An electron, being a particle with an associated magnetic moment, may be pictured as a little spinning top. In the presence of a magnetic field this magnetic moment will align with the field in one of two ways – either spin up or spin down. The quantization of this spin magnetic moment was first observed in 1920 by Otto Stern and Walter Gerlach.

Atoms of silver from the source, a silver furnac shown at the left hand side of the diagram below, emerged, passed through the slits to form a narrow beam and passed through a non uniform magnetic field formed by two specially shaped magnets, onto a photographic plate.

Using classic physical laws we would expect a single continuous line up the photographic plate, as the electron spin would classically be oriented in all directions, with the atom experiencing a forcewhereis the component of the magnetic dipoleoriented along the- axis andis the gradient of the magnetic field, which is of course, oriented along the– axis.

In atoms usually the electrons are located in such way that electrons are paired – spin up paired with spin down so that there is no net spin, but in the silver atom there is an electron in the outermost shell will no other electron to pair with so the silver atom as a whole has a net spin due to this electron. The atoms are separated into two beams of atoms – one beam of which has the aforesaid electron with spin oriented up and one which has spin oriented down.

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.