Middle of the Nineteenth Century. 239 



and completed by James Prescott Joule, of Manchester, in 1841. 

 Joule, who believed* that heat is producible from mechanical 

 work and convertible into it, measuredf the amount of heat 

 evolved in unit time in a metallic wire, through which a 

 current of known strength was passed; he found the amount 

 to be proportional to the resistance of the wire multiplied by 

 the square of the current- strength ; or (as follows from Ohm's 

 law) to the current-strength multiplied by the difference of 

 electric tensions at the extremities of the wire. 



The quantity of energy yielded up as heat in the outer 

 circuit being thus known, it became possible to consider the 

 transference of energy in the circuit as a whole. " When," 

 wrote Joule, " any voltaic arrangement, whether simple or 

 compound, passes a current of electricity through any substance, 

 whether an electrolyte or not, the total voltaic heat which is 

 generated in any time is proportional to the number of atoms 

 which are electrolyzed in each cell of the circuit, multiplied 

 by the virtual intensity of the battery : if a decomposing cell 

 be in the circuit, the virtual intensity of the battery is reduced 

 in proportion to its resistance to electrolyzation." In the same 

 year hej enhanced the significance of this by showing that the 

 quantities of heat which are evolved by the combustion of the 

 equivalents of bodies are proportional to the intensities of their 

 affinities for oxygen, as measured by the electromotive force 

 of a battery required to decompose the oxide electrolytically. 



The theory of Koget and Faraday, thus perfected by Joule, 

 enables us to trace quantitatively the transformations of energy 

 in the voltaic cell and circuit. The primary source of energy 

 is the chemical reaction : in a Daniell cell, ZnjZn SOJCu S0 4 |Cu, 

 for instance, it is the substitution of zinc for copper as the 

 partner of the sulphion. The strength of the chemical affinities 

 concerned is in this case measured by the difference of the heats 

 of formation of zinc sulphate and copper sulphate ; and it is 



*Cf. p. 33. 



t Phil. Mag. xix (1841), p. 260 ; Joule's Scientific Papers i, p. 60. 

 I Phil. Mag. xx (1841), p. 98 : cf. also Phil. Mag. xxii (1843), p. 204. 



