län (county) of central Sweden, extending north of Lake Mälar. Its area includes the southwestern part of the traditional landskap (province) of Uppland and the eastern part of Västmanland. A fertile plain in the southeast rises northward to the edge of hilly Bergslagen district and is drained by the Arboga River, the Kolbäcks River, and several smaller rivers. Dairying and market gardening are pursued on the plain, while mining and metalworking are centred on Fagersta, in Bergslagen district. The proximity of iron ore, with lesser deposits of copper, lead, and zinc, influenced the development of Västerås (q.v.), the capital, and Köping as engineering centres. Area 2,554 square miles (6,614 square km). Pop. (2005 est.) 261,005.