how a beam of atoms first split in two

The Mechanism

In February 1922, at the Physikalisches Institut at the University of Frankfurt, Otto Stern (then 33) and Walther Gerlach (then 32) ran an experiment Stern had conceived in 1921. They heated silver in a small oven so atoms streamed out as a thin beam, then sent the beam through the slot of a non-uniform magnetic field designed by Gerlach, and let it land on a glass collector plate. Classical physics predicted the beam should spread into a smear: a continuous spread of orientations for the silver atoms' magnetic moments. Bohr-Sommerfeld theory predicted something stranger — that the orientation would be *quantized*, allowed only in discrete directions, splitting the beam into a discrete number of separate spots. On the night of 7-8 February 1922, working alone in a smoke-filled lab, Gerlach finally developed a plate and saw two clean parallel deposits of silver. The beam had split in two. Gerlach photographed the splitting under a microscope, mounted the print on a postcard, and mailed it to Niels Bohr in Copenhagen with the line, in translation: *"Attached is the experimental proof of directional quantization. We congratulate you on the confirmation of your theory."* Stern's cheap, sulfur-rich cigars are credited with helping the result: their fumes blackened the silver into a visible deposit, which on a clean plate would have been invisible. The experiment was the first direct experimental confirmation that the orientation of angular momentum in a magnetic field is quantized, not continuous, and is now read as the first observation of what would later be called electron spin (Uhlenbeck and Goudsmit, 1925).