("HANGE OK ELASTIC CONSTANTS OF FERROMAGNETICS. G*.) 



Tn cast cobalt (Fig. 81), the change of the coefficient of ex- 

 pansion at low temperature increases with the field, reaches a 

 maximum, and then decreases. As the temperature becomes 

 higher, the amount of the maximum gradually lessens ; the posi- 

 tion of the maximum shifts towards lower fields and the curve at 

 last cuts the zero line. At a sufficiently high temperature, the 

 change of expansion is always negative. 



The change of the coefficient of expansion in annealed cobalt 

 (Fig. 82) is rather abnormal, and its maximum amount is consi- 

 derably large, being about 2 9^ of the coefficient itself. The 

 change of the mean coefficient of expansion between the ordinary 

 and liquid air temperatures steadily decreases with the field, 

 though its amount is small. As the temperature rises, the amount 

 of the diminution increases rapidly and then decreases. Here the 

 course of the curve shows a minimum decrease of the change. 

 As the temperature is further increased, a considerable amount of 

 increase is observed, which steadily increases with field. It 

 attains a maximum and again decreases, till it changes its sign. 



The values of da thus obtained may possibly differ from the 

 values obtained by heating the specimens in a constant magnetic 

 field. They are the changes of « by magnetization, when the 

 temperature is first raised and then the field applied. However, 

 any experiment for the determination of the change of «, in which 

 the thermal expansion is directly observed in magnetic field, 

 must be welcomed, as affording a counterpart for the analogous 

 comparison. 



Again, if we put, in the last equation, the tension T per 

 unit area for //, we get 



\ m )t V 9« Jo öt 



