Illustration for article titled How to Use a Slide Rule
Photo: HOerwin56

Analog WeekJust because ‘there’s an app for that’ doesn’t mean you have to use it. This week we’re going analog, reminding ourselves that we can live—and live _well_ —without smartphones, and seeing what’s worth preserving from the time before we were all plugged in 24/7.  

I’ve always felt a little left out of the traditional nerd stereotypes. I don’t wear glasses, my lady clothes have no real pockets so I can’t use a pocket protector, and I was born well after the reign of the slide rule. But in the spirit of Analog Week, I’m trying to learn.

A slide rule is a multi-purpose calculator tool. It’s what people used to do math in the days before calculators and now ever-present phones and computers. It looks like a ruler, but has a slide-y part in the middle. You can use it to quickly multiply and divide large numbers, and if you are a slide rule whiz you can even do exponents, roots, and trigonometry.


You don’t have to track down vintage slide rules to play along: there’s a virtual slide rule right here. Slide rules can get very fancy, but the basic type has a body, one slide (the thing running down the middle), and a cursor that gives you a line so you can accurately line up the body and the slide with each other.

Illustration for article titled How to Use a Slide Rule
Screenshot: Simulated Pickett N909-ES Slide Rule

See those letters next to each scale of numbers? To multiply or divide, we can use the C scale (on the bottom of the slide) together with the D scale (right next to it, on the bottom part of the body). Ready?


How to multiply

Let’s say you want to multiply two numbers, perhaps 32*45. Look at your slide rule. The C scale is on the slide, and the D scale is on the body.

Now look at the numbers on each scale. There are…a lot of numbers. In the example here, you can see the C and D scales both have the number 1, then the number 1 again, then numbers up through 9, and then, finally, 2. All of those numbers in between actually represent 1.1, 1.2, 1.3, and so on. So when we go ahead with our example, make sure that if you’re looking for number 3, that you find actual number 3 and not 1.3.



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