STERN'S-GERLACH'S EXPERIMENT
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.
