The joint at the head of the compasses (see [fig. 175]) is the most important feature. It should hold the legs firmly in any position, so that in going over a circle several times only one line will result. It should allow the legs to move smoothly and evenly, and should be capable of adjustment.
Fig. 180.—Fig. 181.—Fig. 182.—Fig. 183.—Fig. 184.
As shown in [fig. 174], one leg has a hinge or joint, and a needle point, which can be regulated by a thumb screw; the other leg has a socket or recess into which interchangeable parts can be inserted. The four figures to the right of the compasses show the parts which are provided with shanks or insertion pieces. [Fig. 180] and [fig. 181] represent compasses specially used for making small circles, and work too minute for the larger instruments described above.
To do work of this nature easily a pair of spring dividers are frequently used. This instrument has one point attached to a spring, which is regulated by a screw, so that very slight changes in the space may be made with ease.
Compasses specially used for putting in fine circles and dimensions are called “bows.” When a pen point it is a “bow pen,” with a pencil point a “bow pencil,” and if with needle point a “bow dividers.” [Fig. 180] is a “bow dividers”, this fitted with screw for fine adjustment in one leg, [fig. 181], is called a “hair-spring bow dividers”; for small details, bows with steel spring legs without any joint are used; these are called “steel-spring bows.”
Fig. 185.—Fig. 186.—Fig. 187.
SPRING BOWS.
These were originally developed from the common form of compasses, with a single spring leg; later, the demand for smaller sizes made changes necessary, and spring bows are now made symmetrical, both sides of the bow being made to “spring.”