British physicist. Son of a brewer, it accepted his instruction at the house. John Dalton was her chemistry teacher. Joule did not take again the management of the brewery, it was satisfied to install its laboratory in the family company.

It started by being interested in the electromagnetic machines, and in particular in the electromagnets, by attending William Sturgeon. He undertook to establish the measurements based on good definitions of the sizes and the use of quite selected units. He showed that the electromagnets attract each other by forces proportional to the square of the intensity of the current and that this one is simply proportional to the chemical power concerned to obtain this current. This bringing together led it to believe in a possibility of using the electromagnets to produce mechanical power indefinitely, it with what it had to give up.

It turned then to the study of the heating effect of electricity. He undertook to establish the law which bears its name, (just like the phenomenon itself is called “Joule effect”), and which states that produced heat is proportional to the resistance and the square of the intensity of the current (1840). He proceeded by measuring the temperature of the water in which the driver passed, taking a small number of measurements and concerning very small temperature variations thanks to the precision of its thermometers. Such was constantly its experimental step: to test by small series of measure a very simple, linear or quadratic law. The reception of the result by Royal Society was hardly encouraging.

Joule then explored in a systematic way the relations between the modes of production chemical of an electric current and the heating effects. That amounted seeking equivalents with heat in the chemical field. Joule thought of moving these problems while not making more depend the current on a chemical reaction but by obtaining its production by a mechanical way.

An electromagnetic system - locked up in a container filled with water - was heated by the current, itself obtained by actuating the machine of outside. Joule could thus determine the mechanical equivalent, in terms of work, of a quantity of heat given (1843).

It is not that in 1847 qu ' it gave an account of the now traditional experiment, in which mechanical work is directly transformed into heat by frictions of the blades in the liquid. In addition, it proceeded, following the example of Mayer, under investigation of the expansion and the compression of the air. It checked that the expansion of a gas which is not accompanied by a production of work makes at constant temperature.

By exposing its results in the cultivated mediums of Manchester, Joule delivered also the design of the heat to which its reflections had led it. It had taken party for the atomic design of the matter, in which heat is explained by the agitation of the atoms. It communicated also its work with the Academy of Science of Paris like to British Association. It accepted there the support of William Thomson, who encouraged it to persevere in her research and which submitted the project to him to study together the variation of thermodynamic behavior between real gases and perfect gases.

In 1852, the two scientists established, thanks to the experimental skill of Joule, that the slow relaxation of a real gas through a porous wall, therefore with constant enthalpy, causes a cooling of this one (Joule-Thomson effect). In 1850, Joule had been elected member of Royal Society. But it did not take part truly any more in the development of thermodynamics, of good part because its weak mathematical formation did not enable him to follow work which, more and more, made a thorough use of it.

In 1878, Joule was brought to determine a precise and unquestionable value equivalent between heat and work at ends of metrology. It was its last scientific work. And the energy work unit of the international system was called the joule in its honor. Heat is expressed in the same unit, the calorie being worth 4.18 joules.

Among other research an interpretation of the law of Mariotte by the kinetic theory of gases appears. It also calculated the mean velocity of the molecules (1848).