Lusuerg sector, 1701.
© Science Museum / Science & Society Picture Library
Made by D Lusuerg of Rome. Sets of drawing instruments were often made to order for wealthy clients, and were regarded more as an adornment and status symbol than as tools for practitioners. 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.