The Kaufmann Posographe is a fascinating mechanical contraption which was used by photographers in the early 20th Century to set their exposure times. Back then exposures were measured in seconds, and everyone in the picture had to freeze ("pose") for the entire time. Film plates were expensive and difficult to handle, so do-overs were avoided as much as possible. Over 100,000 French versions of the Posographe were sold alone, as well as tens of thousands of English-language versions. (Although this program is in English, the far more common French Posographe is depicted.)
This animation was originally created based on an article in the Oughtred Society's bi-annual journal, Spring 2019 issue Kaufmann Posographe (pdf), and then the lead programmer purchased an antique Posographe in order to make more accurate measurements and higher resolution photographs, to check the placement of some hidden connections, and lastly, to test the accuracy of the model against an actual Posographe. It's very close!
Kaufmann invented and patented several versions of the Posographe and a couple of other related mechanical contraptions. The mathematical theory behind how the Posographe works was conceived around the same time, and Kaufmann apparently had a number of correspondences with the mathematician who invented the theory. Although the theory is not explained here, the math behind the Posographe is illustrated with connecting lines, colored brass plates (or not, if you prefer!), circles indicating intersecting positions of the pieces, and various additional ways of exploring "under the hood."
This program has fairly extensive math operating in the background, so it's best to have lots of battery life if running the program while on a batteries, especially if you turn on the "auto-move" feature and simply watch it run -- which is a lot of fun to do, by the way!
If using a keyboard, the following keys are available: