88
THE QUANTUM [vi. 2
The element to be examined was heated in a small electric
oven, surrounded by a cooling chamber, and the metallic vapour
escaping through an aperture in one wall of the oven passed
through a system of slits S x and S 2 so as to produce a narrow
stream of atoms, all travelling in the same direction. This
stream passes through a non-homogeneous magnetic field
produced by the pole pieces of an electromagnet, the pole pieces
having the form shown at b on the right of Fig. 14. One pole
piece is in the shape of a blunt wedge, the angle of the wedge
being 90° in some cases and 6o° in others. The opposing pole
piece is provided with a groove parallel to the edge of the wedge.
It was found that this arrangement gave a suitable non-uniform
magnetic field. The stream of atoms having passed through
the magnetic field, which in one form of the apparatus had a
length of 3 cms. and in another about 4! cms., fell on a glass
plate. The apparatus was enclosed in a gas-tight vessel which
could be evacuated. The main difficulty in the earlier experi-
Fig. 14.—Experiments of Gerlach and Stern.
ments was the adjustment of the various parts in a straight line
so as to give a rectilinear stream. This was overcome by
reconstructing the apparatus so that the essential parts were
fixed firmly to the magnet poles. In these later experiments
the amount of silver deposited was so much increased that the
displaced images could be seen without development, but no
trace of a null line, which would be expected on the classical
theory, could be obtained even after development.
It was found that under normal conditions copper and gold
atoms behaved like silver atoms and showed the separation into
two streams. The measurements indicate monatomic structure
and a magnetic moment equal to that of the Bohr magneton.
The atom of thallium has a smaller magnetic moment and
appears to correspond to the case of ± 3 of a Bohr magneton.
Lead, tin and iron atoms are not affected by the magnetic field,
so apparently the atoms are non-magnetic. In the case of
nickel the values indicated for the magnetic moment are ± 2
Bohr magnetons and also zero.
