In 1795, a Frenchman named Nicholas Jacques Conté figured out how to mix powdered graphite with clay so that the material could be formed into rods that would be hardened when fired.This was not only a breakthrough in production, it also allowed pencil makers to vary the quality of the mark made by the rod—the greater the percentage of graphite to clay, the softer the rod and the darker the mark.A Harling "Woolwich" case of electrum instruments (missing large compass, dividers & pencil bow).Under the large protractor there is a pair of vulcanite set squares and in the bottom compartment there are a set of marquois scales and a number of scale rules, all signed Harling.Scott they ceased making instruments to the traditional pattern in 1929 and changed to flat pattern ones.In 1964, having merged with Blundell Rules Ltd to form Blundell Harling, they moved to the Blundell premises in Weymouth.This page contains slide rules that are very rare, hard to find or manufactured in the 1800's, earlier and later, but prior to WWII, using turn of the century materials and process. These may include Boxwood, Ivory (Bone), Brass and in many instances were hand-divided by an artisan.
The long joint head of the compass is well made with two steels but the other hinged joints are much cruder being just brass.
The graphite would be glued in place, hand-trimmed to remove excess graphite, and then a second piece of wood would be glued on top so that the graphite, which is soft, was fully enclosed.
The wood would then be smoothed, polished, and varnished to bring out the grain.
Some slide rules have been made for special use, as for aviation or finance.
Those slide rules have special scales for those applications, as well as normal scales.