atomic physics

In 1896, Pieter Zeeman placed sodium flame between magnet poles and watched spectral lines split. The experiment revealed the electron's charge-to-mass ratio and earned the 1902 Nobel Prize in Physics.

In 1914, James Franck and Gustav Hertz fired electrons through mercury vapor and watched energy disappear in exact 4.9 eV steps — the first direct proof of quantized atomic energy levels.

Robert Millikan suspended tiny oil drops between charged plates to measure the electron's charge. Here is exactly how he did it, what he found, and the controversy that followed.

In 1922, Stern and Gerlach fired silver atoms through a magnetic field and watched the beam split in two. Here is exactly what they saw and why it rewrote physics.

In 1897, J.J. Thomson proved cathode rays are particles 1,800 times lighter than hydrogen. Learn how his cathode ray tube experiment discovered the electron.