13.5 Stern-Gerlach Apparatus

A constant magnetic field will exert a torque, but no net force on a magnetic dipole like an electron; if you think of the dipole as a magnetic north pole and south pole close together, the magnetic forces on north pole and south pole will be opposite and produce no net force on the dipole. However, if the magnetic field strength varies with location, the two forces will be different and a net force will result.

The Stern-Gerlach apparatus exploits this process by sending a beam of atoms through a magnetic field with spatial variation, causing the atoms to deflect upwards or downwards depending on their magnetic dipole strength. The magnetic dipole strengths of the atoms will be proportional to the relevant electron angular momenta, (the nucleus can be ignored because of the large mass in its gyromagnetic ratio), and that will be quantized. So the incoming beam will split into distinct beams corresponding to the quantized values of the electron angular momentum.

The experiment was a great step forward in the development of quantum mechanics, because there is really no way that classical mechanics can explain the splitting into separate beams; classical mechanics just has to predict a smeared-out beam. Angular momentum in classical mechanics can have any value, not just the values $m\hbar$ of quantum mechanics. Moreover, by capturing one of the split beams, you have a source of particles all in the same state without uncertainty, to use for other experiments or practical applications such as masers.

Stern and Gerlach used a beam of silver atoms in their experiment, and the separated beams deposited this silver on a plate. Initially, Gerlach had difficulty seeing any deposited silver on those plates because the layer was extremely thin. But fortunately for quantum mechanics, Stern was puffing his usual cheap cigars when he had a look, and the large amount of sulphur in the smoke was enough to turn some of the silver into jet-black silver sulfide, making it show clearly.

An irony is that that Stern and Gerlach assumed that that they had verified Bohr's orbital momentum. But actually, they had discovered spin. The net magnetic moment of silver’s inner electrons is zero, and the lone valence electron is in a 5s orbit with zero orbital angular momentum. It was the spin of the valence electron that caused the splitting. While spin has half the strength of orbital angular momentum, its magnetic moment is about the same due to its $g$-​factor being two rather than one.

To use the Stern Gerlach procedure with charged particles such as lone electrons, a transverse electric field must be provided to counteract the large Lorentz force that the magnet imparts on the moving electrons.