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INDIANA UNIVERSITY, DEPARTMENT OF PHYSICS, P309 LABORATORY Laboratory #12: Franck-Hertz Experiment: Electron Spectroscopy Goal: Verify Bohr's hypothesis that atomic excitation energies are quantized by repeating an experiment which was first carried out by J.Franck and G.Hertz (Verband Deutscher Physikalischer Gesellschaften, 16 (1914) 457), 12 years before quantum mechanics was developed. Equipment: Franck-Hertz tube, oven, thermostat (NEVA), power supply -50V DC, 6.3V AC (NEVA mains rect.), electrometer (Keithley 414A), variable 15 resistor, variable battery supply, 100:1 voltage divider, 2 instrumentation amps (Vernier), LabPro Box (Vernier). (A) The physics V o Δ V Figure 1 Electrons are ejected from a heated cathode and accelerated toward a grid that is at a positive potential V o relative to the cathode. Some electrons pass through the grid and reach the plate, which is kept at a slightly lower potential V p =V o Δ V . The current due to the electrons which reach the plate is measured by a picoAmmeter. The tube is filled with mercury vapor. The Hg density depends on the tube temperature. The density can be calculated from the pressure p that is given by the law of Clausius-Clapeyron: T C B T A p log / log + + = (1) Here, the vapor pressure p is in Torr, the temperature in K, the logarithm is to base 10, and the
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