Hertzel

Meet Hertzel, a tube of mercury vapour who only accepts energy in exact packets.

In 1914 James Franck and Gustav Hertz fired electrons through mercury gas. As they raised the voltage, the current rose, then fell sharply at about 4.9 V, then rose and fell again every 4.9 V or so.

The dips happen because a mercury atom can only take in energy of about 4.9 eV, the gap to its first excited state. An electron with less than that just bounces off. With 4.9 eV or more, it can hand that exact amount over and slow down.

The excited atoms then give out light of wavelength λ = hc/ΔE ≈ 253 nm, ultraviolet. Glowing bands appear at evenly spaced places in the tube.

This was direct proof that atoms have separate energy levels, as Bohr had said. Franck and Hertz shared the 1925 Nobel Prize in Physics for it. Each loss is the same fixed amount, which showed that atoms take in energy only in set steps.