SCIENCE AND PRACTICE. 



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cially mentioned. It is from this alloy that, at present, 

 almost exclusively resistance-coils are manufactured. The 

 conducting power of a specimen when hard-drawn, as deter- 

 mined by Dr. Arndsten, is only 10,532 times as great as 

 that of pure mercury at C., whilst that of a specimen 

 when annealed, according to Dr. Siemens, is still less. 

 Another advantage which this alloy has in common with 

 most of the others is that temperature exerts a comparatively 

 small effect upon its resistance. The conducting power c at 

 a temperature i, according to Arndsten, of this alloy, whose 

 conducting power at C. is 100, is expressed by 



c = 100 0,0387 t + 0,0000557 P 



which is very little over a tenth part of the change found 

 for the pure metals. 



33. Metals annealed. The degree of hardness or softness 

 of a metal or alloy affects materially its conducting power. 

 That of a hard- drawn wire is not the same as when the wire 

 has been made hot and let cool again ; and to the fa,ct that 

 not sufficient importance was attached to this property has 

 been justly attributed the differences between the results of 

 different observers. 



Dr. Matthiessen has found it necessary, in order to obtain 

 comparable results, not only to heat wires to 100 C. before 

 measuring their resistances, but even to keep them during 

 several days at that temperature before their resistances 

 became constant below that point. 



The conducting powers of the metals and alloys are 

 increased by annealing. 



Peltier first pointed out this phenomenon in the behaviour 

 of copper ; and Matthiessen repeated his experiments with 

 copper and silver, with the following results : 



