560 Prof. Thomson on the Applications of Mechanical Effect 



Taking as the thermal unit the quantity of heat required to 

 raise the temperature of a grain of water by 1° Cent., we find 

 5 7*2 13 as the heat generated in the silver wire in one second, 

 of which the mechanical equivalent is 44758 x 57*213. Divi- 

 dmg this bjj the SQ^ of the strength of the cupent^weftad 



^Mo!>..!m,' "h '■:' 15276000 '\'---lj-''i''.l 



for the absolute resistance of the silver wire ; and by multiplying 

 by '74964, we deduce 



11451000 



for the absolute resistance of the mercury conductor. Multi- 

 plying each absolute resistance by the sectional area of the con- 

 ductor to which it coiTcsponds, and dividing by the length ; and 

 again, multiplying each resistance by the mass, and dividing by 

 the square of the length, we obtain the following results with 

 reference to the specific resistances of silver and mercury at about 

 10° Cent, of temperature. 



A .m 



16* The ^'conducting powers ^^ of metals, as ordinarily de- 

 fined, are inversely proportional to their specific resistances re- 

 fen-ed to unity of volume. Hence, according to the preceding 

 results, the conducting powers of silver and mercury at about 

 10° Cent, of temperature are in the proportion of 1 to •01744. 

 According to the experiments of M. E. Bccquercl (Dove's Reper- 

 torium, vol. viii. p. 193), the conducting powers of silver and 

 mercury at 0^ Cent, are in the proportion of 1 to •017387; and 

 at 100^ Cent., of 1 to -022083 : at 10° Cent, they must there- 

 fore be nearly in the proportion of 1 to '01786, which agrees 

 veiy closely with the preceding comparative result. Again, accord- 

 ing to M. BccquerePs experiments, the conducting powers of 

 silver and copper are, — 



