Rotape Beam Compass
Instead of a hook at the end, this tape has a lead holder for 2mm leads. Use the included pin to anchor the Rotape at the center point of your circle (or at the center of the circle whose arc you want to draw), adjust the tape to the radius you want, and voila! Much easier than springing a batten or tying a knot at the exactly correct length. The Rotape is also capable of drawing lines parallel to an edge. Works for radii from 3-1/8 inches to 72 inches.
Notice to residents of California: Click here to view Proposition 65 warning