Messrs. H. Nagaoka and K. Honda on Magnetostriction. 289 



(b) (Theory and Experiment). — Hydrostatic pressure 



produces small increase of magnetization in nickel. 



(c) (Theory and Experiment). — By the application of 



small' longitudinal pull, there is increase of mag- 

 netization in iron till it reaches a maximum in 

 moderate fields, thence to diminish till the magneti- 

 zation becomes smaller than in the unstretched 

 condition. 



(d) (Theory and Experiment). — By the application of 



longitudinal pull, there is decrease of magnetiza- 

 tion in nickel till it reaches a minimum in moderate 

 fields, thence to increase gradually but not in such 

 a degree as to reach a value greater than in the 

 unstretched condition. 



II. Strains caused by magnetization deduced from the effects 



of stress. 



(a) (Theory and Experiment) . — Magnetization produces 



increase of volume in iron (the value assigned by 



theory being about 15 times greater than the 



observed numbers). 

 (6) (Theory). — Magnetization produces small increase of 



volume in nickel (to a degree which is within the 



errors of experiment). 

 (Experiment). — Magnetization produces decrease of 



volume in nickel, 

 (c) (Theory and Experiment). — Magnetization produces 



increase of length in iron till it reaches a maximum 



in about H = 30, thence to diminish gradually with 



increasing field. 

 {d) (Theory and Experiment). — Magnetization produces 



continuous diminution of length in nickel. 



Experiments show that the coefficients k, k 1 , k" are all 

 functions of the strain, but KirchhorFs theory makes the 

 change of magnetization proportional to the strain. Strictly 

 speaking, the present theory is a rough approximation and 

 will perhaps only hold when the strain is infinitely small. 

 We cannot, therefore, expect that such a theory can explain 

 the relations between the strains caused by magnetization and 

 the effects of stress on magnetization in all their qualita- 

 tive and quantitative details. In the present investigation, 

 we have taken care to measure such effects as will be most 

 conformable to theory. We have thus found out that, ex- 

 cepting the theoretical deduction as to the effect of hydrostatic 

 pressure on the magnetization of iron, there are no serious 



