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?

**Answer**

I found the answer by googling “slide rule markings”! It took me straight to the Glossary of the International Slide Rule Museum, which gives C its own entry:

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.

And there is, of course, so much more at that site.

**Attribution***Source : Link , Question Author : timtfj , Answer Author : TonyK*