46 Prof. Nagaoka and K. Honda on Magnetostriction 



of measurement and the nature, of the sample. The minute- 

 ness of the effect necessitated precautions against diverse 

 sources of error, such as the non-homogeneity of the mag- 

 netizing field, and the non-uniformity of temperature. All 

 these different sources of error, however intricate they may 

 at first sight appear, can, by properly arranging the measuring 

 apparatus, be eliminated. In the present investigation we 

 have used the method of observing the change of length and 

 of volume already described in our former papers*, with 

 slight modifications. 



Apart from these instrumentalities, the diversity in the 

 character of magnetostriction with different samples is hardly 

 to be avoided. Experiments by Rhoadsf with rolled and 

 stretched sheets of iron sufficiently prove that the treatment 

 of ferromagnetic bodies has great influence on the change of 

 length accompanying the magnetization. In our former ex- 

 periments on the magnetostriction of iron, steel, and nickel, 

 the soft iron was what may be practically considered homo- 

 geneous, but the nickel ovoid was turned into shape from a 

 thick plate. It thus seemed advisable to repeat the experi- 

 ments with more homogeneous metals. 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 J. Unlike other 

 experimenters, we tested cobalt in the present investigation in 

 the cast and annealed states, and found an extraordinary 

 difference in the change of length. 



The curious property of irreversible nickel-steel as regards 

 magnetization has been known for a long time by the experi- 

 ment of Hopkinson. The question of magnetostriction in 

 reversible nickel-steel was a tempting subject of investigation, 

 especially in connexion with the remarkable small thermal ex- 

 pansion possessed by the metal, and its practical utility in the 

 construction of scales and other instruments, w r hich will not be 

 affected by the variation of temperature. Moreover it was very 

 interesting to examine the nature of the magnetostriction in 

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

 change by magnetization is of opposite character in weak 

 fields, but similar in strong. A simple conjecture may suggest 

 that the changes produced by magnetization are according 



* Nagaoka, Phil. Map:. Jan. 1894 ; Wied. Ann. liii. p. 487 (1894) ; 

 Nagaoka & Honda, Phil. Mag. Sept. 1898, April 1900; Nagaoka, 

 Rapports presentes au Congres International de Physique, Paris, ii. p. 536 

 (1900). For literature on magnetostriction see Rapports. 



t Pthoads, Phys. Rev. vii. p. 65 (1898) ; Phil. Mag. Nov. 1901. 



\ Nagaoka, Wied, Ann. liii. p. 487 (1894). 



