Cased Instruments, Continued
Use a compass to draw arcs and circles. A standard compass is 6 inches long
and it has two legs. One leg holds the needlepoint and the other leg holds the
nib. The nib may be a pen nib, a pencil attachment, or another needlepoint.
The legs are often jointed above the nib to maintain perpendicularity to the
drawing surface. The joint also reduces uneven wear and friction to the nib.
To draw a circle larger than 12 inches, add an extension bar at the joint.
Figure 2-23 shows a pivot joint compass.
Figure 2-23. A pivot
The type of hinge on a standard compass indicates the quality of the
instrument and its ability to hold a set. There are three types of hinges.
Figure 2-24 depicts the three types of compass hinges.
Continued on next page