A beam of silver atoms split cleanly in two, and revealed electron spin

In 1922 Otto Stern and Walther Gerlach fired silver atoms through a magnetic field, expecting a smooth spread across the detector. Instead the beam split into two distinct spots -- the first direct evidence that atomic properties like spin are quantized rather than continuous.

— Walther Gerlach and Otto Stern, Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld — Zeitschrift fur Physik, 1922