Bow-compass

 An arcograph.  [1913 Webster]

 A small pair of compasses, one leg of which carries a pencil, or a pen, for drawing circles. Its legs are often connected by a bow-shaped spring, instead of by a joint.  [1913 Webster]

 A pair of compasses, with a bow or arched plate riveted to one of the legs, and passing through the other.  [1913 Webster]