graphs published by Franck and Hertz (see figure) show the dependence of the electric current flowing out of the anode  upon the electric potential between the grid and the cathode.
* At low potential differences—up to 4.9 volts—the current through the tube increased steadily with increasing potential difference. This behaviour is typical of true vacuum tubes that do not contain mercury vapour; larger voltages lead to larger "[[space-charge limited current]]".

* At 4.9 volts the current drops sharply, almost back to zero.
* The current then increases steadily once again as the voltage is increased further, until 9.8 volts is reached (exactly 4.9+4.9 volts).
* At 9.8 volts a similar sharp drop is observed.
* While it is not evident in the original measurements of the figure, this series of dips in current at approximately 4.9 volt increments continues to potentials of at least 70 volts.<ref name=Thornton2 />

Franck and Hertz noted in their first paper that the 4.9&nbsp;eV characteristic energy of their experiment corresponded well to one of the wavelengths of light emitted by mercury atoms in [[Electric discharge in gases|gas discharges]]. They were using a quantum relationship between the energy of excitation and the corresponding [[wavelength]] of light, which they broadly attributed to [[Johannes Stark]] and to [[Arnold Sommerfeld]]; it predicts that 4.9&nbsp;eV corresponds to light with a 254&nbsp;nm wavelength.<ref name=FH1 /> The same relationship was also incorporated in Einstein's 1905 photon theory of the [[photoelectric effect]].<ref name=Pais /> In a second paper, Franck and Hertz reported the optical emission from their tubes, which emitted light with a single prominent wavelength 254&nbsp;nm.<ref name=FH2 /> The figure at the right shows the spectrum of a Franck–Hertz tube; nearly all of the light emitted has a single wavelength. For reference, the figure also shows the spectrum for a mercury gas discharge light, which emits light at several wavelengths besides 254&nbsp;nm. The figure is based on the original spectra published by Franck and Hertz in 1914. The fact that the Franck–Hertz tube emitted just the single wavelength, corresponding nearly exactly to the voltage period they had measured, was very important.<ref name=Brandt />