Determinations of Magnetic Susceptibility. 



443 



that both the cast-iron bar and the steel bar are greatly inferior to the 

 soft iron bar in respect to magnetisability. This is indeed unexpected, 

 and in some measure astonishing, remembering that the steel pianoforte 

 wire was not at all inferior in this respect to the soft iron wires, at 

 least for higher magnetising forces. The difference that is found in 

 the maximum intensity of magnetisation and the minimum mag- 

 netising force corresponding to that magnetisation between the soft 

 iron bar and the wires is, however, no doubt, chiefly due to the effects 

 of the dimensions of the bar. 



Another point of interest lies in the " return curves."* They show 

 that in the case of each bar the magnetisation of the bar did not 

 reverse until the magnetising force exceeded a certain value on the 

 negative side of the zero, and that this value is considerable even in 

 the case of the soft iron bar, considerably greater in the case 

 of the cast iron, and still greater — greater by a vast amount — in the 

 case of the steel bar. A complete curve for the residual mag- 

 netism was only obtained, or at least only shown, for the cast 

 iron ; but the fact that those points in the return curves corresponding 

 to the zero magnetising force represent the maximum residual 

 magnetism of the corresponding bars, will give us a rough indica- 

 tion of what might be the residual magnetism curves for the other 

 bars. 



I have now given the general explanations and discussions of all 

 the results of the experiments, and as 1 fear space does not permit me 

 to enter into a fuller discussion of all the details of the results and of 

 the inferences that can be drawn from them, I am obliged to leave 

 them untouched. There is, however, one very interesting and 

 important conclusion which can be derived from the results and which 

 I cannot help noting specially, as it illustrates the beauty of this 

 magnetometric method, and that is, in regard to the change in the 

 distribution of magnetism of the wires or bars due to the corresponding 

 change in the magnetising force to which they are subjected. It has 

 already been said that one way to study the results given in the 

 Tables I, II, &c, is to trace curves in the manner explained. Now, it 

 is easy to get two such curves as (1) and (2) of the Plate 10 for each 

 set of the results, one representing the effect due to both the magnetism 

 of the wire or bar, and the coil carrying a current, and the other 

 representing the same due to the coil alone. If we draw another 

 curve such that its abscissa at every point of it is the difference of the 

 abscissae at the same point of the two curves, we obtain a curve 

 representing the effect due to the magnetism of the wire or bar alone. 

 The curve representing the effect of the coil alone can be easily 



* Compare these curves with those given in Sir William Thomson's paper re- 

 ferred to before, " Phil. Trans.," 1879, Plates 8 and 9. 



