The 1922 Stern–Gerlach experiment showed that the angular momentum of silver atoms has discrete, quantized values.
Otto Stern and Walther Gerlach sent a beam of silver atoms through a nonuniform magnetic field. Classical physics suggested that the beam might spread into a continuous band, because atomic magnetic moments could point in any direction. Instead, the beam split into two distinct parts.
The separated pattern demonstrated space quantization, later understood through quantum mechanics as the two possible spin projections of the silver atom’s outer electron. The experiment therefore provided a direct early illustration of quantum behavior.
The result is commonly described as measuring electron spin, but the original apparatus acted on the magnetic moment of whole silver atoms. The two-beam pattern was the central observation, not a continuous smear.