The best-known of Joule's experiments was that in which a brass paddle consisting of eight arms rotated in a cylindrical vessel of water containing four fixed vanes, which allowed the passage of the arms of the paddle but prevented the water from rotating as a whole.
Barrett in 1882 that a nickel bar contracts when magnetized, nothing of importance was added by Joule's results for nearly forty years.
According to Joule's observations, the length of a bar of iron or soft steel was increased by magnetization, the elongation being proportional up to a certain point to the square of the intensity of magnetization; but when the " saturation point " was approached the elongation was less than this law would require, and a stage was finally reached at which further increase of the magnetizing force produced little or no effect upon the length.
From data contained in Joule's paper it may be calculated that the strongest external field Ho produced by his coil was about 126 C.G.S.
The object of the present article is to illustrate the practical application of the two general principles - (I) Joule's law of the equivalence of heat and work, and (2) Carnot's principle, that the efficiency of a reversible engine depends only on the temperatures between which it works; these principles are commonly known as the first and second laws of thermodynamics.
Clausius (1850), applying the same assumption, deduced the same value of F'(t), and showed that it was consistent with the mechanical theory and Joule's experiments, but required that a vapour like steam should deviate more considerably from the gaseous laws than was at that time generally admitted.
Joule's experiments on the equivalence of W and H were not sufficiently precise to decide the question.
Ultimately the discrepancy was traced to an error which, not by Joule's fault, vitiated the determination by the electrical method, for it was found that the standard ohm, as actually defined by the British Association committee and as used by him, was slightly smaller than was intended; when the necessary corrections were made the results of the two methods were almost precisely congruent, and thus the figure 772-55 was vindicated.
In addition, numerous other researches stand to Joule's credit - the work done in compressing gases and the thermal changes they undergo when forced under pressure through small apertures (with Lord Kelvin), the change of volume on solution, the change of temperature produced by the longitudinal extension and compression of solids, &c. It was during the experiments involved by the first of these inquiries that Joule was incidentally led to appreciate the value of surface condensation in increasing the efficiency of the steam engine.
Joule's final result was 772.55 foot-bounds at Manchester per pound-degreeFahrenheit at a temperature of 62° F., but individual experiments differed by as much as I %.
Measure is equivalent to 4.177 joules per calorie at 16.5° C., on the scale of Joule's mercury thermometer.
Joule'S Scientific Papers (London, 1890); Ames And Griffiths, Reports To The International Congress (Paris, 1900), " On The Mechanical Equivalent Of Heat," And " On The Specific Heat Of Water"; Griffiths, Thermal Measurement Of Energy (Cambridge, 1901); Callendar And Barnes, Phil.
Employing Joule's value of the mechanical equivalent of heat, then recently published, in connexion with the value of the ratio of the specific heats of air S/s=I.