English-Style Sector

This boxwood instrument has two rectangular arms with flat edges and is held together by a circular brass hinge that is heavily tarnished. According to the accession file, off-gassing from a celluloid rectangular military protractor received with the object in 1974 caused the damage before 1990. The scales run from top to bottom on each arm. On one side, each arm has a sine scale, running from 10 to 80 degrees; a tangent scale, running from 50 to 75 degrees; and a second tangent scale, running from 10 to 45 degrees. Spanning both arms on the outer edge are three scales: log tangent, running from 2 to 45 degrees; log sine, running from 1 to 70 degrees; and logarithmic numbers, running from 1 to 10 twice. The top face of the instrument has a scale of equal parts that divides one foot into 100 increments and runs from 90 to 10. There is no maker's mark.

The other side has a double scale along the fold line for regular polygons, labeled POL and running from 12 to 4 sides. Each arm has a scale of equal parts, running from 1 to 10 and labeled L; a secant scale, running from 40 to 75 and labeled s; and a scale of chords, running from 10 to 60 and labeled C. The scales for dialing typically found on 18th-century English-style sectors are not present. Spanning both arms on the outer edge is a scale of inches, running from 12 to 1 and divided to tenths of an inch.

This object likely dates to the 19th century. Like ivory sectors from the time period, such as MA.321755, this instrument has small brass pegs embedded into the scales at frequently-used points, in order to protect the soft wood from metal dividers digging into the surface. In part because they were less expensive to produce, instrument makers probably made a larger number of pocket-sized wooden sectors than ivory sectors to place in cases of drawing instruments. However, since the material is not as durable as ivory, fewer examples of these objects survived.

References: Thomas Kentish, A Treatise on a Box of Instruments and the Slide Rule (London: Relfe & Fletcher, 1847), 39–61; Nathan Zeldes, "Ivory Sector," History of Computing, http://www.nzeldes.com/HOC/Sector.htm.

Currently not on view