Magnetic Permeabilii}) of various AUo//s 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 tliese elements 

 should vary in the order we have found, are problems tliat 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 ^jarticles 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. Tlie 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 liighest 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 approxinmtely 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, wliicli 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. Nearl}' all Dr. Hopkinson's specimens had, how- 

 ever, a much liigher percentage of impurities than ours, and hence had a resistance 

 above the normal: see Phil. Trans.., 1885, Part II., p. 463. 



TRANS. EOT DUB. SOC, N.S. VOL. VII., TAKT IT. Q 



