320 Magnetisation of Iron at High Temperatures. [Feb. 14, 



These curves illustrate what has been long known, that rise of tem- 

 perature causes increase of induction if the magnetising force is 

 small, but diminution of induction if the force is great. 



In Curve IV the abscissae are temperatures, the ordinates are the 

 ratios of induction to magnetising force or permeabilities for a force 

 of 4*0, and of 0*3 C.Gr.S. units, the data being supplied from the 

 preceding and other curves. The latter curve brings out a most 

 remarkable feature. For this force the permeability increases some- 

 what steadily to a temperature of about 640 C., its rate of increase 

 then rapidly accelerates, till it attains a maximum of 11,000 at a 

 temperature of 727 C. ; at 737 C. the permeability is practically 

 unity, or the magnetisability of the material has entirely disappeared. 



Regarding the iron as made up of magnetic molecules the axes of 

 which are directed to parallelism by magnetic forces, the results are 

 expressed by saying that the magnetic moment of the molecule 

 diminishes with rise of temperature, at first slowly, but very rapidly 

 as the point is approached at which magnetism disappears ; on the 

 other hand, the facility with which the particles are directed con- 

 tinually increases, at first slowly, but at high temperatures very 

 rapidly. The effect is that at a temperature of 720 C. an exceedingly 

 small force is competent to turn the axes of nearly all the molecules 

 in a direction parallel to the magnetising force. 



The estimates of temperature given herein must be accepted as 

 provisional, and subject to revision. The actual temperatures are 

 undoubtedly materially higher, as I have not yet taken into account 

 the part of the secondary wire outside the furnace. 



