This is a planimeter. It can measure areas. To test it, move the diamond shaped point once around the circle. You can reset the display with the green dot on the lower right.
The device is actually very simple. A rolling wheel is fixed in a perpendicular way at the hinge. The wheel partly roles and partly glides, so that it measures movements perpendicular to the second arm only. You can read out the position of the disk on the green display. I added a numerical output on the lower left.
This has been realized with the functions d and sum, which in combination can be used to integrate changing values.