34 



Mr. G-. Barlow. Effects of Magnetisation [June 18, 



Results. — The corresponding values of A<£, H, and I for the process 

 of ascending reversals of magnetisation in nickel, iron, and steel are 

 contained in Table II. The change of resistance is always an increase, 

 and is about eight times greater in nickel than in iron or steel for the 

 same field. If we take A<£ as ordinate and H as abscissa, the curves 

 obtained will be found to have the same characteristics as the ordi- 

 nary magnetisation curves, except that the points of inflection are 

 not so strongly marked. The result is quite different if I be taken as 

 abscissa. In this case we obtain for the three metals a curve which 

 has no point of inflection, but which resembles in general form a semi- 

 cubical parabola, the curve being remarkably flat near the origin and 

 then rising very steeply. Figs. 2 and 3 show the relation of A<£ to I for 

 nickel and iron respectively, the abscissa being the square of the 

 magnetisation. The curve for steel differs very little from that of 

 iron. 



For the three metals it is found that the relation A<£ = al 4 , suggested 

 by the experiments of Gray and Jones, is not generally satisfied (i.e., 

 the curves in figs. 2 and 3 are not parabolas), although it is nearly 

 true for strong fields in the case of iron. In weak fields the variation 

 of resistance appears to depend on a lower power of the magnetisation, 

 and in order to represent the results for the three metals by the same 

 formula it was found necessary to adopt the more general expression 



A</> = aP + bV + d 6 , 



which must only be regarded as a convenient method of expressing 

 the results. The relative values of the coefficients a, b, and c, are as 

 follows : — 



Nickel a : b : c = 19 : 57 : -4 



Iron '. a : b : e = 74 : 3 : 31 



Thus the term containing I 4 is relatively much more important in 

 nickel than in iron. 



An examination of the hysteresis effects in the three metals led to 

 some interesting results which will be best understood by reference to 

 fig. 4. This curye exhibits the variation of resistance in the nickel 

 wire for a cycle of magnetisation corresponding to a double reversal 

 of a magnetic field of about 165 c.g.s. The resistance hysteresis 

 loops for the three metals possess the same characteristic features. As 

 the field diminishes from its maximum value, H = + 165 to zero, the 

 descending branch of the curve lies above the ascending, and in zero 

 field the two branches intersect at the point which represents the 

 residual change of resistance. Then as the field is reversed to small 

 negative values, the resistance continues to diminish until at a certain 

 critical value of the field the curve reaches a minimum. After passing 

 this point the curve rises rather steeply (especially in iron and steel), 



