In the 1922 Stern–Gerlach experiment, what happened to a beam of silver atoms passing through a non-uniform magnetic field?

The 1922 Stern–Gerlach experiment split a beam of silver atoms into two beams in a non-uniform magnetic field.

Otto Stern and Walther Gerlach sent neutral silver atoms through a sharply varying magnetic field and detected where the atoms landed on a glass plate. Instead of forming one broadened stripe, the beam separated into two distinct parts. This showed that the atoms had only certain allowed orientations of angular momentum.

The result became an important early demonstration of space quantization and is now commonly explained using electron spin. The original experiment was performed before spin was fully understood, so Stern and Gerlach initially interpreted the result in terms of quantized orbital angular momentum.

A common mix-up is saying that the experiment directly discovered the electron’s intrinsic spin. It revealed discrete magnetic behavior, while later quantum theory identified electron spin as the key explanation. The two-way splitting is also different from the three outcomes expected for a spin-1 particle.