The Ott Derivimeter, an instrument for finding the derivative of a curve drawn on paper. Sold by the Ott company in the early 20th century, though I don't know when mine was made.
This is episode 84 of my video series about old calculating methods.
OTT Hydromet company history video: https://www.youtube.com/watch?v=lgax3bnP-RE
Chris Staecker webarea: https://faculty.fairfield.edu/cstaecker
#calculus #elsa