James Prescott Joule (December 24, 1818–October 11, 1889) was an English physicist, born in Salford, near Manchester.

James Joule

Joule studied the nature of heat, and discovered its relationship to mechanical work (see energy). This led to the theory of conservation of energy (the First Law of Thermodynamics). The SI unit of work, the joule, is named after him, and is pronounced to rhyme with "tool." He worked with Lord Kelvin to develop the absolute scale of temperature, made observations on magnetostriction, and found the relationship between the flow of current through a resistance and the heat dissipated, now called Joule's law.

Biography

Early years

The son of a wealthy brewer, Joule was tutored as a young man by the famous scientist John Dalton. He was fascinated by electricity, and he and his brother experimented by giving electric shocks to each other and to the family's servants. Joule ran the brewery as an adult, and science was merely a serious hobby. His work on energy can be traced to his attempt to build an electric motor that would replace steam engines.

Energy

Joule's ideas about energy were not accepted at first, partly because they depended on extremely precise measurements, which had not previously been common in physics. His best-known experiment involved the use of a falling weight to spin a paddlewheel in an insulated barrel of water, whose increased temperature he measured. He claimed to be able to measure temperatures to an accuracy of 1/200 of a degree Fahrenheit, which his contemporaries did not believe possible. Joule's experiments complemented the theoretical work of Rudolf Clausius, who is considered by some to be the coinventor of the energy concept.

Further resistance came because Joule's work contradicted the widespread belief that heat was a fluid, the "caloric", that could be neither created nor destroyed, whereas Joule claimed that heat was only one of many forms of energy, and only the sum of all the forms was conserved.

Joule was proposing a kinetic theory of heat (he believed it to be a form of rotational, rather then translational, kinetic energy), and this required a conceptual leap: if heat was a form of molecular motion, why didn't the motion of the molecules gradually die out? Joule's ideas required one to believe that the collisions of molecules were perfectly elastic. We should also remember that the very existence of atoms and molecules was not widely accepted for another hundred years.

Although it may be hard today to understand the allure of the caloric theory, at the time it seemed to have some clear advantages. Carnot's successful theory of heat engines had also been based on the caloric assumption, and only later was it proved by Lord Kelvin that Carnot's mathematics were equally valid without assuming a caloric fluid.

Energy conservation

Although the discovery of energy conservation was one of the keystones of the new science of thermodynamics, Joule and his contemporaries did not initially understand that thermodynamic processes could be irreversible. Instead, they interpreted energy universe, in which the same processes could be repeated indefinitely by recycling the same energy. This view was only later shown to be invalid with the discovery of the second law of thermodynamics and the concept of entropy.

Death and afterwards

Joule died at home in Sale and is buried in the cemetery there. The gravestone is inscribed with the number 772.55 which was the amount of work, measured in ft lb, Joule determined experimentally to be required to raise the temperature of 1 lb of water by 1° Fahrenheit. Many biographical articles on Joule state he and John Dalton are buried in Westminster Abbey. This is not true in either case (although Joule has a memorial in the North Choir Aisle at the abbey).