French sector, 1751-1774.
© Science Museum / Science & Society Picture Library
A bras sector, made by Canivet, Paris, France. French sectors are les complicated than English versions. They were used for calculations involving proportions of length, area and volume. Sectors are composed of two flat arms hinged together that can be adjusted to form any angle. Each arm is engraved with logarithmic and trigonometrical lines, which radiate from the centre of the hinge.