Circular Slide Rule
Regular readers of this blog (all five of them) will know that, Selective Luddite™ that I am, I'm a fanatic about slide rules. I keep a slide rule on my desk, and often use it in preference to a calculator. My interest in slide rules dates back to my high school days, in the early 70s, when they were still teaching us in math class how to use slide rules. I got a few slide rules when I was in high school, and I still have a couple of them, including the slide rule pictured here: a Pickett circular slide rule, model 109ES.
This really is something of an oddity. The advantage of a circular slide rule, of course, is that you can never run "off the end" of a scale. Also, it's much more compact: only 4 inches in diameter, my circular slide rule has scales more detailed than those on a standard 10-inch slide rule.
The front has EI, CI, C, D, A, and K scales. The back has L, DI, and D scales, plus half a dozen sine and/or tangent scales. Looking back, I wish I'd thought to buy a slide rule with log-log scales on it: somehow that idea never occurred to me in high school.
Then I went with calculators, which were just coming out and becoming affordable around that time, and for many years I was an apostate from the world of slide rules. Then I began rediscovering those slipsticks, at second-hand stores and antique stores and garage sales. Over the past 10 or 12 years I've built up something of a slide rule collection. But I've never run across another circular slide rule like that one I got back in high school.