CHANGE OF LENGTH BY MAGNETIZATION. ]j 



after passing the state of no contraction, it is changed to an 

 elongation, which again increases with the temperature up to a 

 maximum, and then gradually diminishes. At such a high tem- 

 perature as 103J: o C, we could still observe a considerable elongation of 

 the metal. To judge from the course of the curves, the temperature 

 at which the elongation at last vanishes is nearly the same as in the 

 cast cobalt. It is interesting to observe that the curve of the length 

 change at a temperature near 450°C is similar to that of iron at 

 ordinary temperature. The cobalt slightly elongates in weak fields ; 

 but it contracts in strong fields. At a temperature higher than 

 500°C, the cast and annealed cobalts resemble each other in their 

 behaviour in respect of the change of length. 



The curves (Fig. 12) showing the relation between the change 

 of length and the temperature, present a peculiar feature. They have 

 generally a maximum point and a minimum ; in low fields, however, 

 two small maxima and minima are observed. They also pass 

 through a point (46-i°C) in the axis of the temperature. It follows 

 then that there is a certain temperature, at which the change of length 

 in the annealed cobalt nearly disappears for all magnetizing fields, 

 and that the change occurs in an opposite sense in every field, accord- 

 ing as the specimen is heated above or below that temperature. It 

 appears then that annealed cobalt undergoes some molecular change 

 at that temperature. 



General remarks. On comparing the above results in soft iron, 

 tungsten steel, cast and annealed cobalts, we notice the remarkable 

 fact that the changes of length of these metals, at ordinary tem- 

 perature so very different from each other, assume, at sufficiently high 

 temperatures, an extraordinarily simple character ; they tend to 

 become proportional to the magnetic force, a fact which has no doubt 

 an important bearing in the theory of molecular magnetism. In Fig. 



