The Franck{Hertz Experiment
Michael S. Turner, James W. Waters
In 1912 Niels Bohr developed his model of the atom, in which electrons are allowed only certain discrete orbits. Transitions between these orbits may take place only when the atom absorbs or emits quantized packets of energy E = hc. This explains the spectral lines given o ® by an excited element, such as described by Rydberg's formula for the series of spectral lines for hydrogen. Less than a year after Bohr published his theory, J. Franck1 and G. Hertz2 proved it experimentally, earning themselves the 1925 Nobel Prize in Physics. They boiled electrons o ® a hot ¯lament K, used a potential Ub to accelerate them through a vapor of an element toward a grid A (v. Fig. 1), and looked for them on a plate M, with a small retarding potential V0 = 1.5 V, to stop electrons whose kinetic energy was below a certain value. Figure 1: Circuit diagram for the Franck-Hertz experiment If the kinetic energy of the electrons is too small to excite the atoms in the vapor, the electrons collide elastically and lose almost no energy, as required by the conservation of energy and momentum, and a large current is detected at plate M. If, on the other hand, the electrons gain just enough kinetic energy 1James Franck, American (German-born) physicist, 1882-1964 2Gustav Hertz, German physicist, 1887-1975, nephew of Heinrich Hertz, who discovered radio waves
•Jan 1, 2002•0 citations