windmill, apparatus that harnesses wind power for a variety of uses, e.g., pumping water, grinding corn, driving small sawmills, and driving electrical generators. Windmills were probably not known in Europe before the 12th cent., but thereafter they became familiar landmarks in Holland, England, France, and Germany. The typical Dutch windmill, also called the tower type, has a huge tower of stone, brick, or wood, in contrast to the German, or post, mill, the distinctive feature of which is that the whole building revolves on a central post. At the top of either type there is a revolving apparatus to which four to six arms are attached. The arms, usually 20 to 40 ft (6-12 m) long, bear sails constructed of light wood, or of canvas attached to a frame. A small fan serves as a rudder to keep the wheel facing the wind.

More modern American windmills have high towers of light steel girders; at the top is a wheel with many sheet-metal concave and "warped" vanes (sails) about 4 ft (1.2 m) long. The wheel is kept automatically facing the wind by a broad tail geared to a shaft. They have been widely used for pumping water in rural parts of the United States. Such windmills can also be used to generate about one kilowatt of elecricity.

Larger windmills, such as the modern propellerlike wind turbines, can have rotors (the blade assembly) that span 200 ft (60 m) or more. These wind turbines, often joined together in wind farms, can produce 1.5 MW or more of electricity and can serve as a significant source of electric energy in plains and coastal areas (including offshore locations). Wind turbines have been most extensively used in Europe, where Denmark, for example, is undertaking to generate 50% of its electricty using wind power by 2030. As many as 2,000 small wind turbines are used in Inner Mongolia to provide local electric power to nomadic people.

Machine for harnessing the energy of the wind using sails mounted on a rotating shaft. The sails are mounted at an angle or are given a slight twist, so that the force of wind against them has two components, one of which, in the plane of the sails, causes rotation. Like waterwheels, windmills were among the original prime movers that replaced human beings as a source of power. Their most important traditional use was for grinding grain, though in certain areas their use in land drainage and water pumping was equally important. Windmill use became increasingly widespread in Europe (particularly the Netherlands) from the 12th century to the early 19th century, but thereafter slowly declined. Interest in windmills for generating electric power revived in the 1970s. Seealso wind power.

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