This is something I saw on one of the Blogs I frequent:
(Re)Produce Accurate Arcs
Sometimes you can make a serviceable arc with just a couple of nails and a thin piece of wood, but what if you need to exactly replicate an arc or produce an arc of a known radius? You break out Hoyle’s Acu-Arc adjustable ruler.
The Acu-Arc ruler lets you determine the radius of existing curves, produce arcs of a known radius, and find center points of curves and center lines. You can adjust the 12″ flexible edge to reproduce curves with a 6-3/4″ to 200″ radius, or a 17mm to 500cm radius in the metric model. A sliding knob locks the curve in place, and a handy scale lets you read or set the radius of the arc.
Hoyle makes the Acu-Arc ruler with plastic that’s rugged enough to stand up to everyday use, but it apparently bursts into flames or something when hit by direct sunlight, according to the manufacturer’s all-caps warning: “KEEP YOUR ACU-ARC RULER AWAY FROM DIRECT SUNLIGHT!”
The Acu-Arc adjustable ruler runs between $35 and $50
Acu-Arc Adjustable Ruler [Hoyle]