© Wikimedia Commons, GNU Free Documentation License, Version 1.2. Author: Peng, 1 July 2005.
In 1922, Otto Stern and Walther Gerlach performed a landmark experiment with silver atoms. These atoms have a single electron in their outermost shell, and thus the magnetic moment of the atoms is essentially that of the unpaired outer electron. Stern and Gerlach directed a beam of silver atoms through a region of the inhomogeneous magnetic field indicated by the N-S field lines on the commemorative plaque shown here. As the atoms passed through the magnetic field, they were deflected from a straight path as their internal magnetic moments interacted with the external field. Stern and Gerlach expected to find a wide spread of possible paths, corresponding to all the possible orientations of the atomic magnetic moments. They expected to see a uniform blob on their atom detector placed after the magnet. Much to their surprise, the beam split into two distinct parts, indicating that the angular momentum associated with the electrons' magnetic moment had the value 1/2, with projections of along the direction of the external field. The values of the orbital angular momentum, l, as discussed in Unit 5, are 0, 1, 2, 3... with projections running from -l to +l in steps of one, and thus 1, 3, 5, 7... different projections on a fixed axis. Two projections and an angular momentum of 1/2 were startling discoveries. (Unit: 6)