# What does the mysterious constant marked by C on a slide rule indicate?

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 $$\pi$$, which is marked on most of the scales, this mysterious $$C$$ only appears on the C and D scales, which are the main ones used for multiplication and division.

If you need me to, I can give more explanation of the various scales on the rule and how calculations are done. That might give some clues as to what $$C$$ is for.

I'm sure the instructions explained what $$C$$ was, but I last saw those in the 1970s.

Has anyone any idea what $$C$$ is and why it would be useful on a slide rule?

• Please tag as appropriate—I'm not sure if the right tags and can't yet create a new "slide-rules" tag. – timtfj Nov 30 '18 at 2:59
• Funnest question in forever. – Randall Nov 30 '18 at 3:03
• Did anyone else here buy their kid a slide rule when he/she went off to engineering school. My daughter ended up with two (we are a family with many engineers). She dutifully sat through my instructions on slide rule use (twice), but I don't think she paid any attention. – Flydog57 Nov 30 '18 at 17:34
• Shouldn't this go in retrocomputing.SE? ;-) – shoover Nov 30 '18 at 18:34
• Thanks for the question ! I took my slide rule (I did buy it in 1958) and, thanks to you, I nticed that this $C$ was present ! I never noticed. Shame on me ! – Claude Leibovici Dec 5 '18 at 4:31

C - Gauge mark found on the C and D scales denoting $$\sqrt{4/\pi} = 1.128$$ for calculating the area of a circle and the volume of a cylinder. Place the C mark on the C scale over the diameter of a circle on the D scale. The area of the circle is found above the index on the A scale. If this is the base of a cylinder, without moving the slide, move the cursor to the height of the cylinder on the B scale. The volume is read on the A scale. This gauge mark was rendered obsolete with the advent of multi-lined cursors.
• I've just tried calculating $\sqrt{4/\pi}$ on the slide rule and the result does line up nicely with the $C$ mark, so this is definitely it!. Thanks! – timtfj Nov 30 '18 at 3:23