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Summary Article: Joule, James Prescott (1818-1889)
from The Hutchinson Dictionary of Scientific Biography

Place: United Kingdom, England

Subject: biography, physics

English physicist who verified the principle of conservation of energy by making the first accurate determination of the mechanical equivalent of heat. He also discovered Joule's law, which defines the relation between heat and electricity, and - with William Thomson (Lord Kelvin) - the Joule-Thomson effect. In recognition of Joule's pioneering work on energy, the SI unit of energy is named the joule.

Joule was born in Salford on 24 December 1818 into a wealthy brewing family. He and his brother were educated at home between 1833 and 1837 in elementary mathematics, natural philosophy, and chemistry, partly by the chemist John Dalton. Joule was a delicate child and very shy, and apart from his early education he was entirely self-taught in science. He does not seem to have played any part in the family brewing business, although some of his first experiments were done in a laboratory at the brewery.

Joule had great dexterity as an experimenter, and was able to measure temperatures very exactly indeed. At first, other scientists could not credit such accuracy and were disinclined to believe the theories that Joule developed to explain his results. The encouragement of Lord Kelvin from 1847 changed these attitudes, however, and Kelvin subsequently used Joule's practical ability to great advantage. By 1850, Joule was highly thought of among scientists and became a fellow of the Royal Society. He was awarded the Society's Copley Medal in 1866 and was president of the British Association for the Advancement of Science in 1872 and again in 1887. Joule's own wealth was able to fund his scientific career, and he never took an academic post. His funds eventually ran out, however. He was awarded a pension in 1878 by Queen Victoria, but by that time his mental powers were declining. He suffered a long illness and died in Sale, Cheshire, on 11 October 1889.

Joule realized the importance of accurate measurement very early on and exact quantitative data became his hallmark. His most active research period was between 1837 and 1847 and led to the establishment of the principle of conservation of energy and the equivalence of heat and other forms of energy. In a long series of experiments, he studied the quantitative relationship between electrical, mechanical, and chemical effects and heat, and in 1843 he was able to announce his determination of the amount of work required to produce a unit of heat. This is called the mechanical equivalent of heat (currently accepted value 4.1868 joules per calorie).

Joule's first experiments related the chemical and electrical energy expended to the heat produced in metallic conductors and voltaic and electrolytic cells. These results were published between 1840 and 1843. He proved the relationship, known as Joule's law, that the heat produced in a conductor of resistance R by a current I is proportional to I2R per second. He went on to discuss the relationship between heat and mechanical power in 1843. Joule first measured the rise in temperature and the current and the mechanical work involved when a small electromagnet rotated in water between the poles of another magnet, his training for these experiments having been provided by early research with William Sturgeon, a pioneer of electromagnetism. Joule then checked the rise in temperature by a more accurate experiment, forcing water through capillary tubes. The third method depended on the compression of air and the fourth produced heat from friction in water using paddles that rotated under the action of a falling weight. This has become the best-known method for the determination of the mechanical equivalent. Joule showed that the results obtained using different liquids (water, mercury, and sperm oil) were the same. In the case of water, 772 ft lb of work produced a rise of 1°F in 472 cu cm/29 cu in of water. This value was universally accepted as the mechanical equivalent of heat. It now has no validity, however, because as both heat and work are considered to be forms of energy, they are measured in the same units - in joules. A joule is basically defined as the energy expended when a force of 1 newton moves 1 metre.

The great value of Joule's work in the establishment of the conservation of energy lay in the variety and completeness of his experimental evidence. He showed that the same relationship held in all cases that could be examined experimentally and that the ratio of equivalence of the different forms of energy did not depend on how one form was converted into another or on the materials involved. The principle that Joule had established is in fact the first law of thermodynamics - that energy cannot be created nor destroyed but only transformed.

Because he had not received any formal mathematical training, Joule was unable to keep up with the new science of thermodynamics to which he had made such a fundamental and important contribution. However, the presentation of his final work on the mechanical equivalent of heat in 1847 attracted great interest and support from William Thomson, then only 22 and later to become Lord Kelvin. Much of Joule's later work was carried out with him, for Kelvin had need of Joule's experimental prowess to put his ideas on thermodynamics into practice. This led in 1852 to the discovery of the Joule-Thomson effect, which produces cooling in a gas when the gas expands freely. The effect is caused by the conversion of heat into work done by the molecules in overcoming attractive forces between them as they move apart. It was to prove vital to techniques in the liquefaction of gases and low-temperature physics.

Joule lives on in the use of his name to measure energy, supplanting earlier units such as the erg and calorie. It is an appropriate reflection of his great experimental ability and his tenacity in establishing a basic law of science.

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