Franck-Hertz Experiment

Introduction

The Franck-Hertz experiment verifies that the atomic electron energy states are quantized by observing maxima and minima in transmission of electrons through mercury vapor. The variation in electron current is caused by inelastic electron scattering that excites the atomic electrons of mercury. The 1925 Nobel prize in physics was awarded jointly to Franck and Hertz for their discovery of the laws governing the impact of an electron on an atom.

Equipment

        Franck-Hertz tube

        Klinger experiment manual

        Heater

        Electrical circuit to provide voltages

        Power supplies

        Oscilloscope or digital voltmeter

        XY recorder

        Thermometer

Procedure

Set up the experiment. Turn off all voltages and then connect the circuit and filament power supply to the F-H tube. NEVER APPLY VOLTAGES TO THE TUBE UNLESS IT IS IN THE OVEN AT THE DESIRED TEMPERATURE. MAKE SURE THE FILAMENT VOLTAGE DOES NOT EXCEED 6.5V, AS YOU CAN DESTROY THE TUBE BY ACCIDENTALLY CONNECTING THE 0-30V CATHODE VOLTAGE TO THE FILAMENT CONNECTIONS.

Turn on the oven and experiment with the oven temperature. The voltages to the tube may be applied after a stable operating temperature is achieved.

Vary the anode voltage from 0 to 30 V, recording appropriate voltages and currents in your notebook (or using the XY plotter). Measure the temperature periodically and adjust the oven as required. Verify the quantization of atomic electron energy states of mercury by observing the maxima and minima of an electron current passing through a gas of mercury atoms. Determine how the atomic electron energy states of mercury affect the transmission of electrons. By heating the tube to different temperatures, determine how temperature affects the number density of mercury atoms, the mean free path of a transmission electron, and the kinetic energy of a transmission electron.

References

D. W. Preston and E. R. Dietz, The Art of Experimental Physics, Wiley, New York (1991).