English-Style Sector

This ivory instrument has two rectangular arms with flat edges and is held together by a circular brass hinge. The scales run from top to bottom on each arm. On one side, each arm has a sine scale, running from 10 to 90 degrees; a tangent scale, running from 45 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 11 to 1 and divided to tenths of an inch. The sector likely dates to the 19th century. Compare to 1985.0580.06, MA.333937, and MA.335351.

Before slide rules were widely used by engineers and draftsmen, instrument makers often included a sector in a set of drawing instruments. Users opened the sector to a desired angle, measured distances between numbers with a pair of dividers, and made calculations according to the principles of similar triangles. The donor reported that her husband's ancestor, Lt. Col. Alexander Matheson (b. 1788), was the original owner of this drawing instrument. He settled in Perth, Canada, after serving in the British military during the War of 1812.

Reference:

Thomas Kentish, A Treatise on a Box of Instruments and the Slide Rule (London: Relfe & Fletcher, 1847), 39–61.

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