6 3 8 



CONSTANTS OF MAGNETISATION. 



Tempering has a much greater importance than any other 

 physical property. While the maximum of ft is 4600 for iron 

 heated to redness and then cooled slowly, this value is only 2250 

 for tempered metal, and rises to 6260 when it is reheated after 

 tempering. 



For nickel and cobalt, Rowland* found the curious result that 

 at a temperature of 220 the curve of permeability becomes much 

 closer, with a higher maximum ; in other words, the variations of 



300 



200 1 



100- 



moo 



3000 



5000 



7000 



Fig. 248. 



permeability are more rapid and the maximum intensity of mag- 

 netisation diminished. 



The experiments on nickel have been completed and extended 

 by Mr. Perkins, f The curves of Fig. 248 represent the results for 

 several different temperatures. The dotted curves hold for tem- 

 porary magnetism ; they all start from the same origin as the 

 corresponding curves of the total magnetisation in other words, 

 that, as for iron, the magnetism developed by weak magnetising 

 forces is purely temporary. 



* ROWLAND. Phil. Mag. [4], Vol. XLVIII., p. 321. 1874. 



t PERKINS. American Journal of Science, Vol. xxx., p. 218. 1885. 



