2 ASHWORTH, Intrinsic Field of a Magnet. 



a pressure and by putting a limit to the density. In 

 ferro-magnetism it is generally admitted that there is a 

 mutual attraction of the molecular magnets, and that there 

 is a limit to the intensity of magnetisation, and these are the 

 counterparts of the leading ideas in Van der Waals' theory. 



This view of ferro-magnetism requires that there 

 should be an intrinsic field analogous to the intrinsic or 

 molecular pressure in a liquid, and, further, if the quantity 

 analogous to the gas constant R is to be treated as 

 constant over the whole range of temperature as in the fluid 

 equation, then this intrinsic field must be very large, and 

 in this respect is analogous to the large intrinsic pressure 

 in a liquid. 



In a recent paper {Phil. Mag., February, 1914)^ I have 

 shewn that the ferro-magnetic equation in its main features 

 is in agreement with experimental facts, and it is easy to 

 deduce from the data there given that the intrinsic field 

 must be of the order of several millions of gausses, which 

 it is needless to say is far greater than any magnetic field 

 yet produced in air. 



Prof. P. Weiss has developed a kinetic theory of 

 magnetism based on Curie's views, and has found it 

 necessary to introduce into his theory a molecular field 

 which must be of the same large order. He claims to 

 have established the existence of this field by showing 

 that the gradual increase of the specific heats of the ferro- 

 magnetic bodies up to their critical temperatures and the 

 rapid change at that point to the normal value is exactly 

 explained by the energy absorbed in the diminution and 

 final destruction of such a field due to thermal agitation of 

 the molecular magnets.' But so far the magnitude of 



2 "The Anhysteretic Properties of Iron and Nickel." Fhil. Mag., Feb., 

 1914. 



» P. Weiss and P. N. Beck, /. de Pkys., 1908, (4), t. vii., p. 249. 



