The Stern Gerlach Experiment

The theory of the quantisation of the spin angular momentum of an electrons of atoms placed in a magnetic field needed to be experimentally proved. In 1920 (a few years before the concept of the spin was coined into physics) Otto Stern and Walter Gerlach conducted an experiment, in which they noticed this property of the electron.

The silver atoms from the source, which was a furnace filled with boiling silver, got to the vacuum, where narrow slits formed a flat beam of them. Then the beam got into heterogeneous magnetic field and fell on a photographic plate. By classic thinking we would expect a single image of the beam on the plate. However, actually the beam going through the heterogeneous magnetic field was dissipated, and gave the image of two separated lines on the plate. The silver atoms from the source, which was a furnace filled with boiling silver, got to the vacuum, where narrow slits formed a flat beam of them. Then the beam got into heterogeneous magnetic field and fell on a photographic plate. By classic thinking we would expect a single image of the beam on the plate. However, actually the beam going through the heterogeneous magnetic field was dissipated, and gave the image of two separated lines on the plate.

This phenomenon can be explained just by the quantisation of the spin angular momentum.

Electrons in the atom are situated in such way, that in each subsequent pair of the electrons one of them has the upward spin, and the second one the downward spin. So the total spin of such pair equals nought. In the silver atom, on the outer shell there is a single electron, however. And so its spin can't be counterbalanced by the electron having the opposite spin.

A spinning electron creates some magnetic dipole moment (which is something like a kind of a tiny magnet). In a magnetic field the dipole is influenced by the moment of a force, which turns the dipole making its direction the same as the direction of the field B. In a heterogeneous magnetic field except the moment of the force causing the rotation of the dipole another force influences the dipole. If the dipole is directed the same as the magnetic field, then it is pulled inside the area of the stronger field. Or if the dipole is directed the other way than the direction of the field, then the dipole is pushed out of the area of the stronger field.A spinning electron creates some magnetic dipole moment (which is something like a kind of a tiny magnet). In a magnetic field the dipole is influenced by the moment of a force, which turns the dipole making its direction the same as the direction of the field B. In a heterogeneous magnetic field except the moment of the force causing the rotation of the dipole another force influences the dipole. If the dipole is directed the same as the magnetic field, then it is pulled inside the area of the stronger field. Or if the dipole is directed the other way than the direction of the field, then the dipole is pushed out of the area of the stronger field.

And so the silver atom having one electron on the outer orbit can be pulled in or pushed out of the stronger magnetic field. If the spin of the electron equals +1/2 then the atom is pushed out; or if it's -1/2 then it is pulled in. That is why when passing through a heterogeneous magnetic field a beam of the silver atoms gets split into two beams. Each of them consists of the atoms having one kind of the spin of the outer electrons.And so the silver atom having one electron on the outer orbit can be pulled in or pushed out the stronger magnetic field. If the spin of the electron equals +1/2 then the atom is pushed out; or if it's -1/2 then it is pulled in. That is why when passing through a heterogeneous magnetic field a beam of the silver atoms gets split into two beams. Each of them consists of the atoms having one kind of the spin of the outer electrons.

In 1927 Phipps and Taylor conducted an experiment, which was similar to the Stern-Gerlach one. However, this time hydrogen was used instead of silver. In this experiment the split into two beams took place once again.

Later on also other kinds of atoms having a single electron on the outer shell were studied this way (copper, gold, sodium, potassium). Each time in the photograph the image of the double beam was obtained.

Not only the electrons have the spin in the atom but also the nucleons. But the proton and the neutron have much bigger masses than the electron (saying more exactly about 1836 times bigger). And the magnetic dipole moment is inversely proportional to the mass of the particle. So the moments of the proton and the neutron are very small in comparison with the moment of the electron. Stern, Frish, and Easterman measured those tiny magnetic dipoles in the later experiments.Not only the electrons have the spin in the atom but also the nucleons. But the proton and the neutron have much bigger masses than the electron (saying more exactly about 1836 times bigger). And the magnetic dipole moment is inversely proportional to the mass of the particle. So the moments of the proton and of the neutron are very small in comparison with the moment of the electron. Stern, Frish, and Easterman measured those tiny magnetic dipoles in the later experiments.