Determinations of Magnetic Susceptibility. . 439 



and (a) and (b) are symmetrically placed with respect to the hori- 

 zontal axis, each to each, shows that the ultimate effect of " Ons and 

 Offs/' is to magnetise the wire to the same degree of intensity, under 

 the same circumstances, whether the magnetising force be in one or 

 in the opposite direction. On the other hand, the curve (c) shows 

 that when the magnetising force is so high as 60 units or so the wire 

 seems to lose its retentiveness, so much so, that the reversal of the 

 polarities of the wire by the reversal of the force is so complete that 

 the operation of " Ons and Offs " produced no permanent effect ; but 

 that when the magnetising force is below that value the simple reversal 

 of the force is not so effective as to annul the permanent effects of 

 " Ons and Offs," or even to reverse the polarities of the wire. It is 

 obvious that the excess of the intensity of magnetisation represented 

 by the curve (&) over that represented by the curve (/), correspond- 

 ing to any magnetising force, is a measure of the retentiveness of 

 the wire for that magnetising force. 



Remarks so very similar to those made on the curves in the 

 Diagram I apply to the corresponding curves in the Diagram II that 

 it is quite unnecessary to mention them. The comparison of the two 

 sets of curves in the two diagrams, however, presents many points of 

 interest. The curves (a) and (&) in these diagrams show that for 

 some low magnetising forces the intensity of magnetisation of the 

 "Bright Wire" is greater than that of the "Dark Wire;" this is, 

 perhaps, not because the former is more susceptible of magnetisation 

 than the latter, but chiefly because of the fact that there is for each 

 wire a certain amount of pull (used for " Ons and Offs ") which would 

 give a maximum effect on the magnetisation of the wire, and that a 

 weight of 12 kilogs. is nearer that value for the bright wire than a 

 weight of 8 kilogs. is for the dark wire. As regards the critical point, 

 we see that it is about 15 units in the case of the dark wire, while it 

 is about 10 units in the case of the bright wire ; bat this point is no 

 doubt different, not only for different kinds of wire but also for 

 different amounts of the pull. But it is in the curve (c) that the 

 chief interest lies. The comparison of the curves (c) and (e) in the 

 two diagrams shows that the effect of reversing the magnetising force 

 on the change or reversal of magnetisation is considerably less in the 

 case of the bright wire than in the case of the dark wire, both which 

 must doubtless be accounted for by supposing that the one (tolerably 

 soft iron) has a greater coercive force than the other (exceedingly 

 soft iron) , as might be expected. 



The comparison of the curves in the Diagram III with those in 

 the Diagram I or II is also interesting. The most striking point ia 

 that, unlike the case of soft iron wires, there is no such thing as 

 critical point in the case of steel wire, as the curves (a) and (6) in the 

 Diagram III point out; for every magnetising force the intensity of 



