■^^sV^ 



ib the MeasurenMfdfilledW'mtive Forces. 561 



at 0° in the proportion of 1 to '91617, 

 at 100° ... 1 to -91030, 



and therefore 



at 10' 



1 to '915. 



mm 



Hence the specific resistance of copper at about 10° Cent. refeBv 

 red to unity of volume, may be found by dividing that of silver 

 by '915 ; and from the preceding result, it is thus found to be 

 2*080. Multiplying this by 3810500, the weight in grains of a 

 cubic foot of copper, (found by taking 8*72 as the specific gra- 

 vity of copper,) we obtain for the *' specific resistance ^^ of copper 

 the value 7925800. 



16. Weber, in first introducing the measurement of resistances 

 in absolute units, gave two experimental methods, both founded 

 virtually on a comparison of the electromotive forces with the 

 strengths of the currents produced by them, in the conductors 

 examined ; and he actually applied them to various conductors^, 

 and obtained results which, reduced to British units, are shown 

 in the following table. The first four numbers in the second 

 column are deduced from M. Weber^s results, on the hypothesis 

 that the specific gravity of each specimen of copper is 8*72. 

 The only numbers given on the authority of M. Weber are the 

 first four of the column headed "Specific resistance.^^ The 

 specific resistances derived above from Mr. Joule^s experiments 

 are shown in the same table for the sake of comparison. 



The great discrepancies among the first four numbers of the 

 third column, each of which is probably correct in three of its 

 significant figures, show how very much the specific resistances 

 of the substance of different specimens of copper wire may differ 

 from one another. The specific resistance of copper (No. 5), 

 deduced indirectly from Joule's absolute by means of BecquerePs 

 relative determinations, agrees very closely with that of the elec- 

 trolytically precipitated copper (No. 4) expei^imented on by 



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