Magnetic Permeability of various Alloys of Iron. 99 
Why the conductivity of a metal is so much reduced by the presence of a 
small quantity of certain other elements, and why the effect of these elements 
should vary in the order we have found, are problems that await explanation. 
Whether the results are due to the production of a back electro-motive force 
from the contact of dissimilar elements in the alloy, as suggested by Lord 
Rayleigh, or to the intermixture of badly conducting particles produced by the 
chemical union of a portion of the metal with the added element, or to other 
causes, are questions upon which our experiments will, we hope, ultimately 
throw some light. Our results appear to give some support to Lord Rayleigh’s 
conjecture ; it will, moreover, be observed that the greatest increase in resist- 
ance is: produced by the addition of those elements having the lowest atomic or 
molecular weight. We hope to return to this question in another paper. 
Some experiments on the electric resistance of various alloys of iron or steel 
have recently been made by M. Le Chatelier, and are published in the Comptes 
Rendus for June 13th, 1898. The specimens he used were in the form of short 
bars, 20 cms. long, and of square section, 1 cm. on the side. The total number 
of specimens was not very large, and the amount of impurities present in some of 
them was considerable. M. Le Chatelier finds that the increase of resistance in 
steel produced by adding one per cent. of the following bodies to iron is:—for 
silicon, 14 microhms; carbon, 7; manganese, 5; nickel, from 3 to 7 microhms; 
and for one per cent. of chromium, tungsten, or molybdenum the increase of 
resistance produced was very small. These results, however, are obtained by 
taking the increase of resistance between the lowest and highest percentages of the 
added element in each series of alloys; as the limiting percentages in the speci- 
mens examined by M. Le Chatelier were widely different (varying from 0-06 to 
1:6 per cent. in the case of carbon, and 0°24 to 13 per cent. in the case of 
manganese), his results are not strictly comparable with each other. We find, 
however, that the resistance he obtained for particular specimens is practically the 
same as that found by ourselves for a similar alloy, with approximately the same 
amount of impurities. 
The late Dr. Hopkinson, F.R.S., also determined the electric resistance of a 
few specimens of manganese, silicon, chromium, and tungsten steels, which had 
been subject to different thermal treatment. The general order of conductivity 
he found agrees with our results, and so does the specific resistance of particular 
specimens, when those of approximately the same composition and in the same 
physical state are compared. Nearly all Dr. Hopkinson’s specimens had, how- 
ever, a much higher percentage of impurities than ours, und hence had a resistance 
above the normal: see Phil. Trans., 1885, Part II., p. 463. 
TRANS, ROY DUB. SOC., N.S. VOL. VII., PART IV, Q 
