father and grandfather, who came to Salford from Youlgreave in Derbyshire, were brewers, but the former disposed of the business in 1854, owing to failing health. As a boy Joule was delicate, and in consequence received his early education at home till he reached the age of sixteen. In 1835 he began with his brother Benjamin to study under Dalton, who was then president of the Manchester Literary and Philosophical Society. Dalton taught the boys algebra and geometry, and had just introduced them to chemistry when an attack of paralysis disabled him. But from this distinguished chemist Joule received his first inducement to undertake the work of an original scientific investigator. A room in his father's house was allotted to him as a laboratory, and he began electrical and magnetic experiments, which bore their first fruit in a published paper ‘On an Electro-magnetic Engine’ (Sturgeon, Annals of Electricity, 1838). Various other papers on magnetism and electro-magnetism followed; one of these, ‘On Electro-magnetic Forces’ (ib. 1840), describes almost the earliest attempt known to measure an electric current in terms of a unit. A unit current is defined by Joule as one which, if allowed to pass for an hour through a water voltameter, will decompose nine grains of water. In a lecture delivered at Manchester in February 1841 (ib. vol. viii.) Joule showed that the efficiency of the most nearly perfect electro-magnetic motor that he had contrived was, per lb. of zinc used in the battery, about one-fifth of the efficiency of a good Cornish pumping-engine per lb. of coal. ‘This comparison,’ he concluded, ‘is so very unfavourable that I confess I almost despair of the success of electromagnetic attractions as a means of power.’ The same lecture contains an account of his experimental discovery of the important fact, ‘suggested by an ingenious gentleman of this town,’ that an iron bar is increased in length on being magnetised. When Joule read his first paper—‘On the Electric Origin of the Heat of Combustion’—before the Manchester Literary and Philosophical Society (2 Nov. 1841), Dalton attended, and for the first time in his life moved a vote of thanks to the author. Joule was elected a member of the society 25 Jan. 1842, and was elected librarian in 1844, honorary secretary in 1846, a vice-president in 1851, and president for the first time in 1860. He regularly attended the society's meetings, and throughout his life found there his most congenial society.
In a paper ‘On the Production of Heat by Voltaic Electricity’ (Proc. R. S. 17 Dec. 1840) the first of the great laws with which Joule's name is imperishably connected was announced. The experiments are given in detail in the ‘Philosophical Magazine’ (xix. 260). Ohm in his work ‘Die galvanische Kette,’ 1827, had introduced and defined the accurate notions to which we now give the names of electro-motive force, current, and resistance, and had stated the law which goes by his name. Fairly satisfactory methods of comparing resistances had been devised, and Joule himself by his improvements had made the tangent galvanometer an accurate instrument for the measure of current. The fact that a current produced heat in a conductor through which it passed had been frequently observed, and Davy (Phil. Trans. 1821) had experimented on wires of different materials but of the same dimensions, arranging them in order according to the magnitude of the heat produced. Joule, however, in the paper now under consideration, was the first to announce the definite law that ‘when a current of voltaic electricity is propagated along a metallic conductor the heat evolved in a given time is proportional to the resistance of the conductor multiplied by the square of the electric intensity,’ i.e. electric current. In the same paper he showed that the law applies, when proper allowance is made for certain disturbances, to heat produced in electrolytes. The paper also contained the first reference to a ‘standard of resistance;’ this consisted of a coil of ten feet of copper wire .024 inch in thickness.
These experiments contained the germs of Joule's second great discovery, the equivalence of heat and energy, which he fully developed later. But he had already made it clear that the energy set free in the battery is also proportional to the resistance of the circuit and to the square of the current.
Joule embodied further results of his researches in important papers on the electro-motive forces of various forms of voltaic cells and the heats of combination of the materials of the cells. The results of his experiments down to 1843, and of the theoretical conclusions drawn from them, are summed up in a paper ‘On the Heat evolved during the Electrolysis of Water’ (Mem. Manchester Lit. and Phil. Soc. vol. vii.), and they still form an exposition of the leading principles of the energetics of the electric current. In reading these researches it must be remembered that the intensity of the current—in Ohm's words, its ‘Spannung’—is what we now call electro-motive force. The most important of his conclusions may be quoted: ‘Third—Hence it is that, however we arrange the voltaic apparatus, and whatever cells of electrolysis we include in the circuit, the whole
