51 Prof. Nagaoka and K. Honda on Magnetostriction 



of the same magnitude as that of thermal expansion, which, 

 as is well known, is of very minute amount. Experiments 

 in losv fields show that the magnetostriction plays no important 

 part in the use of nickel-steel scales; only in measurements 

 of extreme accuracy it will be necessary to add a very small 

 correction to the measured values, according as the scale is 

 placed in the magnetic meridian or perpendicular to it. As 

 will be seen from the curves of elongation (fig. 4) the difference 

 in a metre will generally be less than ^ fi for measurements 

 made in the said directions. 



Fig. 4. 



»ft- 6 









~-&' 













-V 







<^ 







^-h 



" lb 



10 



a' — nickel-steel wire (45 p. cent. Ni) 



annealed. 

 a" — Ditto, unannealed. 



b'= nickel-steel wire (35 p. cent. Ni) 



annealed. 

 b" = Ditto, unannealed. 



One distinct feature in the curves of elongation is the 

 effect of annealing. In both 45 and 35 per cent. Ni the 

 wire elongates much more in the annealed than in the hard 

 drawn state, so that in cases where the change caused by mag- 

 netization is to be feared we shall be able to eliminate the 

 errors due to magnetostriction in a considerable degree by 

 using the unannealed metal. 



§ 4. Effect of Mechanically Elongating Cobalt 

 and Nickel-Steels. 



A remarkable feature in the magnetostriction is the re- 

 ciprocal relation between the strain caused by magnetization 

 and the effect of stress on magnetization. We have already 

 examined the different changes from this standpoint for iron 

 and nickel. In the present experiment we made special 

 examination into the effect of longitudinal pull on the mag- 

 netization of cobalt and nickel-steels in the same light. 



Magnetometric measurement of the change of magnetiza- 

 tion of a cast cobalt cylinder by loading shows decrease in 

 low fields (see fig. 5). As the field strength is increased the 

 amount of the decrease reaches a maximum and then gradually 

 lessens. Ultimately the field at which the longitudinal pull 

 does not affect the magnetization is reached. When this 

 stage is over the magnetization increases by loading, so that 



