CHANGES OF TEMPERATURE DURING MAGNETISATION. 631 



In like manner also a stretched metal wire should become 

 cooled when it is twisted to a greater extent, if we assume as 

 certain that the coefficient of torsion diminishes as the tem- 

 perature rises. A twisted wire ought also to become more 

 heated, independently of the external work, when it is allowed 

 to untwist. 



In any case, the amount of energy H, absorbed or disengaged, 

 may be deduced from the external work and the properties of a 

 body. Without dwelling further on the purely calorific phenomena 

 which could thus be deduced from Carnot's principle, we shall 

 examine some deductions from equation (6) relative to electrical 

 or magnetic phenomena. 



648. CHANGES OF TEMPERATURE DURING MAGNETISATION. 

 The relations we have already pointed out between the changes of 

 temperature and the coefficients of magnetisation enable us to 

 predict the following results : 



i st. If we work at a temperature below redness, but so high that 

 the coefficient of magnetisation is decreasing, a piece of soft iron 

 should become heated when it is slowly brought near a magnet, and 

 cooled when it is removed. We assume that the motion is slow, so 

 as to avoid the influence of induction currents. 



The reverse would be the case at ordinary temperatures, if the 

 coefficient of magnetisation, as seems probable, increases with the 

 temperature. 



2nd. Cobalt should behave like iron that is, become cooled 

 when it is brought near a magnet at the ordinary temperature ; and 

 become heated, on the contrary, if we work at a higher temperature 

 than that of the maximum of magnetisation. 



3rd. For nickel, there is no maximum of magnetisation ; at all 

 temperatures this metal ought to become heated when it is brought 

 near, and cooled when it is moved away from a magnet. 



More generally, nickel and cobalt at ordinary temperatures 

 ought to become cooled when the motion requires an external 

 work opposed to that of the magnetic forces. For nickel at any 

 given temperature, and for the two former metals, at temperatures 

 higher than those of the maximum of magnetisation, any displace- 

 ment which requires a work opposed to magnetic actions produces, 

 on the contrary, a heating of the body. 



4th. In a magnetic field, a crystal becomes cooled when its axis 

 of greatest magnetic induction, or of least diamagnetic induction, 

 passes from a direction parallel, to a direction perpendicular to that 

 of the field. 



