# Rubber Band “Slide Rule” Doesn’t Slide, But Rotates

Around here we mostly enjoy slide rules. We even have our own collections including some cylindrical and circular ones. But [Mathologer] discusses a recent Reddit post that explains a circular slide rule-like device using a wheel and a stretchable rubber band. While it probably would be difficult to build the actual device using a rubber band, it can do wonders for your understanding of logarithms which still show up in our lives when, for example, you are calculating decibels. [Dimitri] did simulate the rubber band for you in software.

The idea is that a perfect rubber band has numbers from 0 to 10 evenly marked on it. As you rotate a wheel attached at the 10 mark, the rubber band stretches more and more. So the 10 and the 9 have relatively little space between them, but the 1 and the 2 are much further apart. The wheel’s circumference is set so that the 1 will exactly overlay the 10. What this means is that each spot on the wheel can represent any number that differs only by a decimal point. So you could have 3 mean 0.03, 300, or — of course — 3. Of course, you don’t need to build the wheel with a rubber band — you could just mark the wheel like a regular circular slide rule.

If you’ve never really learned why a slide rule works or you don’t know how to work one, you’ll find the explanation in the video very intuitive and enlightening. You do have to have a rough idea of the order of magnitude of the answer you expect, but that’s not so hard with practice.

Of course, if you flatten the circle out, you get a regular slide rule. You can see some of my collection — but oddly, none of my circular ones — in an old post from 2015. If you want to make your own, we suggest you leave the rubber band in the drawer and check out [Dylan’s] work.

## 11 thoughts on “Rubber Band “Slide Rule” Doesn’t Slide, But Rotates”

1. Am I the only one who entered 25.8069 into a calculator and squared it?

1. Bryantherobotman says:

Nope, but in my defence I said to myself “that’s too big to be the root of 66”.

2. bob says:

i took the speculative execution route and got the square root of the evil number instead.

3. Paul says:

Maybe. I saw 25×25=625 and thought “yeah, that’s about right”
25.807 is closer to 666 though.

1. DennisC says:

Seems the shirt truncated instead of rounded
sqrt(666) = 25.8069758011

4. Tommy says:

Well, I squared it, but with a slide rule…

5. Prowler50mil says:

Nope, I did too. Mainly to double check, “That’s not quite enough, right?”

6. Steven-X says:

No, because of you I went ahead and did it was well.

And in sheer confidence, it’s the same number used as my luggage lock combination.

2. Paul says:

So, this is Zeno’s rubber band?

3. Truth says:

Their T-shirt is old, the current thinking is that 24.8193 is now the root of Evil.

4. Miroslav says:

Root of Evil = 43.738 :)

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