A rare clockmaker’s sector, with a tangent screw for fine adjustment, dating from the second half of the 19th century. The sector is unsigned.
 

The sector is 14 inches long and made of brass. The pinion sizing section of the scale is graduated from 6 to 12 at 0.25 intervals; the wheel sizing scale is graduated from 13 to 110 at one unit intervals and numbered at each interval. Unusually, the pinion area has a steel insert in the brass to reduce wear and thus improve accuracy.
 

The sector is a proportional gauge and was used principally for the rapid sizing of wheels and pinions. It can also be used for sizing spring barrels and their arbors. Detailed information on the construction and use of the sector is given in The Watch & Clock Makers’ Handbook Dictionary and Guide by F. J. Britten. The use of the sector was also described in the Horological Journal for January 1928, page 96.
 

The sector, as a proportional measuring tool, was known in the seventeenth century. Thomas Jump, a Prescot clockmaker, is believed to be the first person to adapt the sector for use in clock and watch making a few years before 1800. See Antiquarian Horology, December 1983 page 406.
 

This sector is very similar to those advertised in tool catalogues by Levy, Dreyfus & Co. (1886) and Ezra Bowman (1895), so may be American in origin. [See Crom: Horological Shop tools 1700 – 1900 pages 304 and 325]