The definition of the joule as J = kg⋅m 2⋅s −2 has remained unchanged since 1946, but the joule as a derived unit has inherited changes in the definitions of the second (in 19), the metre (in 1983) and the kilogram ( in 2019). This definition was the direct precursor of the joule as adopted in the modern International System of Units in 1960. The ratification of the definition at the ninth General Conference on Weights and Measures, in 1948,Īdded the specification that the joule was also to be preferred as the unit of heat in the context of calorimetry, thereby officially deprecating the use of the calorie. The joule was explicitly intended as the unit of energy to be used in both electromagnetic and mechanical contexts. The joule was now no longer defined based on electromagnetic unit, but instead as the unit of work performed by one unit of force (at the time not yet named newton) The Giorgi system was approved by the International Committee for Weights and Measures in 1946. In 1935, the International Electrotechnical Commission (as the successor organisation of the International Electrical Congress) adopted the " Giorgi system", which by virtue of assuming a defined value for the magnetic constant also implied a redefinition of the Joule. Joule died in the same year, on 11 October 1889.Īt the fourth congress (1893), the "international ampere" and "international ohm" were defined, with slight changes in the specifications for their measurement, with the "international joule" being the unit derived from them. "Such a heat unit, if found acceptable, might with great propriety, I think, be called the Joule, after the man who has done so much to develop the dynamical theory of heat." Īt the second International Electrical Congress, on 31 August 1889, the joule was officially adopted alongside the watt and the quadrant (later renamed to henry). The naming of the unit in honour of James Prescott Joule (1818–1889), at the time retired but still living (aged 63), is due to Siemens: Wilhelm Siemens, in his inauguration speech as chairman of the British Association for the Advancement of Science (23 August 1882) first proposed the Joule as unit of heat, to be derived from the electromagnetic units Ampere and Ohm, in cgs units equivalent to 10 7 erg. The erg was adopted as its unit of energy in 1882. The cgs system had been declared official in 1881, at the first International Electrical Congress. As with every SI unit named for a person, its symbol starts with an upper case letter (J), but when written in full it follows the rules for capitalisation of a common noun i.e., " joule" becomes capitalised at the beginning of a sentence and in titles, but is otherwise in lower case. The joule is named after James Prescott Joule. This relationship can be used to define the watt.
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