The orange part is connected to the yellow part by a revolution joint, to the violet part by another revolution joint. It is also connected to the pink part via blue conrod and to green part via another blue conrod.
Move the pink part to get desired radius, tight the screw to fix the ruler and draw the arc.
The drawn arc consists of many line segments and is not smooth. To overcome that adding elastic bands on the ruler’s upper and lower surfaces is recommended.