The Franck-Hertz experiment is an experiment that demonstrates the quantum states of atoms, and can easily be carried out at home using equipment from analogue radios.
It works in the following way; inside a gas-filled tube a voltage is applied, and as the voltage increases it creates peaks and troughs in current, rather than a continuous rise in current as may be expected.
This is because the energy of the electrons increases to a level where it matches the energy level of the gas, and as the energy increases beyond that, the current dips again as the electrons require more energy to move to the next energy level.
This experiment was one of the first to demonstrate the quantum nature of atoms.
