Magnetic Perme,ability of various Alloys of Iron. 85 



Notwithstanding that coppei' is at least six times the conductivity of iron, 

 alloying iron with a small percentage of copper does not appear to increase its 

 conductivity materially, if at all. This indefinite conclusion is due to the fact 

 that the specimens tested contained a large percentage of carbon or manganese 

 (and 1149 A contained aluminium), so that the true conductivity of a pure alloy 

 of copper and iron is not shown in these results. The only comparison possible 

 is between 1264 A and B, where an increase of 0'9 per cent, of copper diminishes 

 the resistance half a microhm, equivalent to 0"6 microhm for one per cent, of 

 added copper ; but even here this small decrease may be due to the rather lower 

 carbon and manganese present in 1264 B. 



These copper steels were drawn into wire (about No. 20 B. W. Gr.) and 

 annealed, the specific resistance at 15° C ; and the temperature-coefficient between 

 10° C. and 120° C. of each specimen was then determined with the following 

 results. The percentage composition is given in tlie preceding table, and the 

 results are here arranged in the order of increasing resistance. 



Specific Kesistance of Coppee Stuels (Tested in Form of Wii-e). 



It will be noticed that the specific resistance of these annealed wires, as 

 directly determined, agrees fairly well with the resistance as deduced from the 

 conductivity of the same alloy in the form of a rod, given in the last column of 

 the table in Group 8 ; the somewhat lower resistance in the former case is 

 doubtless due to slight differences in the annealing of the two series of speci- 

 mens. 



For the convenience of those who prefer results given in terms of resistance 

 rather than conductivity, the curves shown in fig. 6 have been drawn. Here the 

 ordinates are sp. resistances in microhms per c.c. (at about 16° C), and the 

 abscissse the various percentages of the elements named which were added to the 

 iron. These curves are, of course, merely the reciprocals of those shown in 

 Plate II., the results having been deduced from the conductivity in the manner 

 already described (p. 72). As the determinations of the conductivity of the 

 specimens in the annealed state were not finished when the figure was engraved, the 



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