Stern–Gerlach experiment

The 1922 experiment showing that a beam of silver atoms passed through a magnetic field splits cleanly in two instead of smearing out: direct evidence that atoms can point only in certain directions.

In February 1922 Otto Stern and Walther Gerlach in Frankfurt passed a thin stream of atoms from heated silver between the poles of a specially shaped magnet and let it land on a glass plate^[1]. If the atoms, each a tiny magnet, pointed in every possible direction, the trace would be smeared out. The result was two sharp lines. At the time this was taken as proof of the "space quantization" predicted by the Bohr–Sommerfeld theory.

It later turned out that the two-way split came not from the electron's orbital motion that Bohr's theory had in mind, but from electron spin, proposed in 1925. The prediction had been right; the reason it was right had been mistaken.

The trace on the plate was so faint that at first nothing could be seen. The story goes that sulphur in the smoke of Stern's cheap cigar reacted with the silver and turned it black, bringing the lines into view^[2].