Page:Proceedings of the Royal Society of London Vol 69.djvu/501

Rh estimating the conductivity, the results obtained can only be regarded as a more or less close approximation to the true value.*

A more formidable difficulty is the impossibility of obtaining such a collection of alloys free from the admixture of disturbing impurities. Slight variations in the amount of these impurities, especially in the amount of carbon present, produce in the low percentages a profound effect on the conductivity and also on other physical properties of these alloys. However, in most of the specimens Mr. Hadfield has succeeded in reducing these impurities to a lower amount than any other large collection of alloys yet made. Carbon, manganese, and silicon were the impurities most commonly present, and the sum of these in the specimens selected was, as a rule, under 1 per cent.

There were sixty-eight specimens of these alloys having various per- centages of a single element added to iron ; these were as follows (the element with which the iron is alloyed being named in the first column) :

Manganese. ... 18 specimens ranging from $ to 1HJ per cent, of Mn.

Carbon 13 ^. H ,- C.

Nickel 12 i 31 Ni.

Tungsten 4 1 15 W.

Chromium.... 3 2 9 Cr.

Copper 3 H 3 Cu.

Aluminium... 3 I 2k Al.

Silicon 2 2i 5| Si.

In addition to these there were fifty-two other specimens having two or more of the above elements alloyed to iron in various pro- portions.

In every case, with the doubtful exception of copper,! a decrease in conductivity was found to result from alloying iron with another metal, even when that element, as in the case of aluminium, had itself a conductivity far higher than that of the iron. From the conduc- tivities of the specimens, their specific resistances were calculated ; these were plotted against the percentages of the added element. A series of fairly smooth curves were thus obtained for each alloy. These curves are shown in fig. 1. It will be noticed that the addition of silicon or aluminium to iron produces the greatest increase in electric resistance and tungsten the least, and that a remarkable change in the electric resistivity of nickel steels occurs at high percentages. There is obviously no connection between these curves and the electric conduc-

were drawn into wire, and the specific resistances of which were measured in the ordinary way, the results obtained corroborated the values found from the rod*.
 * It was satisfactory, however, to find that in a dozen or more specimens which

t The specimens of copper-iron alloy were very few, and the results masked by the varying impurities they contained; very little alteration of the conductivity appeared to be produced by the addition of copper to iron.