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Alisha Vira added The_basic_apparatus_of_Franck__.tex
over 8 years ago
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The basic apparatus of Franck Hertz experiment is shown in Figure 1. Argon tube and neon tube are used in this experiment. The tubes are highly evacuated and are filled with atoms. Within each tube, there is a filament, a control grid, an accelerating grid, a cathode and an anode. An accelerating voltage is applied between cathode and accelerating plate to provide electrons with kinetic energy. A retarding voltage is applied between anode and accelerating plate to attract more electrons to the anode. Control grid is necessary for argon and neon, but will be unnecessary for some atoms such as mercury. The function of the control grid is to attract more electrons from the filament. Electrons are emitted from the filament when the heating voltage is applied. As we increase the accelerating voltage, electrons are gaining more and more kinematic energy and they are colliding with atoms elastically until the kinematic energy reaches the first excitation level of the atom. When the first excitation level is reached, inelastic collisions occur and the electron will transfer all the kinematic energy it has to the atom. This corresponds to a dip in current detected by the anode due to the sudden decrease of electrons that reach the anode. As the accelerating voltage keeps increasing, there will be more dips, corresponding to multiple integers of inelastic collisions happened in the tube.
