The longimeter, invented by Hugo Steinhaus, is a device for measuring the length of a curve drawn on paper.
It's a strange grid on transparency that is laid over the curve. The grid is constructed so that the number of times the curve crosses the grid is the length of the curve in millimeters.
Here's a short video I made about the Steinhaus Longimeter.
This is part of my series of videos about antique computational devices. Here is the full series playlist
Here are some PDFs of longimeters which are meant to be printed on 8½ x 11 transparency paper.
These PDFs were generated by tikz in this LaTeX file: