384 MAGNETIC INDUCTION. 



405. INFLUENCE OF TEMPERATURE. Temperature has a pro- 

 nounced influence on the value of the coefficient k; there is 

 nevertheless considerable uncertainty as to the law of the variation. 



The fact longest known, and best marked, is that soft iron loses 

 almost all magnetic properties at a red heat. This is also the case 

 with nickel at a temperature of 300 degrees ; but with cobalt it only 

 occurs near the temperature at which copper melts. 



If we only take into account temperatures between - 20 and 150 

 degrees, we find that the inducing power of iron is virtually constant, 

 although there is reason to think that it increases at first and passes 

 through a maximum; that that of nickel decreases continuously; and 

 lastly, that that of cobalt constantly increases. In the case of this 

 latter metal there is certainly a maximum towards a red heat. 



Heat acts also on crystalised magnetic or diamagnetic bodies in 

 such a way as not only to diminish the coefficients, but also to 

 diminish the magneto-crystalline properties which are closely con- 

 nected with the difference of these coefficients. In the case of 

 bismuth the difference of the coefficients diminishes by one-half 

 between 30 and 140 degrees ; and for iron carbonate by two-thirds 

 between the same limits of temperature. 



