The crucial experiment demonstrating the quantization of angular momentum was carried out by Stern and Gerlach using an atomic beam, the magnetic moment of the atom was also measured in the experiment.
An atomic beam is a beam of atoms moving with thermal velocities in one direction, this is obtained by heating the element in an oven with a small hole for atoms to escape from, and at a pressure of around 10-2 mm Hg. The pressure outside the oven is kept at 10-6 mm Hg so that the mean free path of the atoms is large compared to the hole in the oven, then no collisions occur, and the atoms escaping from the hole are all moving in one direction. The beam is also passed through collimating slits to further limit its size.
In the Stern Gerlach experiment the atoms (of silver) are deflected in an inhomogeneous magnetic field, obtained with a magnet with a wedge shaped pole piece. The diagram shows this shape, the atomss move along parallel to this sharp edge, the field gradient so produced is then perpendicular to the path of the atom beam. The path length so traversed is around 50 cm in the experiment. The atom beam is captured by a detector (a liquid air cooled target originally - the atoms just condense on it). A field of about a Tesla and a field gradient of about 10 Tesla/cm is needed to give deflections of about 1 mm. So the effect is not large.
The deflection is inversely proportional to the thermal energy of the atoms, and so the trace on the detector plate is spread out according to the temperature of the oven, this limits the accuracy of the experiment. With atoms such as sodium or silver, which are both in 2S½ states, two distinct traces are observed, with deflections appropriate to ± one Bohr magneton. Thus both the existence of spatial quantization and the magnetic moment (the Bohr magneton) associated with the electron spin are confirmed