Franck–Hertz Experiment

A groundbreaking experiment that provided the first direct electrical evidence of quantized atomic energy levels.

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Result

Collector Current Graph

The graph shows collector current versus accelerating voltage. The current rises overall, but repeated inelastic collisions with mercury atoms produce minima and maxima separated by about 4.9 V.

Bohr Model of Mercury

The 4.9 eV spacing in the Franck–Hertz graph corresponds to an excitation energy of the mercury atom.

The Franck-Hertz experiment, performed in 1914 by James Franck and Gustav Hertz.