Jesse Ramsden, Sector in the English Style, 1765–1800, Smithsonian Institution negative number DOR2012-2586.

Jesse Ramsden (1735–1800) was one of the most famous British makers of mathematical instruments. Among his achievements was the invention of a circular dividing engine, which made it possible to mechanically inscribe scales of angles on surveying, astronomical, and drawing instruments.

His workshop also produced this sector, a calculating instrument used in Europe and North America from the 17th to the 19th centuries. Users employed a pair of dividers to measure distances between points on the scales on a sector and make calculations using the principle of similar triangles. They could figure out proportions useful for designing military fortifications and solving other problems involving regular polygons, positioning and loading artillery, working with different types of metals, determining ratios between volumes or between areas, constructing angles, and calculating square and cube roots. English sectors were unique in that they often had scales for making sundials, for trigonometry, for navigation, and, as is visible on this instrument, for computing with logarithms. The latter scales made English sectors a forerunner of slide rules.

This object and other sectors from the Smithsonian's National Museum of American History are now shown and described at the website http://americanhistory.si.edu/collections/object-groups/sectors.