Italian-Style Sector

Description:

This small brass instrument has two hinged arms, each with a steel point at the end. On one side, both arms have a single scale that runs from 8 (near the hinge) to 1 and is labeled Lin. Recta. If 1 is the full length of the line, 2 represents 1/2 the length, 3 is 1/3 the length, and so on. One arm on this side also has a scale marked with the number 3 at both ends. It is 1/3 the length of the instrument. The single scale on the other side runs from 15 to 6 and is labeled Lin. Circularis. It is used to indicate the length of arcs of a circle. If the points are set at opposite ends of the radius of a circle (the distance marked "6"), a polygon inscribed in the circle with sides equal to that distance will have six sides. A polygon inscribed in the circle at the separation marked "7" will have seven sides, and so on.

Instruments of this type were introduced in Italy, probably by Guidobaldo del Monte, and first described by G. P. Gallucci in a book about mathematical instruments published in Venice in 1598. They were used into the seventeenth century, but they were superseded by more complicated sectors. In particular, Galileo expanded on and publicized the arrangement of scales in 1606, and his form became known as the Italian style.

Reference: Galileo Galilei, Operations of the Geometric and Military Compass, trans. and intro. Stillman Drake (Washington, D.C.: Smithsonian Institution Press, 1978), 12.