English: Emission spectra from a Franck–Hertz tube operated as at two voltages. The lefthand spectral lines are obtained at voltages high enough to obtain a glow discharge of the mercury atoms in the tube. The single spectral line on the right is obtained at a voltage just above 4.9 V, which is sufficient to excite the mercury atoms but insufficient to cause a glow discharge. The drawing is based on the original spectra published by Franck and Hertz in 1914.^[1] The labels have been added; the original figure is unlabeled. The single, 254 nanometer wavelength of the Franck–Hertz tube corresponds well to the 4.9 V period of the electrical current through the tube. Franck himself described these features in a 1960 education film.^[2]