What does the mysterious constant marked by C on a slide rule indicate?
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Years ago, before everyone (or anyone) had electronic calculators, I had a pocket slide rule which I used in secondary school until the first TI-30 cane out. Recently I dug it out. Here's a photo of one end of it. As you can see, there's a number $C$ marked at about $1.128$ (times some power of $10$ ; with a slide rule you supply that yourself) on the C and D scales. Reading across to the A scale, its square is about $1.27$ . By the C1 scale (which reads reciprocals of the C scale) its reciprocal is about $0.886$ (times some power of $10$ ). The only two special numbers marked are $C$ and $pi$ . I'm not sure whether it's some frequently used constant that's used (eg) in some branch of engineering, or a number which is useful for some trick for using the slide rule. Unlike $pi$ ,