The theory of spatial quantization of the spin moment of the momentum of electrons of atoms situated in the magnetic field needed to be proved experimentally. In 1920 (two years before the theoretical description of the spin was created) Otto Stern and Walter Gerlach observed it in the experiment they conducted.
The atoms of silver from the source which was the furnace with boiling silver were leaded to the vacuum space. There (thanks to the thin slides) the flat beam of those atoms was created. Then the beam got into non-homogeneous magnetic field and incidenced a photographic plate. Using classic physical laws we would expect the single picture of the beam on the plate. Whereas the beam of the atoms passing through not homogenous magnetic field undergoes splitting. That is why Otto Stern and Walter Gerlach received the two lines on the photographic plate.
The phenomena can be explained with the spatial quantization of the spin moment of momentum. In atoms the electrons are located in such way that in the each next pair of electrons there is one of the upward spin and one of the downward spin. So the whole spin of such pair is equal zero. But in the atom of silver on the outer shell there is a single electron which spin is not balanced by any electron.
The circulating causes some magnetic dipole moment (it's like it was a very small magnet). There is a force moment in the magnetic field influencing the dipole that is turning it until its position is the same as the direction of the field B. There is some other force influencing the dipole in the field. When the dipole is directed the same as the magnetic field then the dipole is pulled by that force in to the space of a strongest field. But if the dipole is directed opposite to the fields direction is pulled by that force out from the space of a strongest field.
So the atom of silver having one electron on the outer shell can be pulled in or out the space of a strongest magnetic field, what depends on the value of the magnetic spin quantum number. When the spin of the electron is equal +1/2 the atom is pulled out and when the spin is equal -1/2 the atom is pulled in. So during passing through the non-homogenous magnetic field the beam of the atoms of silver undergoes splitting into the two beams. Each of them consist of atoms which outer electrons are of the same spin.
In 1927 Phipps and Taylor conducted a similar experiment. This time they used atoms of hydrogen, not silver. They also observed that the beam of atoms undergoes splitting into two ones.
Later scientists conducted experiments using other atoms which have only one electron on the outer shell (cooper, gold, sodium, potassium). Every time there were two lines achieved on the photographic plate.
Of course in the atom not only electrons have spin. The elements of the nucleus also have it. But protons and neutrons are much more heavier than electrons (about 1836 times), and the magnetic dipole moment is inversely proportional to the mass. So the proton's and neutron's magnetic dipole momentum is much smaller than the one of the whole atom. This small magnetic dipole was later measured by Stern, Frisch and Easterman.
