6i8 



NATURE 



[Oct. 26, 1 : 



as unit quantity of electricity, the quantity required to 

 decompose nine grains of water, 9 being the atomic weight 

 of water, according to the chemical nomenclature then 

 in use. 



He had already made and described very important 

 improvements in the construction of galvanometers, and 

 he graduated his tangent galvanometer to correspond 

 with the system of electric measurement he had adopted. 

 The electric currents used in his experiments were thence- 

 forth measured on the new system ; and the numbers 

 given in Joule's papers from 1S40 downwards are 

 easily reducible to the modern absolute system of 

 electric measurements, in the construction and general 

 introduction of which he himself took so prominent 

 a part. It was in 1840, also, that after experiment- 

 ing on improvements in voltaic apparatus, he turned 

 his attention to " the heat evolved by metallic conductors 

 of electricity, and in the cells of a battery during electro- 

 lysis." In this paper and those following it in 1841 and 

 1842, he laid the foundation of a new province in physical 

 science — electric and chemical thermodynamics— then 

 totally unknown, but now wonderfully familiar even to 

 the roughest common-sense practical electrician. With 

 regard to the heat evolved by a metallic conductor carry- 

 ing an electric current, he established what was already 

 supposed to be the law, namely, that "the quantity of 

 heat evolved by it [in a given time] is always proportional 

 to the resistance which it presents, whatever may be the 

 length, thickness, shape, or kind of the metallic con- 

 ductor," while he obtained the law, then unknown, that 

 the heat evolved is proportional to the square of the 

 quantity of electricity passing in a given time. Corre- 

 sponding laws were established for the heat evolved by 

 the current passing in the electrolytic cell, and likewise 

 for the heat developed in the cells of the battery itself. 



In the year 1840 he was already speculating on the 

 transformation of chemical energy into heat. In the 

 paper last referred to and in a short abstract in the Pro- 

 ceedings of the Royal Society, December, 1840, he points 

 out that the heat generated in a wire conveying a current 

 of electricity is a part of the heat of chemical combina- 

 tion of the materials used in the voltaic cell, and that 

 the remainder, not the whole heat of combination, is 

 evolved within the cell in which the chemical action takes 

 place. In papers given in 1841 and 1842, he pushes 

 his investigations farther, and shows that the sum of 

 the heat produced in all parts of the circuit during 

 voltaic action is proportional to the chemical action 

 that goes on in the voltaic pile, and again, that the quan- 

 tities of heat which are evolved by the combustion of 

 equivalents of bodies are proportional to the intensities 

 of their affinities for oxygen. Having proceeded thus 

 far, he carried on the same train of reasoning and experi- 

 ment till he was able to announce, in January, 1843, 

 that the magneto-electric machine enables us to convert 

 mechanical power into heat. Most of his spare time in 

 the early part of the year 1843 was devoted to making 

 experiments necessary for the discovery of the laws of 

 the development of heat by magneto-electricity, and for 

 the definite determination of the mechanical value of heat. 

 At the meeting of the British Association at Cork, on 

 August 21, 1843, he read his paper "On the Calorific 

 Effects of Magneto-Electricity, and on the Mechanical 



Value of Heat." The paper gives an account of an 

 admirable series of experiments, proving that heat is. 

 generated (not merely transferred from some source) 

 by the magneto-electric machine. The investigation 

 was pushed on for the purpose of finding whether a 

 constant ratio exists between the heat generated and 

 the mechanical power used in its production. As the 

 result of one set of magneto-electric experiments he 

 finds S38 foot lbs. to be the mechanical equivalent of the 

 quantity of heat capable of increasing the temperature oS 

 one pound of water by one degree of Fahrenheit's scale. 

 The paper is dated Broomhill, July, 1843, but a post- 

 script dated August, 1S43, contains the following sen- 

 tences:— "We shall be obliged to admit that Count 

 Rumford was right in attributing the heat evolved by- 

 boring cannon to friction, and not (in any considerable 

 degree) to any change in the capacity of the metal. I 

 have lately proved experimentally that heat is evolved by 

 the passage of water through narrow tubes. My appa- 

 ratus consisted of a piston peiforated by a number of 

 small holes, working in a cylindrical glass jar containing 

 about 7 lbs. of water. I thus obtained one degree of heat 

 per lb. of water from a mechanical force capable of 

 raising about 770 lbs. to the height of one foot, a result 

 which will be allowed to be very strongly confirmatory of 

 our previous deductions. I shall lose no time in repeat- 

 ing and extending these experiments, being satisfied that 

 the grand agents of nature are, by the Creator's fiat, 

 indestructible, and that wherever mechanical force is 

 expended, an exact equivalent of heat is always ob- 

 tained." 



This was the first determination of the dynamical 

 equivalent of heat. Other naturalists ani experimenters 

 about the same time were attempting to compare the 

 quantity of heat produced under certain circumstances 

 with the quantity of work expended in producing it ; and 

 results and deductions (some of them very remarkable) 

 were given by Seguin (1839), Mayer (1842), Colding 

 (1843), founded partly on experiment, and partly on a 

 kind of metaphysical reasoning. It was Joule, however, 

 who first definitely proposed the problem of determining 

 the relation between heat produced and work done in 

 any mechanical action, and solved the problem directly. 



It is not to be supposed that Joule's discovery and the 

 results of his investigation met with immediate attention 

 or with ready acquiescence. The problem occupied him 

 almost continuously for many years ; and in 1878 he gives 

 in the Philosophical Transactions the results of a fresh 

 determination according to which the quantity of work 

 required to be expended in order to raise the temperature 

 of one pound of water weighed in vacuum from 6o° to 

 61° Fan., is 772*55 foot lbs. of work at the sea-level, and in 

 the latitude of Greenwich. His results of 1849 an d 1878 

 agree in a striking manner with those obtained by Hirn 

 and with those derived from an elaborate series of ex- 

 periments carried out by Prof. Rowland at the expense 

 of the Government of the United States. 



His experiments subsequent to 1843 on the dynamical 

 equivalent of heat must be mentioned briefly. In that 

 year his father removed from Pendlebury to Oak Field, 

 Whalley Range on the south side of Manchester, and 

 built for his son a convenient laboratory near to the house- 

 It was at this time that he felt the pressing need of accu- 



