The measurement of the higher levels of 63P2and 61P1of the mercury atom in the Franck–Hertz experiment has been introduced into the junior and senior laboratory course by using a homemade tetrode Franck–Hertz tube. The main structure of the tube is described. The optimum operating conditions are in the temperature range between 130 and 150 °C and the collector currents are of the order of 10−9A. The additional observations of the famous Franck–Hertz experiment in the laboratory course will give the students more familiarity with the quantum behavior of atoms.