The Franck-Hertz demonstration

A positive grid is added to a cathode ray tube between the cathode and the anode. The tube is filled with low pressure gas. The tube was not completely evacuated. The positive voltage on the grid is less than the anode voltage.

As the grid voltage is increased, the anode current increases as expected, but the voltage-current curve is found to be, not the expected curve but a wavy line, with peaks that are evenly spaced.

Different gas fillings, (mercury vapor and neon etc.), give similar peaks at different separations. The gas in the tube causes the anode current to vary as electrons transfer their energy by collisions to gas atoms, that became excited, and then an instant later, emit the energy as light photons.

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