Photograph of a glass vacuum tube (2.7 cm diameter) used for the Franck-Hertz experiment in teaching laboratories. The tube was manufactured by 3B Scientific (part number U8482170). There's a small drop of mercury inside; the vapor pressure of the mercury is controlled by the temperature of the tube. The glowing orange dot labeled C is the hot cathode, which emits electrons. The grid labeled G is a metal screen that lets most electrons pass. The metal disk labeled A is the anode that collects the electrons.