About a year and a half ago, my dad mentioned that he saw a planimeter at a flea market. He told me that a planimeter a mechanical device that historically was used to calculate areas. I looked it up breifly, but wasn’t able to get a grasp of how it worked and the mystery sort of lurked about somewhere far in the back of my brain, nestled right up around the neck perhaps. In any case, the device came up again around Christmas time. Both times, all I understood was that somehow by simply tracing the planimeter around the perimeter of any closed curve, it mechanically could calculate the enclosed area. This time, I thought I would turn to Gemini and ask for an “explain like I’m 5” summary of how a planimeter worked. Still there was a disconnect, and either it was too hand-wavy or showed Green’s theorem and started talking about flux. This time, the mystery never made it back to brain-hiberntation and this youtube video and simulation originated from the quest to finally try to understand this amazing device from the proof Jakob Amsler, inventor of the polar planimeter, used to prove to the engineering community that his planimeter accurately could calculate areas.