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In the Franck-Hertz experiment electrons in a vacuum tube containing a small amount of vapor are accelerated towards an anode grid. Beyond the grid is a second electrode held at a small negative potential with respect to the anode, so that only electrons with a certain minimum energy can reach the second electrode. The current arriving at this electrode is monitored as the accelerating voltage is increased. The schematic shows the arrangement. In general, the current rises with the accelerating voltage, but a series of superposed decreases is also observed as electrons acquire enough energy to excite atoms of the vapor. The energy lost in the excitation leaves the electron with too little energy to reach the second electrode. From the spacing of successive minima, the ionization potential of the vapor atoms can be derived. The original apparatus used mercury vapor heated to about 180C. This apparatus uses argon at room temperature, which is much easier to control, and up to five minima can be easily obtained with manual plotting. The argon tube is enclosed in the metal casing of the unit, which also shields it from stray environmental voltages.
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