I was curious about circular slide rules, and found one on eBay for a few bucks, so what the heck. It turned out that it was a give-away from a company that used to be in my town, so I ended up having a nice conversation with the seller about his history with the company and so forth. He was a long retired engineer.

Anyway, I started playing with ratios, and noticed that the thing is inaccurate! Further inspection and hypothesizing led me to discover that the scales were not printed concentrically with the mechanism. It's very slight, and not critical to my work in any way, so I treat it as part of the learning experience.