Born Jan. 19, 1736, in Greenock, Scotland; died Aug. 19, 1819, in Heathfield, England. British inventor of the universal steam engine. Member of the Royal Society of London (1785).

Beginning in 1757, Watt worked as a scientific instrument-maker at the University of Glasgow, where he became familiar with the properties of steam and, using D. Papin’s boiler, undertook a careful study of the dependence of the temperature of saturated vapor on pressure. In 1763–64, while working on a model of the steam engine designed by T. Newcomen, Watt concluded that the large consumption of steam in the engine was caused by the cooling of the cylinder walls down to the temperature of the cooling water as it entered the cylinder. He theorized that steam consumption could be reduced by separating the steam condenser from the cylinder, and in 1765 he built an experimental engine with a cylinder diameter of 16 cm, and, in 1768, the first large steam engine. The latter, however, proved to have a number of defects—the condenser was not successful, and the piston required additional sealing. In 1769, Watt obtained a British patent for methods of reducing steam consumption and, consequently, fuel in combustion engines. The patent covered a number of new technical proposals, such as the use of a steam jacket for maintaining temperature in the cylinder and a design for a rotary steam engine. Actual construction of the steam engine was completed in 1774, and subsequent tests showed that Watt’s engine was more than twice as efficient as the best Newcomen engines. In order to ensure steady engine operation, Watt used a centrifugal governor connected to a gate valve on the outlet steam pipe. Watt retained the balance wheel as the driving gear but used the sun and planet gear to double the speed of shaft rotation. He thoroughly investigated steam operation in the cylinder and was the first to design an indicator for this purpose. In 1782 he obtained a British patent for an engine that used steam expansively. Watt introduced the first unit of power—the horsepower—and his own name later came to designate a second unit of power—the watt.

Because of its efficiency, Watt’s steam engine became widely used and played an enormous role in the transition to mechanical production. “The great genius of Watt, ” wrote K. Marx, “lies in that fact that, in the patent he obtained . . ., his steam engine is not presented as an invention for specific purposes only, but as a universal engine for heavy industry” (K. Marx and F. Engels, Soch., 2nd ed., vol. 23, p. 389).

After 1784, Watt was primarily concerned with improving the manufacture of steam engines at his plant.