Yesterday, while walking a bit around Zurich with A. Saha (one of the new postdocs, who will arrive at ETH in September) I found a slide-rule in an antique shop; since I had never actually seen one before, and it was very cheap, I bought it immediately. It is in very good shape, and seems to be fairly sophisticated, and I hope to learn a bit how to use it for the fun of it.
I was interested to see that π is indicated on some of the scales, but then we also noticed another mark indicating a value between 1.74 and 1.75, highlighted by a symbol which I had never seen before (click for larger picture):
Trial-and-error led us to the conclusion that this number is simply
and so is to be used to convert between degrees and radians. Further searches revealed a number of pictures of (essentially) identical slide-rules with a small tick to indicate this constant. However, I haven’t found a picture yet with the strange symbol. Is it really standard? Has anyone seen it before? Is it somewhere in Unicode?