Franck–Hertz Apparatus

This apparatus is used to determine the first excitation energy of mercury (~4.9 eV). It consists of a mercury-filled vacuum tube with embedded electrodes and a thermostatically controlled oven, in which the vapour pressure is set by the temperature; varying the accelerating voltage and measuring the tube current produces the Franck–Hertz curve that documents the quantised energy levels. In atomic and quantum physics pupils record current–voltage variations, discuss inelastic collisions and show that the electrons' energy loss in the tube is a whole multiple of mercury's first excitation energy (~4.9 eV). Safe handling and correct heating are key routines in the exercise. Tube dimensions (ø x l): 2.6 cm x 13 cm.