Constrained ball line drawing aid - photo

[Altair]

Constrained ball line drawing aid.




Previously:

Drawing a perfect circle on a chalkboard - GIF
Drawing three right angles on a sphere - GIF
Electronic touchscreen blackboard - GIF

[Toolmaker51]

I had to go look. Constrained ball might apply to a sphere in the tip, akin to ballpoint pen. The more surprising feature is the display atop the tool. I thought it marked specific pen size of the silicone retainer....... no it's an odometer!
The wheel operates an encoder, distance readable above, in the display window. Many drivers in Europe had a similar tool, used to find distance between points on a map; a small wheel geared to an indicator in Km, increments and multiples.
They're known as curvimeter or more correctly opisometer. And all relate to that used by surveyors, which probably was first.

Few, maybe very few things are so unique to not be used elsewhere.