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\section{experiment}  A typical arrangement of the Franck-Hertz experiment consists of an electron-emitting filament and a means of accelerating electrons to a variable potential. These accelerated electrons then bombarded atoms of the element, which are usually in a gaseous state. In order to detect the excitation of atoms, the current of electron beam can be observed. Specifically, it is expected that if the electrons have been accelerated to a potential that is equal to the discrete energy of the first excited level, some of atoms of the element will become excited, the bombarding electrons will lose most of their energy, and the collision is inelastic. If a small retarding potential exists before the anode, which is used to collect remaining electrons, electrons that have lost most of their energy will not be able to overcome it, and thus will not reach the region of collection. This should correspond to a decrease in current of the electron beam.