Franck-Hertz's experiment is one of the best ways to show the quantum properties of atoms. The experiment verifies the excitation energy of the Hg atoms through collisions with free electrons. Franck-Hertz tubes consist of an oxide cathode, gallery anode and a collection electrode. The evacuated tube contains a drop of Hg. Before starting the measurements, the pipe must be heated to 200 °C, partly to increase the pressure of the Hg steam and partly to obtain a sufficient probability of collision. When the temperature in the tube has become constant, the cathode is heated and a direct voltage which can be varied in the range 0 to 50 V is connected between the cathode and the anode. When the counter voltage is set to about -1,5 V and the anode voltage increases slowly from 0 V, the electrons will soon reach speeds in the acceleration range of the tube so that an increasing part of the primary current reaches the collector electrode. As the velocity of the electron flux approaches the lowest excitation level of the Hg atoms (4,9 eV), the electrons of the Hg atoms are capable of leaving the atoms and thus losing some of their velocity. The collector current drops to a minimum value. The Hg atom then emits its 253,7 nm ultraviolet line and then returns to its normal energy level. As the acceleration voltage increases further, the current reaching the collector electrode will increase again. This process is repeated periodically many times.
