300 Dr. C. Fromme on the Magnetism of Steel Bars. 



expected — since, further, I believed I should find the in- and 

 outbendings of one curve in the others also, I am of opinion 

 that these differences really have their foundation in the nature 

 of the thing — that therefore, for forces which do not alter the 

 'permanent magnetism of a steel bar, k exhibits a behaviour the 

 more regular the greater the permanent magnetization is, the 

 more the molecular magnets are oriented one with another. 



§ 12. Returning now to the starting-point of the experi- 

 ments described in the two preceding sections — the fact that, 

 on repeated employment of the same magnetizing force, steel 

 attains a maximum of residual magnetism, which may be de- 

 signated the saturation-moment corresponding to the force, 

 — we have now to determine whether the number of the im- 

 pulses requisite for attaining the maximum can be brought 

 into a precise relation to the affecting factors. Such a rela- 

 tion would perhaps be found, if a steel bar in constantly equal 

 external and molecular conditions could be submitted to vary- 

 ing magnetizing forces. But now, since every prior magne- 

 tizing force changed the condition of the bar, and so each 

 succeeding one finds the bar in another state, while, though 

 residual magnetism can indeed be got rid of by annealing, the 

 bar is thereby altered in hardness and external form, 1 have 

 only been able in general to lay down the proposition that the 

 saturation-moment is readied more quickly with the softer than 

 with the harder kind of steel. Instead of this rule, however, we 

 may put the more general one, The number of the impulses in- 

 creases with the ratio of the attainable residual moment (moment 

 of saturation) to the original moment (that already present be- 

 fore the action of the force) ; for this ratio is in general greater 

 with the harder steel. 



M. Bouty, in a work * which came under my notice after 

 the close of these investigations, has endeavoured to represent 

 by an empiric formula the quantity of the residual magnetic 

 moment after each single impulse. 



He magnetizes thin steel needles both by constant and by 

 induction-currents, and believes that the increment of the re- 

 sidual magnetism after the latter must be expressed by another 

 empiric formula. 



For the magnetization by a constant current he chooses the 

 formula 



^ = A -?> a) 



which determines the magnetic moment y after the #th im- 



* " Etudes sur le Magnetisme," Annates de VJEcole Normale, 1875, No. 

 1 ; Phil. Mag. Feb. and March 1875, vol. xlix. pp. 81, 180. 



