FRANCK-HERTZ EXPERIMENT

Film : The Franck-Hertz Experiment

Length(min.):30, Color: No, Sound: Yes , a PSSC Film

Byron L. Youtz, Reed College with an epilogue by James Franck

The aim of the film is to show the existence of discrete energy states of atoms. The classical Franck-Hertz experiment is performed, which establishes that the smallest energy that an electron can impart to an atom of mercury in an inelastic collision is 4.9 electron volts.

This film should be used with Sections 34-1 and 34-2 of the PSSC text.

Professor Youtz points out that the light emitted by excited mercury atoms appears in the form of line spectra. Since mercury atoms (or any other kind) can lose only discrete amounts of energy - proportional to the frequency of their spectral lines one might predict that these atoms can only gain discrete amounts of energy. This should be true whether this energy is supplied by photons, as in the absorption of light, or by any other means. It is noted that this hypothesis was originally tested in 1914 by James Franck and Gustav Hertz, who studied the energy transferred in inelastic collisions between electrons and mercury atoms. Professor Youtz then describes the apparatus he will use to repeat that experiment. The process of energy transfer by inelastic collisions, which lies at the heart of the experiment, is illustrated with the help of a mechanical model.

In a specially designed tube , electrons are accelerated through mercury vapor and are detected by an ammeter in the anode circuit. Professor Youtz shows that with very little mercury vapor in the tube the current rises steadily as the accelerating voltage is increased. If the density of the mercury vapor is increased, he predicts a steady rise in anode current until the energy of the electrons reaches the minimum value for an inelastic collision to occur. At this point the current should decrease as the accelerating voltage is increased.

A pen recording of the anode current vs. the accelerating voltage shows the current rising and falling at regularly spaced intervals of 4.9 volts.

From an examination of the data it is concluded that 4.9 electron volts is the smallest amount of energy which can be absorbed by a mercury atom. It is calculated that this is the same amount of energy that is lost by a mercury atom when it emits a photon in the 2537 Angstrom spectral line. This experiment implies that the mercury atom can exist only in states of discrete rather than continuous energies.

In an epilogue, Professor Franck discusses another experiment in which he and Hertz established that mercury atoms, excited by the bombardment of 4.9-ev electrons, emitted light of only one wave length - 2537 Angstrom.