V:><il\'3: fQ f]^Q Measurement of Electro-motive Fc^ceh'^^ 559 



if / be the length, co the area of the section, and m the mass (or 

 weight) of a conductor, k its absolute resistance, and €r„ and a 

 the specific resistances of its substance referred respectively to 

 unity of volume and to unity of mass. ojiojI) ;»ii) iufl .-)nri 



14. The absolute resistance of a certain silver wire, and of 

 a column of mercury contained in a spiral glass tube, may be 

 determined from experimental data extracted from a paper of 

 Mr. Joule^s laid before the French Institute [Comptes Rendus, 

 Feb. 9, 1846), and communicated to me by the author. In 

 four experiments on the silver wire, and in four similar expe- 

 riments on the mercury tube, a current measured by a tangent 

 galvanometer was passed through the conductor, and, in each 

 experiment, the quantity of heat evolved during ten minutes was 

 determined by the elevation of temperature produced in a mea- 

 sured mass of water, the temperature of the conductors during 

 all the experiments having been nearly 50° Fahr. The mean 

 result of the four experiments on each conductor is expressed 

 in terms of the square root of the sum of the squares of the tan- 

 gents of the galvanometer-deflections, and the mean quantity of 

 heat evolved in ten minutes. The weight of the silver wire in 

 air and in water, the weight of the mercury contained in the 

 glass tube, and the exact length of each conductor, were deter- 

 mined a short time ago, at my request, by Mr. Joule, and the 

 areas of the sections of the conductors have been deduced. The 

 same galvanometer having been used as was employed in the expe- 

 riments on electrolysis, referred to in the " Note on Electro-che- 

 mical Equivalents," contained in this Volume of the Magazine 

 (p. 429), and the experiments at present referred to having also 

 been made at Manchester in 1845, the strength of the current 

 in absolute measure is found by multiplying the tangent of de- 

 flection by -28186. The various experimental data thus obtained 

 are as follows : — 



-«0A 



his standard conductor was "10 feet long and '024 of an inch thick;'* but 

 there must be some mistake here, as it will be seen below that this is about 

 double what we might expect it to be. I have found 2*17 for the specific 

 resistance of copper referred to unity of volume, according to the experi- 

 ment described in § 9, on a wire stated to be 2 yards long and -i^i^n of an 

 inch thick; and 178 and 1*98, according to the experiments described in 

 §§9 and 11, on a wire stated to be 2 yards long and -gLth of an inch thick; 

 also 77 for that of iron, in a wire stated (§ 11) to be two yards long and 

 :jVth of an inch thick. It is to be remarked, however, that no attempt was 

 made by Mr. Joule to determine the sections of his wires with acciuracy, 

 and that the " thicknesses" are merely mentioned in round numbers, as 

 descriptive of the kinds of wire used in his different experinieii^.^ .^^ ^^^^ 



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