CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 
139 
The last term is zero if each molecule has no initial moment, as Langevin’s theory 
of diamagnetism requires. Leaving aside the subscript 0 and using the equations of 
the electromagnetic field 
_ 3E Z _ _ 9H. 
and 
dx 3 y 
dt 
+ + = o 
dx d y 3 * 
3H Z , 3H Z , 3H Z , 3H Z <m z 
—r 2 + u • + V . ^T— + W . —r • - 
dx dy dz at 
ct 
we get, following Langevin, 
e 2 d 
M -=-4^-fd H - A ) + 57J A 
' 8./(P,) _ 8/(P,y 
dx dy 
Integrating this equation from time O (TI = 0) to time t (H = H z ) we find 
AM. = . H.A+ ~ r 
4771/ 4:771 Jo 
'k™ - 
dt, 
where AM. is the magnetic moment produced in the molecule during this interval. 
The second term on the R.H.S. of (6) will depend upon the molecular configuration of 
the substance and implies a modification of the electron circuits which will change 
their self-induction. Now any change of the self-induction may be represented by a 
small change of the intensity of the applied magnetic field. 
We may write 
/(P) = a . P,* 
where “ a ” characterizes the grouping of the molecules and therefore 
/3P V 3 PA 3 /jfTT \ 
=a \jx id) = ~ a st {m ’ ) ' 
where a . ML is the elementary change in the external field during a small interval 
of time St. 
Therefore 
AM. = 
e * H.A——. fi(MI z ). dt 
Joct 
4m 
2 H,A 
4m 
1 + 
4m 
a. AH. 
H. 
( 6 ) 
where a . AH Z is the total variation of H z due to establishing this magnetic field when 
the electron orbits are modified by the presence of neighbouring molecules. 
* Lorentz, ‘Theory of Electrons,’ p. 137. 
