Mechanical equivalent of heat

Before the 1840s heat and work were separate currencies with no exchange rate. James Prescott Joule established one, most famously with a paddle wheel driven by falling weights that stirred an insulated water bath while he measured a temperature rise of a few hundredths of a degree. His 1850 figure, 772.692 foot-pounds per BTU, is within a fraction of a percent of the modern value — an extraordinary piece of experimental work that established the first law of thermodynamics and eventually got the unit of energy named after him.

Today no measurement is involved. The calorie is simply defined in joules, so the mechanical equivalent of heat is exactly 4.1868 J/cal for the international-table calorie used in steam tables, and exactly 4.184 J/cal for the thermochemical calorie used in chemistry. In modern terms J = 1 and the constant is a historical artefact — but it is still embedded in the BTU, the horsepower-hour and every imperial steam table in use.