406 ON MAGNETS. 



unequal; it is more particularly produced near the surface, where 

 the cooling is very rapid, so that the maximum action of the coercive 

 force is in the superficial layers. The inductive action and the 

 demagnetising force, manifest themselves then in conditions quite 

 different from those met with in homogeneous bodies. 



432. INFLUENCE OF TEMPERATURE. The magnetism induced 

 by the action of a magnet on itself, is perhaps the simplest way of 

 explaining the influence of temperature. 



It is natural to assume that rigid magnetism is not altered by 

 small changes of temperature, for the magnetisation resumes its 

 original value when the magnet regains its original temperature ; it 

 is difficult to suppose that rigid magnetism can repair its losses, for 

 all the internal actions tend to diminish it. On these considerations, 

 the temporary enfeeblement of magnetism will be simply due to an 

 increase in the induced magnetism, and therefore the coefficient of 

 magnetisation must at first increase with the temperature. 



For higher temperatures, above 100 for instance, magnetism 

 undergoes a distinct diminution ; the rigid magnetism itself has 

 therefore changed. In these conditions we cannot say whether the 

 coefficient of magnetism continues to increase with the temperature, 

 for the enfeeblement is due to a double cause. As iron and steel at 

 a red heat are no longer attracted by a magnet, we must assume that 

 the coefficient of magnetisation then becomes null, or at any rate 

 is extremely small. 



It appears then that for iron and steel the coefficient of 

 magnetisation must at first increase with the temperature and 

 then diminish to zero, passing through a maximum at a definite 

 temperature. 



If this is the case, a bar magnetised at a lower temperature 

 than the maximum must lose magnetism when it is heated, and 

 the converse must take place with a bar magnetised at a lower 

 temperature than that of the maximum. 



Experiment shows that this is the case with cobalt. For iron 

 and steel the facts hitherto known agree partially with this mode of 

 view; but there are too few experiments made under well defined 

 conditions, to enable us to judge how far it agrees with the truth. 

 Everything seems to indicate that the true phenomena are more 

 complex. 



