The Prytz planimeter is a simple mechanical device that historically was used to approximate areas of plane regions. It also serves as a starting point to explain topics in differential geometry and sub-Riemannian geometry. In this article, we present a mathematical description and analysis of the planimeter and show how the analysis of its motion leads to concepts such as connections and sub-Riemannian geodesics.
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Figure 3. Some examples of planimeter paths. The blue line follows the tracer end $ p(t) $ and the red line the chisel end $ q(t) $. The circular arc segment between $ q(0) $ and $ q(T) $ with center in $ p(0) = p(T) $ is shown as a dashed line. The exact values are $ A = \pi\approx 3.14159 $ for the circle, and $ A = 2\sin\left(\frac{\pi}{5}\right)\approx 1.17557 $ for the star
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An illustration of the moving segment theorem. The area between the two shapes is swept out twice, once in the positive direction and once in the negative direction
A Prytz planimeter
Some examples of planimeter paths. The blue line follows the tracer end
An illustration of the planimeter and the coordinates used. The chisel end
Examples of sub-Riemannian geodesics