PURPOSE: Demonstrate that the bound electrons in an atom can only occupy discrete energy levels, by determining the quantum of energy such an electron can absorb.

DESCRIPTION: The oven heats the Franck-Hertz tube so that the mercury in the tube becomes a vapor and diffuses uniformly throughout the tube. The cathode filament provides a source of electrons which are accelerated through a variable potential to a perforated anode grid. While the electrons are being accelerated, they collide with the mercury atoms. Some electrons will pass through the grid, encountering a retarding potential until they reach the electrode. The current from the electrode is measured by the picoammeter and displayed using a slave meter on the overhead projector. The electron current can be plotted as a function of the accelerating voltage, indicating the energy levels of the mercury electrons. The circuit along with the accelerating voltage and anode current are displayed on the overhead projector, shown in the photograph at the right.

An excellent and very complete explanation of this experiment by Richard Dickson Piccard of Ohio University will be found at the web site Notes on Modern Physics and Ionizing Radiation section on QM States and Atomic Energy Levels.

SUGGESTIONS: DO NOT TOUCH OVEN WHILE IT IS OPERATING. Carefully follow instruction sheet which comes with apparatus.

REFERENCES: (PIRA 7B30.20)

EQUIPMENT: Franck-Hertz apparatus pre-assembled on base plate, picoammeter, overhead projector with metering projectual.

SETUP TIME: 30 min.