Energy Conversion — J · kJ · kWh · MWh · cal · kcal · eV · BTU

James Prescott Joule (1818-1889) was the son of a Manchester brewery owner who built his physics apparatus in the family brewing-house laboratory. Tutored briefly by John Dalton, Joule had no university affiliation throughout his career — his work was funded by the brewery and presented to the Manchester Literary and Philosophical Society. His 1841 paper "On the heat evolved by metallic conductors of electricity" in the Philosophical Magazine established Joule's first law: P = I²R, the heating effect of electric current. The law underlies every electric resistance heater in the world.

Joule's second great work was the 1843 paddlewheel experiment that measured the mechanical equivalent of heat. A falling weight rotated a paddle in a thermally isolated water bath; the temperature rise gave the conversion between mechanical joules and thermal calories. Joule's 1850 value was 4.155 J/cal, within 0.7% of the modern exact 4.184 J/cal. This established the principle of energy conservation and refuted the "caloric" theory of heat dominant since Lavoisier.

The unit "joule" was proposed at the First International Electrical Congress in Paris in 1881 — the same Congress that named the volt, ohm, ampere, coulomb and farad. The 9th General Conference on Weights and Measures (CGPM) in 1948 made the joule the SI coherent unit of energy, defined as the work done when a force of 1 N acts through 1 m: J = N·m = kg·m²/s².

Cross-domain energy units evolved independently for centuries. The calorie was defined by Nicolas Clément in 1824 as the heat to raise 1 g of water 1 °C. The thermochemical calorie was fixed at exactly 4.184 J by the 5th International Conference on the Properties of Steam in 1956 (London). The British thermal unit (BTU) was defined by British engineers for steam-engine work as the heat to raise 1 lb of water 1 °F; the ISO 31-4:1992 fixed the IT BTU at exactly 1055.05585262 J — the rosette uses this value.

The electron-volt entered physics in 1912 when Robert Millikan and Harvey Fletcher used it to express atomic ionization potentials in their Caltech oil-drop work. As an energy unit it scales naturally with atomic and nuclear processes — 1-10 eV chemical bonds, 13.6 eV hydrogen ionization, MeV nuclear binding, TeV LHC collisions. The 2018 SI redefinition fixed e = 1.602176634 × 10⁻¹⁹ C exactly, so 1 eV is now exactly 1.602176634 × 10⁻¹⁹ J — a defined constant, not a measured ratio.

The kWh entered utility billing in the 1890s as electric power companies in London (Edison, Coffin), New York (Tesla, Westinghouse) and Chicago (Insull) standardized measurement infrastructure. 1 kWh = 3.6 MJ is the universal home-energy yardstick: a US home averages 30 kWh/day, a Tesla 82-kWh pack stores 295 MJ, a Hiroshima bomb released 17,400 MWh = 62.8 TJ. The rosette's 8 spokes span the full range of named energy units in modern engineering, from quantum eV through household kWh to grid-scale MWh and beyond.

Modern energy metrology is anchored by the watt balance / Kibble balance — invented by Bryan Kibble at NPL in 1975 and used to redefine the kilogram via the Planck constant at the 26th CGPM in 2018. Every joule on this rosette is now traceable to natural law through h = 6.62607015 × 10⁻³⁴ J·s exactly. From Joule's 1843 brewery-house paddlewheel to Versailles 2018, the energy unit family has been pinned to fundamental constants — and the rosette puts all 8 named units one click apart.