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.”
Fig. 182 are spring dividers.
Fig. 183 is a spring pencil.
Fig. 184 is a spring pen.
In these figures it will be seen that the two threads, a right and a
left, are moved with one central thumbscrew; in the figs. 185 to 187 a
single screw is used.
In choosing spring bows, care must be exercised to select a sufficiently
strong, stiff spring, as the relation between spring pressure and
thumbscrew is important.