1882.] 



of Magnetic Susceptibility. 



287 



ceptibilitj of the steel wires for low magnetising forces can be greatly 

 increased. Further details regarding these interesting points are- 

 difficult to describe in a few words, but can readily be understood on 

 reference to the paper itself. 



The results of the experiments performed upon the magnetisation 

 of somewhat thick bars (from *9 to '95 square centim. in section) of 

 malleable iron, hard-tempered steel, and cast-iron, are also recorded 

 fully in this paper. The intensity of magnetisation of each bar for 

 various magnetising forces under different circumstances, is shown by 

 means of curves, of which the " direct-curves " represent the results 

 obtained by beginning with a low magnetising force, which was 

 gradually increased to such a high degree of strength as to magnetise 

 the bar to saturation ; while the " return-curves " represent the results 

 arrived at by coming down from a large magnetising force to 

 smaller and smaller forces, passing through the zero, and gradually 

 going up to a large magnetising force on the negative side of the zero. 

 The direct-curves prove that the intensity of magnetisation of the 

 steel bar is slightly greater, at least for high magnetising forces, than 

 that of the cast-iron bar, but is vastly smaller than that of the malle- 

 able iron bar for all magnetising forces. The maximum intensity of 

 magnetisation of the soft iron, steel, and cast-iron bar, is found to be 

 approximately 1,330, 860 and 770 respectively ; while the smallest 

 magnetising force giving that magnetisation is roughly 190, 450,. 

 and 400 respectively. The difference in the intensity of magnetisa- 

 tion of these bars is, no doubt, due to the fact that the soft iron bar 

 is far superior in respect to magnetisability to both the hard-tempered 

 steel bar and the cast-iron bar; although the difference that exists 

 between the soft iron bar and the wires in the intensity of magnetisa- 

 tion for all magnetising forces is probably due mainly to the effects 

 of the dimensions of the bar, as has been mathematically demonstrated 

 by Green. But the chief point of interest lies in the return-curves ; 

 they show that in the case of each bar the magnetisation does not 

 reverse until the magnetising force exceeds a certain negative value, 

 and that this value is considerable even in the case of the soft iron bar, 

 considerably greater in the case of the cast-iron bar, and still greater 

 — enormously greater — in the case of the steel bar. 



An illustration of the beauty of this magnetometric method by 

 means of curves showing the change in the distribution of magnetism 

 in a wire corresponding to the change in the magnetising force 

 to which it is subjected, draws the paper to a close. The curves 

 decidedly show that the magnetisation of the wire for a low magne- 

 tising force is far from being solenoidal, being stronger towards the 

 centre, but that as the magnetising force is made higher and higher 

 the distribution of magnetism in the wire tends more and more to 

 uniformity, until it attains nearly, if indeed not quite, a solenoidal state 



