Rare 19th Century Brass Watchmakers Sector Jump Proportional Sector Rule

Rare 19th Century clockmaker’s sector that is 13.25 inches long and made of patinated brass. The pinion sizing section of the scale is graduated 6,7,8,10, 12, 14, 16; the wheel sizing scale is graduated from 20 to 105 at one unit intervals.
  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 is believed to be the first person to adapt the sector for use in clock and watch making. See Antiquarian Horology, December 1983 page 406.