Peaucellier-Lipkin linkage
This linkage converts motion along a circle
to motion along a straight line.
The two GREEN lines are of equal length and fixed at their common
end point.
The four BLUE lines are of equal length forming a rhombus.
Two opposite vertices of the rhombus are connected to the ends
of the GREEN lines.
Drag the GOLD dot along the circle. Prove that the BLUE dot
moves along a straight line.