THE SMITHSONIAN INSTITUTION. 



373 



tlie two currents comes into play. In one of the closing conductors 

 the rheostat is inserted, hy means of which the resistance in both 

 circuits caj ha made perfectly equal, so that the galvanometer needle 

 remains at zero. Now, if in the other circuit we insert the wire to 

 he determined, then to retain the needle at zero, the resistance of an 

 equivalent number of rheostat coils must be added to the existing 

 resistance. In this way the resistance of the wire is first expressed 

 in rheostat coils. 



It is easily seen that this method is practically the same as that by 

 Wheatstone's differeni^ial resistance-measurer, which, however, has 

 the great advantage that with it any ordinary galvanometer can be 

 used, while Becquerel's method requires one of peculiar construction. 



The silver which Becquerel used in his experiments was reduced 

 from the chloride, and Uie copper was precij)itated electro-chemically 

 and melted. 



The numbers of the last column are computed from experiments 

 which Frick and myself made conjointly by Wheatstone's method. 

 The copper was trom galvanic prec* itation. 



Most of the experiments gave for silver a resistance very near to 

 that of copper, while Eiess and Lenz before him found it considerably 

 less. This great diSerence cannol; depend upon the want of purity in 

 the silver, for that would increase rather than diminish the resistance. 



According to the measurements of Lenz (Pog. Ann. XLIV, 345) 

 the resistance o ' 



Antimony is 11.23 



Mercury is 21.45 



Bismath is 38.47 



§30. Dependence of the resistance of metols on temperature. — Lenz 

 has investigated the influence of change of temperature on the con- 

 ductive capacity of metals. His reports may be found in Poggen- 

 dorff's Annalen, Bd. XXXIV, p. 418, and Bd. XLV, p. 105. We 

 extract from the last-named paper the following results : 



. It is very evident from this table how great the influence of heat is 

 on the conductive capaci y of metals, and also how unequal this influ- 

 ence is in the different metals. For instance, at 100° the last five 



