CONSTITUTION AND TEMPEEATURE ON MAGNETIC SUSCEPTIBILITY. 
207 
(5) Further Discussion of the Nature of the Local Molecular Field 
IN Ferro-magnetic and Diamagnetic Media. 
This subject has already been discussed in Part ITT., pp. 89 and LOO, hut no 
interpretation was then given to the magnitudes of tlie constants N and a\ of the 
local ferro-magnetic and diamagnetic fields. The values of N given by Weiss and 
Beck"^ are 0’38 x 10^ for iron, and I’27 x I0'‘ for nickel. For diamagnetic crystalline 
media a'^ is of the order 2 x 10^. 
Ewing and Lowt have shown that in very strong magnetic fields the relation 
between induction (B) and applied field (FI) may be represented by the equation 
B = H-i-a constant.(l) 
This constant has the value IttI (where I is the saturation intensity) and is equal 
to 21,360 in wrought iron, 6470 in nickel and 16,300 in cohalt. 
In the case of wrought iron 
B = H + 21360.(2) 
Suppose we could apply a field equal to the molecular field, 6'53xl0® gauss for 
iron. The limiting value of the permeability for this field will be from (2) 
=1 
21360 
6-53 X 10«’ 
and the limiting susceptibility per unit volume 
and 
XL = 
/ ^L-I 
Air 
21360 
An X 6'53 x 10*^ 
2-60x 10-b 
- = 0-38 X 10". 
XL 
This is equal to the value of N, the coefficient of the molecular field, as we should 
expect. 
Similar calculations may be made for nickel and cobalt, the limiting susceptibilities 
being respectively 
Xl = 0‘81 X 10 “" for nickel, 
Xl = 2 'Ox 10“" for colialt. 
Now we may ask the question, why is it that, in spite of the fiict that all the 
molecules are ordered into a definite space lattice under the influence of the respective 
molecular fields, the materials still show a finite susceptibility to magnetization ? The 
* ‘ Journ. de Phys.,’ ser. iv., vol. 7, p. 249, 1908. 
t ‘Phil. Trans. Roy. Soc.,’ A, p. 242, 1889. 
