Bras sector, 1600-1700.
© Science Museum / Science & Society Picture Library
A bras sector made by Jacob Lusuerg, Rome. The sector was invented at the end of the 16th century and was one of the most popular calculating instruments for 200 years. They were used primarily for calculations involving ratios, and the angles of triangles. Sectors are composed of two flat arms hinged together that can be adjusted to form any angle. Each arm is engraved with scales on each of its major faces, and the scales are radial from the centre of the hinge. The instrument is equivalent to two radii of a circle and the arc of the circumference that they enclose. Priority of invention is disputed between Galileo Galilei, and the Englishman, Thomas Hood. The use of sectors decreased in favour of mathematical tables.