ox MAGNETOSTRICTION. 3 



geneous metaJs. In addition to this, our investigation did not include 

 the magnetostriction in cobalt, the only specimen hitherto examined 

 being an ovoid, '^ which was broken in two pieces, and firmly fixed 

 together by wrapping thick paper over the broken edge. Unlike other 

 experimenters, we tested cobalt in the present investigation in the 

 c:ist and annealed states, and found an extraordinary difference in the 

 change of length. 



The curious pr(3perty of irreversible nickel steel as regards magne- 

 tization was known for a long time by the beautiful experiment of 

 Hopkinson. The question of magnetostriction in reversible nickel steels 

 was a tempting subject for investigation, especially in connection w^ith 

 the remarkably small thermal expansion possessed by the metal, and its 

 practical utility in the construction of scales and other instruments, 

 which must not be affected by variations of temperature. Moreover 

 it was verv interesting to examine the nature of the magnetostrictions 

 in nickel steel, as it is composed of two substances, whose length 

 changes by magnetization are of opposite characters in weak fields, 

 but similar in strong. A simple conjecture might suggest that the 

 changes produced by magnetization are according to the relative pro- 

 portion of the magnetostriction of the constituents, but the pheno- 

 menon is of a much complex character. 



Associated with the chano-es of leno-th and of volume, comes the 

 Wiedemann effect, which is measured by the amount of torsion, caused 

 bv interaction of circular and lonoitudinal mao-netizations. The 

 measurement of the effect in cobalt must at present be postponed, as 

 the metal can not be Ijrought to a geometrical shape suitable for ex- 

 periment, on account of its brittleness. Investigation of the effect in 

 nickel steel of different percentages presents a phenomenon of the same 



1) Nagaoka, Wied. Ann. 53. 487, 189i ; Rapports présentés an Congrès de Pliysique, Paris 

 2, 538, I'JOO, For literature on magnetostriction, see Rapports. 



