TODAY IS THE BIRTHDAY OF James Prescott Joule

Born 24 Dec 1818; died 11 Oct 1889 at age 70.  

English physicist who established that the various forms of energy - mechanical, electrical, and heat - are basically the same and can be changed, one into another. Thus he formed the basis of the law of conservation of energy, the first law of thermodynamics. He discovered (1840) the relationship between electric current, resistance, and the amount of heat produced. In 1849 he devised the kinetic theory of gases, and a year later announced the mechanical equivalent of heat. Later, with William Thomson (Lord Kelvin), he discovered the Joule-Thomson effect. The SI unit of energy or work , the joule (symbol J), is named after him. It is defined as the work done when a force of 1 newton moves a distance of 1 metre in the direction of the force. 

The Mechanical Equivalent of Heat

AMONG GALILEOS many scientific interests was the quantitative study of thermal phenomena, for which he invented the first practical instrument, a sensitive but inaccurate form of gas thermometer. During the seventeenth and eighteenth centuries substantial advances were made in the field of thermometry, notably by Gabriel Fahrenheit (1686-1736), Ferchault de Reaumur (1683-1757), and Anders Celsius (1701-1744). These experimenters’ developed several improved types of liquid thermometers and established various fixed points (such as the temperature of melting ice) from which thermometric scales could be devised. The difference between temperature and quantity of heat, frequently confused with one another, was made clear in the latter half of the eighteenth century by Joseph Black (1728-1799). He introduced the science of calorimetry, by which he was led to the idea of specific heat, (Which he called "capacity for heat") and he discovered the concept of latent heat: in changes of state such as ice to water or water to steam, he found that large quantities of heat were absorbed without change in temperature.

Despite these advances, the nature of heat was not understood. It was generally regarded as an imponderable fluid, called caloric, made up of minute particles which repelled one another but were attracted by matter. The caloric was supposed to flow from hotter to colder bodies, and the de-velopment of heat by friction, for example, was explained as due to the fact that friction removed some of the caloric and thereby made the body appear warmer. The melting of ice was accounted for in terms of the combination of caloric with the ice to form water, somewhat in the nature of a chemical compound.   Even Black adhered to the caloric theory, although a number of earlier scientists, including Newton and Boyle, had inclined to the view that heat was related to the motions of the particles of bodies.

At the end of the eighteenth century, Benjamin Thompson, Count Rumford (1753—1814), an American who served as minister of war for the elector of Bavaria, conducted what appeared only later to have been convincing experiments regarding the nature of heat.

Rumford's experiments were disregarded, for the most part, until the middle of the nineteenth century, when two quite independent developments took place that led to general acceptance of the mechanical theory of heat. The first was a suggestion by Julius Mayer (1814-1878), in 1842, that heat and work were equivalent and could be converted one to the other, (Actually the principle of conservation of energy, later formalized by Helmholtz in 1847)  and the second was the actual measurement by Joule, during the decade between 1840 and 1850, of the mechanical equivalent of heat (or more appropriately, the heat equivalent of work).

James Prescott Joule was born near Manchester, England, on December 24, 1818, the son of a prosperous brewery owner. It was still the Age of Romanticism and of the first Industrial Revolution, but new social and economic doctrines were in the wind. Among his contemporaries were the economic philosophers John Stuart Mill (1806-1873) and Karl Marx (1818-1883). The growing use of labor-saving devices had caused serious dislocations of the labor supply, and the factories of Manchester were particularly notorious for the appalling conditions accorded its workers. The cultural atmosphere was no less suffocating than the bad air in the factories and tenements that housed the working-class families. In this environment, hardly one designed to inspire cultural pursuits, but shielded from it by wealth and position, Joule spent the major part of his life.

He was educated at his home by resident tutors until the age of sixteen, when he was sent, together with his brother, to study under the famous chemist, John Dalton (1766-1844), who supported himself in part by such tutoring. This arrangement lasted but a short time, owing to the illness of the teacher, but probably it contributed immeasurably to Joule's interest in science. So ended his "formal" education, although he did receive, for a brief period in 1839, some private lessons in chemistry from one John Davies. Because of his financial independence he needed no further conventional training; his experimental researches were a form of entertainment for him. In 1838 Joule converted one of the rooms of his father's home into a laboratory and started his experimental investigations. In the same year he published his first short paper, but it was not until 1840 that he presented an important paper (Philosophical Magazine, 1841, vol. 19, p. 260) to the Royal Society. In this he showed that the rate at which heat is generated by an electric current in a conduc-tor is proportional to the square of the current, the constant relating the two being the resistance of the conductor. For the next ten years Joule con-tinued his thermal experiments, refining his measurements time and again, and reporting his results at frequent intervals to the Royal Society. In 1850 he published a memoir in the Philosophical Transactions which contained his most precise value of the mechanical equivalent of heat, including the famous paddle-wheel experiment. Following the publication of this paper he was elected to fellowship in the Royal Society, and his reputation as a scientist was firmly established.

Joule's researches after 1850, while numerous and significant, did not rank in importance with his measurements of the heat equivalent of work. During this period he performed the well-known porous-plug experiment with William Thomson (1824-1907) (Later Lord Kelvin) to show the cooling effect of a gas due to the separation of its molecules upon expansion. Economic misfortune overtook him toward the end of his life; his investments had declined to the point where his income no longer permitted him to carry on research at his own expense. In 1878 he was granted a pension by the Government of £ 200 per annum, which was continued until his death in 1889. Of the many honors that came to Joule during his lifetime, probably none was more eloquent than the decision of the second International Congress to use his name for the practical unit of energy.

Of the extracts given below, the first was a letter to the editor entitled On the Existence of an Equivalent Relation between Heat and the Ordinary Forms of Mechanical Power, published in the Philosophical Magazine, vol. 27, Series 3 (1845), page 205. The second is taken from the memoir of 1850, Philosophical Transactions, vol. 140, page 61.

Forte abraço,

Prof. Sérgio Torres
#sergiorbtorres

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