Length in Iron, Steel, and Nickel Ovoids by Magnetization. 333 



characteristic as regards the field of no elongation becomes 

 exceedingly pronounced. From the curve of length-change 

 it does not appear that it will ever cut the line of no elonga- 

 tion, even in intense fields, 



Nickel (fig. 1). — The behaviour of an annealed nickel ovoid 

 is nearly the same as observed by one of us. With increasing- 

 field the contraction reaches an asymptotic value, which in 

 the present case is greater than BidwelFs result, obtained 

 from experiments on a nickel wire. This discrepancy is to 

 be sought partly in the difference of shape, and partly in the 

 difference of treatment, as will be clearly illustrated by ex- 

 periments on the change of volume. We have also reason 

 to believe that repeated annealing alters the elastic behaviour 

 of ferromagnetics as regards the strain wrought by mag- 

 netization. 



The curves of elongation in iron and nickel plotted against 

 the intensity of magnetization are given by dotted lines in 

 fig. 1. The change of length at first takes place very slowly, 

 but on reaching saturation the rate of change becomes very 

 rapid. So far as the present experiment goes, the rate does 

 not diminish except in annealed wolfram steel, in which we 

 notice a slight flattening. In nickel we find a slight bend 

 when the magnetization becomes saturated. 



Change of Volume by Magnetization. 



Experiments by several physicists prove that magnetization 

 produces change of volume in ferromagnetics, in contradiction 

 to the popular belief, which is based on Joule's experiment. 

 The alteration of volume accompanying the magnetization of 

 ferromagnetics is generally very small in weak fields ; but, as 

 will be seen from the present experiment, the phemonenon is 

 more marked as the field is made stronger. As we have 

 already remarked, the change of volume as measured by 

 Cantone * with an iron ovoid must have been exceedingly 

 minute, as the magnetizing field was very small. Dr. Knott f 

 has published several papers on the change of internal volume 

 of ferromagnetic tubes, showing that iron, nickel, and cobalt 

 are subject to the change by magnetization. As our result 

 regarding the same question was somewhat different, especially 

 in the case of nickel, we have thought it advisable to settle 

 the discrepancy by fresh experiments. 



Iron and Steel (fig. 2). — Preliminary experiments on an iron 

 ovoid showed that there is considerable increase in the 



* Cantone, Mem. d, H. Accad. d. Lincei, torn. vi. p. 487 (1891). 

 t Knott, Trans. l\oy. Soc. Edin. vol. xxxviii. p. 5'27 (1896) ; vol. xxxix. 

 p. 457 (1898). 



