Length in Iron, Steel, and Nickel Ovoids by Magnetization. 335 



stantially altered its elastic behaviour. The nickel rod which 

 we used in the former experiment was hammered from a 

 nickel plate to a prism of square cross-section. It con- 

 tained 1*75 per cent, of iron, besides traces of manganese 

 and carbon. The ovoids used in the present experiment were 

 prepared from a thick plate, and were nearly pure nickel, the 

 quantity of iron which was present as impurity being im- 

 measurably small. As the material is likely to become 

 homogeneous by repeated annealing, the ovoids were carefully 

 annealed for about 50 hours. The ovoid was wrapped with 

 asbestos and placed in a thick metil tube, the interspace 

 between the ovoid and the wall of the tube being filled with 

 fine charcoal powder. The tube was then placed in a charcoal 

 fire. When the ovoid was annealed in this way, there was 

 some trace of oxidation on its surface. The change of 

 volume after each annealing was examined, with a result that 

 the process of annealing increases the effect. It therefore 

 appears that the previous history of the specimen exercises 

 an important influence on the magnetization and on the 

 dimensions of ferromagnetics as affected by magnetization. 

 The anomaly in the length-change noticed by Bidwell in two 

 specimens of nickel wire is probably not the effect of tem- 

 perature, but is to be ascribed to the cause above stated. 

 In contradiction to our former result with a square prism, the 

 ovoid showed increase of volume. The amount of increase 

 was small compared with the decrease noticed in the previous 

 experiment. Cantone * obtained tolerably large increase of 

 volume in a nickel ovoid ; our former result was nearly half 

 as large, while in the present experiment there is a slight 

 increase. The discrepancy is probably due to the difference 

 of treatment before the specimen was turned to the proper 

 shape for experiment, and also to its chemical composition. 



The volume-change of ferromagnetics considered as a func- 

 tion of the magnetizing field takes place very slowly in weak 

 fields ; it then increases in a more rapid ratio till it reaches 

 the ' Wendepunkt' ; then the change becomes slower, but goes 

 on increasing nearly in a straight line. Up to H = 2000 

 there is no tendency to decrease in the rate of change. With 

 a still stronger field, the increase of volume will probably 

 become more considerable. 



In our former experiment, the range of 'the magnetizing 

 force was confined to a few hundred c.G.s. units. In the 

 present experiment, the increased field-strength unveiled the 

 character of the change of volume considered as a function of 



* Cantone, Atti d, R. Accad. d. Zincei, torn. vi. p. 257 (1891). 



