The existence of electron spin and its value was provided by the Stern–Gerlah experiment in which a beam of atoms is sent through an inhomogeneous magnetic field.
Schematic representation of the Stern–Gerlah experiment. to a force moment tending to align the magnetic moment along the field direction, but also to a deflecting force due to the difference in field strength at the two poles of the particle. Depending on its orientation, the particle will be driven in the direction of increasing or decreasing field strength. If atoms with all possible orientations in the field are present, a sharp beam should be split up into 2J +1 components. In Fig. 2.2 the beam is shown to be split up into two components corresponding to J = 1/2
