CONSTITUTION, AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 2()B 
The fall oil iii the value of as the temperature is reduced, may, in part, he 
explained by the increased value of the molecular field, due to the nearer approach of 
the molecules. As the local molecular field becomes very big, the induced diam;ignetic 
eftect in each molecule will become big, in the same proportion, and this will tend to 
reduce the value of x and make this quantity tend to a limiting value. If the 
moleculai field is of the order 10^ gauss at ordinary temperatures, we have seen that the 
latio of the induced dianiagnetic moment AM to the magnetic moment of the electron 
orbit M, is of the order l/lOO. If the molecular field at low temperatures approaches 
10 gauss, owing to closer proximity of the magnetic elements in neighlioiiring 
molecules, the diamagnetic effect would be comparable with the ferro-magnetic effect.* 
On oui view this does not imply that diamagnetic substances should acquire a large 
diamagnetic susceptibility at very low temperatures. For taking the molecules in 
pairs, locally they are paramagnetic and the action of the local molecular field is to 
1 educe this paramagnetic effect so that the local magnetic moment becomes smaller 
and the susceptibility to an external field tends to zero as in iron. 
Accoidmg to Ewing I experiments carried out to test this effect have neither proved 
1101 chspio'ved this theory, probably because the external fields were not sufficiently 
stiong. But during crystallization we are applying unconsciously to each molecular 
current a magnetic field 500 or 1000 times stronger than the largest field we can 
apply externally, and probably even greater local intensities are attained at low 
tempeiatuies, since the interacting magnetic elements in adjacent molecules may be 
almost touching one another. The mutual induction and temperature effects combine to 
cause X to approach the limit zero at the absolute zero or in very powerful external fields. 
In diamagnetic media we have seen that the constant of the local molecular field a', 
(which corresponds to N in Weiss’s ferro-magnetic field) is of the order 2'5x 10^ and 
the reciprocal of this, viz., -h4x 10“^, is the order of magnitude of the local positive 
limiting susceptibility of a diamagnetic crystalline medium. (At ordinary temperatures 
the diamagnetic susceptibdity per unit volume is of the order -10-«). The parts of 
molecules adjacent to one another in a diamagnetic crystalline medium attract in a 
similai manner to the adjacent parts of molecules of a ferro-magnetic or paramagnetic 
medium. 
In a ferio-magnetic medium, as the temperature is raised, the susceptibility increases 
up to a certain point just below the critical temperature and then falls oft^ rapidly. 
The temperature controls the susceptibility in two ways ; first, by helping the molecules 
to o\ercome the difficulties of orientation, produced by the neighbouring molecules, to 
a^ point just below the critical temperature; second, by overdoing this effect and by 
giving the molecules too much rotational energy, at the critical temperature and above, 
so that the susceptibility to magnetization falls very rapidly. 
* In nickel the molecular field is 6 • 3 x 10® gauss. An applied field of 10® gauss would make a substance 
as diamagnetic as bismuth have a saturation value equal to that of nickel. 
t ‘ Magnetic Induction in Iron and other Metals,’ p. 353. 
