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Franck Hertz

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Spacer This labs demonstrates the quantization of energy absorption in atoms of mercury and helium which are being bombarded by thermal electrons. Student activities include:

a. measurement of an excitation potential (ground state to the 3P1 state) for mercury;

b. measurement of the ionization potential of mercury;

c. measurement of some of the excitation potentials of helium; and

d. measurement of the ionization potential of helium.

The apparatus used for the experimental work on mercury is similar in design and theory to the apparatus used by Franck and Hertz. Using an x-y plotter, the students obtain a plot of the electron current collected at the anode vs the retarding potential. From the position of the dips in the current the excitation potential is calculated.

The apparatus used for the experimental work on helium is slightly different. Rather than looking at the anode electron current as Franck and Hertz did, the students monitor the low energy electrons scattered out of the main electron beam and collected by a slightly positively biased collector ring in a plane at right angles to the main electron beam.

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Student Handouts


Number of set-ups available: 1

Per lab station:

1 x-y recorder/plotter pen
1 Keithley Electrometer
4 DC power supplies
1 helipot
1 digital multimeter
1 Franck-Hertz tube
1 oven 1 1 100 kŻ resister



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