# My new slide-rule

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

$\frac{100\pi}{180}=1.7453292519943295769236907684886127134\ldots$

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?

### Kowalski

I am a professor of mathematics at ETH Zürich since 2008.

## 14 thoughts on “My new slide-rule”

1. Probably comes from “g”, as in “grade”, as in “degree”.

1. andrej says:

“ro” greek means “radian” proposal from Boris Apsen an engineer from exYugoslavia:

“ro” x very litle angle = sin or tan of this very litlle angle

That sign there is onthe faber castell 111/54 ….

2. lukas says:

I think it is an oddball variety of Greek rho. IIRC my slide rule has a similar symbol. I’d have to dig it out and check… it even comes with a manual, so the symbol might be described further.

3. #1,#2: the symbol also reminds me of either “g” or “rho”; but “g” for “grads” seems strange to me since there are 400 grads for 2π radians. “rho” could be for radians.

#3: Yes, the “sliderulemuseum” has a few pictures of the same model as mine, though without the bizarre rho/g symbol as far as I can see…

4. I own a Faber-Castell Novo-Duplex slide rule that shows this symbol on several scales. The “Rechenstab-Anleitung” coming with this slide rule uses a rho from a normal greek font (and not a handwritten one as on the rule itself). It seems that this mark is mainly intended for calculating sin and tan of small angles.

5. name says:

It’s probably just a rho, as Christian Blatter says. However, the symbol as printed looks a bit like the Unicode character U+018D (LATIN SMALL LETTER TURNED DELTA = reversed Polish-hook o), ƍ if your browser supports it. It’s apparently an old phonetic symbol (“labialized alveolar fricative”, whatever that sounds like), so I’m guessing the similarity is just a coincidence.

6. It’s true that it definitely looks like that Unicode symbol, but I agree this must be a coincidence (if I turn the slide rule upside-down, it looks a bit like a Greek delta but not that much; or similarly, it does not look that much like an a lowercase “o” with a hook or cedilla).

7. dt says:

Thank you.

You solved the 45 year riddle of what that character represented on my dad’s slide rule.

8. Mark Davis says:

That mark appears on the C and D scales on both sides of my recently-acquired Faber Castell 2/82 N rule, and also on the CF & DF scales.

According to the FC English instruction manual, in the “Interpretation of scale marks” section on page 25, this mark is ϱ (rho) = pi/180 = 0.01745.

For whatever (Germanic) reason the character looks like the Roman character ‘g’.

9. Mark Davis says:

This “backwards Rho” symbol, representing π/180 (or perhaps 2π / 360) apparently originated with the Dennart & Pape company in 1926.

This version of the Rho symbol is common on Faber-Castell Duplex slide rules, such as the 2/82 and 2/83 models.

10. Mark Davis says:

corrections…