(1736-1819) Scottish engineer who developed the steam engine in the 1760s.
He made Thomas Newcomen's steam engine vastly more efficient by cooling the used steam in a condenser separate from the main cylinder.
Steam engines incorporating governors, sun-and-planet gears, and other devices of his invention were successfully built by him in partnership with Matthew Boulton and were vital to the Industrial Revolution. Watt also devised the horsepower as a description of an engine’s rate of working.
Watt was born in Greenock (now in Strathclyde) and trained as an instrumentmaker. Between 1767 and 1774, he made his living as a canal surveyor. In 1775 Boulton and Watt went into partnership and manufactured Watt's engines at the Soho Foundry, near Birmingham. In 1782 Watt improved his machine by making it drive on both the forward and backward strokes of the piston, and a sun-and-planet gear produced rotary motion. This highly adaptable engine was quickly adopted by cotton and woolen mills.
Watt also invented artistic instruments and a chemical copying process for documents.

ETYM From the distinguished mechanician and scientist, James Watt.
The unit of power equal to the expenditure of 1 joule of energy in 1 second. The power of an electrical circuit is a function of the potential across the circuit and the current flowing through the circuit. If E = potential, I = current, and R = resistance, power in watts can be calculated as I x E, I 2 x R, or E 2/R.
SI unit (symbol W) of power (the rate of expenditure or consumption of energy). A light bulb, for example, may use 40, 100, or 150 watts of power; an electric heater will use several kilowatts (thousands of watts). The watt is named for the Scottish engineer James Watt.
The absolute watt is defined as the power used when one joule of work is done in one second. In electrical terms, the flow of one ampere of current through a conductor whose ends are at a potential difference of one volt uses one watt of power (watts = volts × amperes).
One watt equals 0.00134 horsepower.
A unit of power equal to 1 joule per second; the power dissipated by a current of 1 ampere flowing across a resistance of 1 ohm; SYN. W.