372 



TENTH ANNUAL REPORT OF 



number which denotes how marv times its resistance is greater than 



that of a copper wire of equal dimensions. Eepresenting by s the 



specific resistance of a metal, the absolute resistance iv of a wire with 



r, is 



Z. 0.785 

 IV =z s ^- 



a length I and a radius 



Specif c resistance i«( what Eiess terms electrical retarding force; 

 hitherto the reciprocal value of specific resistance has been indicated 

 by the term capacity for conduction. But in practice it seems ad- 

 visable to use the numerical value of specific resistance instead of 

 capacity for conduction. 



The values found by Buff for sjiecific resistance of silver, copper, 

 and German silver, given above, deserve entire confidence, because 

 they were determined with great care, and by, what is important, a 

 simple and direct method., which is susceptible of the greatest accuracy. 

 The silver was prepared specially for this object in the chemical labo- 

 ratory at Giessen. Tlie copper was jjrepared with great care by the 

 galvanic process but was not entirely free from iron, as ^^nalysis 

 showed that it contained 0.02 per cent, of that metal. The first 

 quality of commercial copper contained 0.22 per cent, of iron ; the 

 second quality, besides a trace of iron, 0.2 per cent, of lead and 0.26 

 per cent, of nickel. 



In the following table the resistances of difi'erent metals- as de- 

 termined by E. Becquerel, (Ann. de chimie et de phys. 3 serie XVII, 

 242 ; Pog. Ann. LXX, 243,) are compared with those found by Kiess, 

 the specific resistance of copper being taken as unity : 



The method by which Becquerel obtained these numbers is essen- 

 tially as follows : His galvanometer, which he terms a differential 

 galvanometer, is formed of two equal but separate wires placed side 

 by side, each three metres long. The ends of the two coils of the 

 multiplier are now so joined to the electro-motor that the current takes 

 opposite directions in them, so that only the difference of strength of 



