The instrument consists of two brass limbs and a thin brass circular arc. The limbs are built up from three riveted sheets, with the arc riveted to one of the limbs and passing through the other.
On one of the limbs the instrument is signed '[star] Robertus [star] Beckit [star] fecit 1597 [star]'. Beckit worked in London and is also known as a map engraver. The inscription shows not only that sectors were being made before the publication in 1598 of the first book on the sector (Thomas Hood's The Making and Use of the Geometricall Instrument, called a Sector), but that Charles Whitwell, the maker advertised by Hood, did not have an exclusive monopoly on the manufacture of the instrument.
On one face, the limbs carry sectoral scales of equal parts from 0 to about 112, divided to 10 and subdivided to 5, 1 and 1/2 and numbered by 10. The other face carries two sets of scales; one for the chords of circles ('Coards of a circle') divided and numbered from 3 to 12 and the other for squares ('Power of Lynes') divided and numbered from 1/2 to 1/10.
The circular arc (or 'circumferentiall limb') carries on one side a scale of degrees up to 110, divided to 10, subdivided to 5, 1, 1/2 and 1/4, and numbered by 10. This degree scale does not use subdivision by diagonals, as illustrated by Hood. On the reverse are three scales used to measure distances between the points of the sector. The scales can measure up to 18 inches and are graduated to read in 6, 8 and 10 parts of an inch. The first (inner) scale runs to 108, divided by 6, subdivided by 3, 1 and 1/2, and numbered by 6; the second runs to 144, divided by 8, subdivided by 4, 2, 1 and 1/2, and numbered by 8; the third (outer) scale runs to 180, divided to 10, subdivided to 5, 1 and 1/2, and numbered by 10.
The reverse of the arc also carries a further feature, not listed by Hood. Inside the inch scales there are three points marked 3, 4 and 5 respectively. They would be used to mark out the length of the sides of the first three regular polygons.
Unlike the sector described in print by Hood, Beckit's example does not have an additional sliding index limb, nor do the sights and mounting pieces survive. However, each limb has a hole near its point and there is also a hole at the centre of the hinge. These would have received the recommended three sights. The movable limb is also pierced so that it can be fixed in relation to the arc, and the separation of the two limbs thereby held constant.
See G. L'E. Turner, "Mathematical Instrument-Making in London in the 16th Century", in S. Tyacke (ed.), English Map-Making 1500-1650 (London, 1983), pp. 93-106, esp. pp. 103-4.