CONSTITUTION AND TEMPEKATURE ON MAGNETIC SUSCEPTIBILITY. 257 
As the corresponding stresses and enei’gies are of the same order in tlie ferro¬ 
magnetic metals, and, further, since it has been shown (see Part II., pp. 148, 145 and 
Part III., pp. 84-87) that the local molecnlar forcive in diamagnetic media is of the 
same order as that in the ferro-magnetic metals, we may conclude that the local 
intensities of magnetization in the two types of media are comparable. 
Since 
i . H,. I == 2 X 10^ ergs, 
and 
10^ gauss (see Part HI., p. 86), 
we find 
and 
1= 400, 
2-5 X lOf 
In a diamagnetic crystalline medium the local forcives are comparable with those 
in iron, and, since the latter medium shows hysteresis in a magnetic field, we may 
enquire whether a similar phenomenon will be shown by diamagnetic media. If the 
diamagnetic molecules are magnetically unsymmetrical, the application of an external 
magnetic field will tend to orientate them.'^ But this will be a difierential effect on 
our conception of a diamagnetic molecule, and thus the tendency of the applied field 
to produce new molecular groupings will he small. We should therefore expect that 
hysteresis due to magnetization will be inappreciable in diamagnetic media. In iron, 
on account of the unbalanced magnetic nature of the molecules or atoms, new 
groupings are actually produced under fields of moderate intensity and the 
formation of these implies a loss of energy which is measured Ijy the area of the 
hysteresis loop. 
If, however, we take a diamagnetic copper wire and subject it to mechanical strain, 
the medium shows mechanical hysteresis. If sufficiently large stresses are employed, 
a permanent set is produced within the individual crystalline grains, new groupings of 
the molecules are formed, and a certain amount of energy is dissipated. All media, 
whether they are ferro-, para-, or diamagnetic, will show mechanical hysteresis. The 
difference from a magnetic point of view lies merely in the compensated nature of the 
diamagnetic molecule as compared with the \mcompensated nature of the ferro¬ 
magnetic molecule, but the local forcives are comparable, so that under mechanical 
stress the mechanical hysteresis effects will be comparable. 
(3) A Comparison of the Elasticities of some Diamagnetic Crystals with 
THOSE OF Crystalline Paramagnetic and Ferro-magnetic Media. 
It is well known that the application of an external magnetic field alters the 
distribution of stress in a mass of iron crystals. On our theory we see how the 
* This orientation in a diamagnetic liquid gives rise to the induced magnetic double refraction. See 
Part III., p. 87. 
2 N 2 
