442 



Mr. R. Shida. 



the soft iron wires when retentiveness is disregarded, can be calcu- 

 lated, if required, from the magnetisation represented by the carves 

 (/), Plates 11, 12, 13. 



The greatest value for the magnetic susceptibility I obtained of soft 

 iron wire is about 730, the corresponding magnetising force being the 

 Glasgow vertical force, and it is probably still greater for smaller 

 magnetising forces; while the magnetic susceptibility of the same 

 wire for so high a magnetising force as 100 units, is only about 13, 

 and still smaller, no doubt, for higher magnetising forces. These 

 results are truly surprising, and will dispel any doubt as to the old 

 view that the value of fi is constant or nearly so for all or a certain 

 range of the magnetising force. 



I will now proceed to explain the curves in the Plate 15 which 

 represent the results for the bars. The "direct curves" show the 

 results obtained by commencing with a small magnetising force which 

 was gradually increased until it is so high as to magnetise the bars 

 very strongly, if not to saturation; while the "return curves" 

 represent the results obtained by coming down from a high mag- 

 netising force to lower and lower magnetising forces, passing through 

 the zero and going up gradually to a high magnetising force on the 

 negative side of the zero. It may be mentioned that the reason why 

 for the steel bar the direct curve was not obtained is because the bar, 

 which was one of those originally intended to be used for Sir William 

 Thomson's new Siphon Recorder, was previously magnetised strongly, 

 and, therefore, the experiment on it was commenced by using a high 

 magnetising force to start with ; and that there is every reason to 

 believe that the direct curve for the steel bar is something like that 

 for the cast-iron bar. 



On comparing the " direct curves " in the Plate 15, we see that the 

 magnetisation of the cast-iron bar is somewhat less for high mag- 

 netising forces than that of the steel bar, and is much less for every 

 magnetising force than that of the soft iron bar ; and that the 

 maximum magnetisation of the soft iron bar is about 1340, that 

 of the steel bar is about 860, and that of the cast-iron bar is only 

 about 770, while the corresponding least magnetising force in the case 

 of the first is only about 190 units, and in the case of the second and 

 third, it is roughly 450 and 400 units or more. Of course, it is not 

 quite right to assume that the above results represent accurate 

 comparisons of the magnetisable qualities of those different kinds of 

 iron and steel, because the bars are not the same in dimensions, which 

 have very considerable effects on the intensity of magnetisation. 

 Still considering that the difference in dimensions between the soft 

 iron bar and the other bars is very small, while in both the maximum 

 intensity of magnetisation and the minimum magnetising force 

 corresponding to it they differ greatly from each other, it is certain 



