230 Dr. D. K. Morris on the Magnetic Properties 



maximum of permeability increases, and the temperature at 

 which it occurs approaches the critical temperature, so that 

 in very small fields the fall of the permeability is exceedingly 

 sudden. 



An examination of the B-H curves at various temperatures 

 (Nos. 6, 7, and 8) shows that as the temperature rises, the 

 initial slope of the curve increases and approaches more and 

 more nearly to the maximum slope. And the critical tempe- 

 rature occurs just when they would coincide, i. e. when the 

 B-H curve would start from the origin with its maximum 

 slope, and the maximum permeability would occur in an 

 infinitely small magnetic field. 



The critical temperature is as near as can be given 795° for 

 specimen A, and 780° for specimen B. For the soft iron 

 wire specimen it was about 770°. 



Above this temperature the induction is proportional to 

 the magnetizing force (curves 4 and 5) indicating the same 

 permeability for all forces, a result in accordance with the 

 experiments of M. Curie. The curve of variation of the 

 permeability with temperature (No. 13, n.) is given with some 

 diffidence as the observations were liable to considerable error 

 (see first part of Tables IV. and V.), and as the results are 

 too remarkable to pass without further corroboration. It 

 indicates a maximum of about //.= 100 at 820°, and another 

 of ^ = 17 at about 1050°; whilst between 920° and 980° the 

 permeability is but 2*3. 



II. Curves of Cyclic Magnetization. — The variations in the 

 form of these curves due to changes of temperature can be 

 studied in the series of curves Nos. 2, 4, and 5, in which the 

 results recorded in Tables II., VI.-VIII. are plotted. 



The process of annealing of specimen A is graphically 

 depicted in the first of these series. 



In series No. 5 it will be noticed how, as the temperature 

 rises and approaches 550°, the inclination of the curves to 

 the vertical axis increases, and how with further rise of 

 temperature the curves become more erect and angular. 

 Those taken at 764°*5 in this series are interesting as showing 

 the magnetic condition of the iron almost immediately below 

 the critical temperature. The coercive force required to re- 

 move the remanent magnetism was little more than -^ 6 of a 

 C.G.S. unit. For the steep-sloping part of the curve the 



value of -==- is about 15000 ; when H is much above 1 unit, 



its value is about 150. The transition from the steep part of 

 the curve to the gently-sloping part is very sudden. 



