deletions | additions       diff --git a/The_basic_apparatus_of_Franck__.tex b/The_basic_apparatus_of_Franck__.tex  index b9a2fe6..3b2daa2 100644  --- a/The_basic_apparatus_of_Franck__.tex  +++ b/The_basic_apparatus_of_Franck__.tex 
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The basic apparatus of Franck Hertz experiment is shown in Fig. \ref{fig:appartus}. 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 not  necessary for argon and neon, but will be unnecessary for some atoms such as mercury. any atoms; however, it add more flexibility to the experiment.  The function of the control grid is to attract more electrons from the filament. Electrons are emitted from the filament when the a  heating voltage is applied.