The angular momentum vector associated with an atomic state can take up only certain specified directions in space with respect to an arbitrarily chosen axis. The concept of space quantization was predicted theoretically by Wolfgang Pauli. In 1922, Otto Stern (German, 1888-1969) and Walther Gerlach (German, 1889-1979) sought to verify this prediction experimentally.

The apparatus of Stern and Gerlach. A beam of silver atoms passes through a nonuniform magnetic field and is deposited on a glass plat. Because the atoms posses a magnetic moment, they will be deflected by the magnetic field. The purpose of the experiment is to demonstrate the reality of space quantization.

Figure shows the apparatus of Stern and Gerlach. Silver is vaporized in an electrically heated “over” and silver atoms spray into the evacuated apparatus through a small hole in the oven wall. The atoms (which are eclectically neutral but have a magnetic moment) are formed into a narrow beam as they pass through a slit in a screen. The beam, thus collimated, then passes between the poles of an electromagnet and, finally, deposits its silver atoms on a glass plate that serves as a detector. The pole faces of the magnet are shaped to make the magnetic field as nonuniform as possible. The beam of silver atoms passes very close to the sharp V-shaped ridge in the upper pole face, where the nonuniformity of the field is greatest.