(born Jan. 19, 1736, Greenock, Renfrewshire, Scot.—died Aug. 25, 1819, Heathfield Hall, near Birmingham, Warwick, Eng.) Scottish engineer and inventor. Though largely self-taught, he began work early as an instrument maker and later as an engineer on the Forth and Clyde Canal. Watt's major improvement to Thomas Newcomen's steam engine was the use of a separate condenser (1769), which reduced the loss of latent heat and greatly increased its efficiency. With Matthew Boulton he began manufacture of his new engine in 1775. In 1781 he added rotary motion (a so-called sun-and-planet gear) to replace the up-and-down action of the original engine. In 1782 he patented the double-acting engine, in which the piston pushed as well as pulled. This engine required a new method of rigidly connecting the piston to the beam, a problem he solved in 1784 with an arrangement of connected rods that guided the piston rod in a perpendicular motion. His application of the centrifugal governor for automatic control of the speed of the engine (1788) and his invention of a pressure gauge (1790) virtually completed the Watt engine, which had immense consequences for the Industrial Revolution. He introduced the concept of horsepower; the watt, a unit of power, is named for him.

Learn more about Watt, James with a free trial on Britannica.com.