232 



Magnetic Properties of Pure Iron. 



Also that for a small magnetising force the permeability rises very 

 remarkably with the temperature, but just the reverse for a force of, 





say, 



40." 



Fm. 6. 



Fig. 6 shows the rise of permeability in relation to temperature 

 when H = 0'3, the maximum permeability observed being 11,100 for 

 a magnetising force of 0'3, and at a temperature of 855 C. (857). 



Fig. 7 contrasts the relation of induction to temperature at a small 

 and a larger magnetising force. 



During the heating of the specimen, the critical point, when the 

 iron suddenly became non-magnetic, was reached at 874 C. (875), 

 and on cooling it became magnetic at 835 C. (838). 



Comparing these results with those obtained with the more 

 ordinary specimens of iron mentioned in Dr. Hopkinsou's paper, we 

 have here 874 C. as against 786 C., while in an experiment on some 

 soft iron wire the critical temperature was 880 C., and for hard piano- 

 forte wire it was 838 C. 



