caloric of the circuit is exactly accounted for by the whole of the chemical changes. Fourth—As was discovered by Faraday, the quantity of current electricity depends upon the number of atoms which suffer electrolysis in each cell, and the intensity depends upon the sum of the chemical affinities. Now both the mechanical and heating powers of a current are (per equivalent of electrolysis in any one of the battery cells) proportional to its intensity. Therefore the mechanical and heating powers of the current are proportional to each other. Fifth—The magnetic electrical machine enables us to convert mechanical power into heat by aid of the electric currents which are induced by it, and I have little doubt that by interposing an electro-magnetic engine in the circuit of a battery a diminution of the heat evolved per equivalent of chemical change would be the consequence, and that in proportion to the mechanical powers obtained.’ If in No 4 above we read electro-motive force for ‘intensity,’ it will be recognised as in accordance with our present knowledge of the subject.
The experimental question referred to in No. 5 was soon submitted to further test, and on 21 Aug. 1843 a paper, the first of a long series on the subject, ‘On the Calorific Effects of Magneto-Electricity and on the Mechanical Value of Heat,’ was read before the British Association at Cork (Phil. Mag. 3rd ser. vol. xxiii.; Collected Papers, i. 123). This remarkable paper describes a number of experiments in which a small electro-magnet was rotated in water in a magnetic field produced either by permanent magnets or by a fixed electro-magnet. The current induced in the moving coils, the total heat generated, and the energy used in maintaining the motion were all measured, and it was shown that the energy used and the heat produced were both proportional to the square of the current. Thus a constant ratio exists between the heat generated and the mechanical power used in its production, so that, to quote from the paper, ‘The quantity of heat capable of increasing the temperature of a pound of water by one degree of Fahrenheit's scale is equal to … a mechanical force capable of raising 838 pounds to a perpendicular height of one foot.’ A postscript to the same paper contains further important statements to the following effect: ‘I have lately proved experimentally that heat is evolved by the passage of water through narrow tubes. … I thus obtain one degree of heat per pound of water from a mechanical force capable of raising about 770 pounds to the height of one foot. I shall lose no time in repeating 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 obtained.’ Thus in 1843, in his small laboratory at Pendlebury, near Manchester, Joule had determined by two distinct methods the physical constant now known as J., or ‘Joule's equivalent,’ and had shown conclusively that heat was a form of energy.
But further experiment was needed. The difference between 838 and 770 was too great to satisfy Joule's desire for exact knowledge. In a paper ‘On the Changes of Temperature produced by the Rarefaction and Condensation of Air’ (Phil. Mag. 3rd ser. May 1845; Collected Papers, i. 171) he described a determination of J. made by observing the heat produced by compressing air and the energy requisite for the compression; the result was 798 foot-pounds. In this paper he obtained the important result necessary to justify his procedure that ‘no change of temperature occurs when air is allowed to expand in such a way as not to develop mechanical power.’
The first series of observations on the development of heat by the friction of water, in which the now celebrated paddle-wheel was employed to stir the water, was communicated to the British Association at Cambridge in 1845. The number obtained was 890 foot-pounds.
A paper ‘On the Heat disengaged in Chemical Combinations’ (Phil. Mag. 4th ser. vol. iii.; Collected Papers, i. 205), though not published till 1852, belongs to the same period. It contains a description of one of the first, if not absolutely the first, really accurate galvanometers. The needle used was half an inch in length, while the coils were one foot in diameter. In 1846, in a paper ‘On the Effects of Magnetism upon the Dimensions of Iron and Steel Bars’ (Phil. Mag. 3rd ser. vol. xxx.; Collected Papers, i. 235), Joule returned to a subject he had discussed five years previously in Sturgeon's ‘Annals,’ and during the following year the fundamental principles of the doctrine of the conservation of energy were clearly stated by him in a popular lecture ‘On Matter, Living Force, and Heat’ (Manchester Courier, 5 and 12 May 1847; Collected Papers, i. 265).
In June 1844 Joule's father moved from Pendlebury to Whalley Range, where he built for his son a convenient laboratory near the house. In this, with the aid of the minutely accurate thermometers made under his direction in 1845 by Mr. Dancer, he was able to carry out more exact experiments on the value of J. as determined by the friction
