124 Bari;ett, Brown & Hadfield — On the Electrical Conductivity and 



therefore in some cases considerable, and the correction which it was necessary 

 to apply was uncertain in amount. The coercive force found in Mdme. Curie's 

 experiments seems abnormally large, but the sj^jecimcns used were highly tempered, 

 and for the most part contained high carbon ; hence a comparison is not possible 

 between these results and our own, except j^erhaps in a few carbon steels, which 

 were in similar ph3'sical states. In these, Mdme. Curie finds the coercive force 

 increases regularly as the amount of carbon present increases, up to 1'2 per cent.; 

 whilst the residual induction, or retentivity, reaches a maximum at 0*5 per cent, 

 of carbon, and then decreases. Both these results we have also found, as will be 

 seen in the table given on p. 106. We may add that our experiments were made, 

 though not published, some years prior to the appearance of Mdme. Curie's paper. 



A careful determination of the magnetic properties of a few carbon steels and 

 alloys of steel with manganese, chromium, and tungsten was made by Dr. J. 

 Hopkinson, the result being given in his classical paper already referred to, p. 99.* 

 Dr. Hopkinson employed the " yoke" method, and used much higher magnetising 

 forces than we have done ; his specimens were few, and as a rule contained high 

 carbon, as well as being in different physical states. Where his specimens and 

 ours are comparable, the results — though obtained by wholly different methods of 

 experiment — are not far apart, his maximum induction is of course higher, owing 

 to his magnetising force being 200, whilst ours was 45 C. Gr. S. units, and in 

 some similar specimens he finds a higher coercive force than we do. 



It is interesting to compare the results obtained by Dr. Hopkinson, Professor 

 J. A. Ewing, and ourselves with a sjDecimen of good annealed iron. Hopkinson 

 used the "yoke" method, with short, turned, cylindrical bars; Ewingf used a 

 wire, the length of which was 400 diameters, and employed the ballistic method 

 of measurement ; we used nearly cylindrical rods, with a length of 200 diameters, 

 and the magnetometric method. Here are the results for magnetising forces 

 of 200, 17, and 45 C. G. S. respectively :— 



The specimen of iron we used was of higher purity than the sample used by 



* Phil. Ti-ans. of the Royal Society, 1885, Part ii., p. 455. f -^-i P- ^23. 



