Magnetization and Magnetic Change of Length. 647 



-as the point of the maximum curvature. The critical 

 temperature so defined is, in the case of Swedish iron, 780° C. 

 for H = 400 c.G.s. It is also to be observed that the critical 

 temperatures for ferromagnetic metals and alloys depend 

 more or less upon the strength of tbe field. 



The magnetization in a stage of descending temperatures 

 falls a little short of the magnetization in ascending tem- 

 peratures for the same field and temperature. But at ordinary 

 temperatures they coincide with each other. Combining the 

 above results with those of the magnetizations at the liquid 

 air temperature, we obtain a hysteresis curve with regard to 

 temperature, whose lower range is considerably extended by 

 the present experiments. 



Annealed Cobalt.— As in the case of Swedish iron, the 

 temperature-curves of magnetization were obtained, and are 

 given in fig. 6 b. 



The magnetization of annealed cobalt at high temperatures 

 was first observed by Professor Nagaoka and Mr. Kusakabe*. 

 The present results generally agree with those obtained 

 by them, but in our case the cooling in liquid air slightly 

 altered the magnetic property. In fig. 6 b the point 

 corresponding to the magnetization in liquid air is also 

 included. 



As the temperature rises from — 186° C. the magnetization 

 in a constant field increases, at first slowly and then rapidly, 

 till it reaches a maximum at about 300° C, after which it 

 decreases. The magnetization reaches a small minimum and 

 then begins to increase, and after passing through another 

 maximum, rapidly decreases, reaching its critical point at 

 1090° O. for 11 = 100. The descending branch of the curves 

 cuts the ascending branch at about 850° C. from downward 

 to upward ; but its general course is similar to that of the 

 ascending curve. The minimum point in the ascending 

 branch is about 450° C, and nearly coincides with the singular 

 temperature observed by us in the change of length by 

 magnetization f : at this temperature the sign of the length 

 change is reversed for all fields. 



It is also to be noticed that the course of the curve beyond 

 the critical point is nearly parallel to the axis of 

 temperature. 



* Nagaoka and Kusakabe, loc. cit. 



t Honda and Shimizu, Jour. Coll. Sci. xix. art. 10 (1903) : Phil. Mag. 

 vi. p. 392 (1903). 



