2QO PRELIMINARY. 



to which the bar has been afterwards subjected. The harder the 

 steel, and the more brittle the temper, the greater is the coercive 

 force. 



We have hitherto implicitly assumed that the magnetisation of a 

 magnet is invariable, and is independent of the forces to which the 

 magnet is subjected ; but this kind of magnetic rigidity is a limiting 

 case which is never realised with perfect completeness. When a 

 magnet placed in a strong magnetic field is in its normal position of 

 equilibrium, its magnetisation slightly increases ; it diminishes on the 

 contrary if it is in the opposite direction. The variations thus pro- 

 duced are generally feeble, and usually transient like those of soft 

 iron in the same circumstances ; these variations may ordinarily be 

 neglected in the case of powerfully magnetised steel bars placed 

 in a magnetic field of no great strength like that of the earth, for 

 instance. 



303. INFLUENCE OF TEMPERATURE. Heat acts also on the 

 magnetism of magnets. A moderate increase of temperature 

 diminishes the magnetisation, but only temporarily, and the magnet 

 resumes its original magnetisation with its original temperature. 

 Within the ordinary variations of the surrounding temperature, the 

 effects produced are sensibly proportional to these variations, so that 

 if M and M t are the magrietic moments of a magnet at the tempera- 

 ture of zero, and of / degrees, we have the ratio 



the coefficient a depending on the nature of the steel. 



A greater degree of heating, above 100 for instance, produces a 

 definite enfeeblement of the magnetisation, and a bar of steel heated 

 to bright redness has usually lost all traces of magnetisation when it 

 returns to the ordinary temperature. 



A rise of temperature produces analogous effects on the magnetic 

 properties of soft iron. At the ordinary temperature the magnetisa- 

 tion induced in iron by a given field, changes but little with variation 

 of temperature, but beyond 100 the diminution of induced mag- 

 netism becomes very rapid. At a temperature beyond red heat, iron 

 no longer possesses the power of being attracted by magnets ; at that 

 temperature it is not even magnetic. 



304. ON MAGNETIC FLUIDS. The physicists of last century, 

 more especially yEpinus and Coulomb, attempted to explain 

 magnetic phenomena by a hypothesis analogous to that of electrical 

 fluids. 



