A form of pulley attached to a tool or part to enable it to be rotated by the cord of a bow or wheel. Ferrules are produced in many forms and sizes, simple ones being small brass pulleys of different diameters and centerhole sizes held on the work either by friction grip or shellac. Split ferrules are made in two halves and screw-tightened on to arbours of different sizes. A loose ferrule or pulley may be used with a 'driver', i.e. A small split bridle, to rotate very small arbours between dead centres of pairs of turns.