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.