The „Directional quantization“ of Spins was shown in in an experiment by Otto Stern and Walther Gerlach in 1922 (Figure 1): In a special magnetic field magnetic particles are deflected upwards the further their north pole points upwards, and downwards the further their north pole points downwards. downward, the further their north pole points downward. If the poles are aligned horizontally, no deflection takes place. deflection takes place. According to classical physics, we would expect that for particles moving through the apparatus, the north poles would point randomly distributed in all possible directions, and accordingly on the screen also all possible deflections are to be observed. Stern and Gerlach, however, could prove in an experiment with silver atoms, that there are only two small points on the screen.

Thus, they were able to demonstrate the "directional quantization" of the spin of the particles: The spin of silver atoms, like that of electrons, can take only exactly two directions in a magnetic field. two orientations in a magnetic field. Thus, one half of the particles is deflected upwards, the other half is deflected the same distance and the other half is deflected the same distance downward, resulting in two points on the screen.