MAGNETISM
vn. 2] MAGNETISM
magnetic moment may be regarded as brought about
increase dco = 0 in the angular velocity given by
eH.
0
2 m
93
by an
7-7
in accordance with the general result stated by Larmor.
Langevin’s theory enables us to calculate the mean square
radius of all the electronic orbits in the atom from the atomic
diamagnetic susceptibility. The actual magnitudes of the
diamagnetic constants give values for the mean square radius
of the same order as those deduced from other evidence for
hydrogen, carbon and nitrogen. For the inert gases the
susceptibilities as at first determined were far too large to be
compatible with the sizes of the atoms suggested by the Bohr
theory, but a direct determination by Wills and Hector * has
removed this discrepancy. The values agree with those pre
dicted by Joos from the electron arrangements.
2. Paramagnetism and Ferromagnetism
In paramagnetic substances the susceptibility is small,
though usually larger numerically than for diamagnetic sub
stances, but it is positive instead of negative. In 1895 Curie
found that the specific susceptibility % (i.e. the susceptibility per
unit mass) is inversely proportional to the absolute temperature
and is independent of the strength of the field. Thus ^T = C
where C is Curie’s constant.
Ferromagnetic substances, including iron, nickel, cobalt
and some alloys, have large positive values for the susceptibility,
and these values depend on the past history of the material.
These substances can be permanently magnetized and show the
phenomenon of hysteresis. The specific susceptibility varies
with temperature in a complicated manner. At a certain
temperature called the “ Curie Point ” ferromagnetic substances
lose most of their magnetic moment and become paramagnetic.
The electronic theory of paramagnetism advanced by Langevin
is strictly applicable only to a paramagnetic gas, such as oxygen.
On this theory the aggregate magnetic moment of the electronic
orbits in the molecule is not zero, and it is assumed that the
molecular magnet tends to turn in a magnetic field in such a
direction that the potential energy in the field is a minimum.
It is not easy to understand the mechanism of this orientation,
as it cannot be produced directly by the field. It is customary
* Wills and Hector, Phys. Rev., vol. 23, p. 209, 1924 ; Hector, Phys.
Rev., vol. 24, p. 418, 1924.