124 Barrett, Brown & Haprietp—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 specimens 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 perhaps in a few carbon steels, which 
were in similar physical 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.G. 8. 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 specimen of good annealed iron. Hopkinson 
used the ‘‘yoke” method, with short, turned, cylindrical bars; Ewing t 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 :— 
Hopkinson. Ewing. Our results 
H = 200. H =17°3. (uncorrected). 
Maximum induction, . ; 19540 13450 17480 
Residual induction, . : 7080 10980 7120 
Permeability H=8, . ‘ 1400 1550 1560 
Coercive force, . ‘ 5 1:63 1:90 1°66 
Energy dissipated, in ergs, . 10290 9300 11090 
The specimen of iron we used was of higher purity than the sample used by 
* Phil. Trans. of the Royal Society, 1885, Part 1., p. 495. + Ib., p. 523. 
