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 provids 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.

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


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

SETUP TIME: 30 min.

Go back to Lecture-Demonstration Home Page.