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Calculator of the day: the slide rule

This is the first in a series of six posts on Calculator of the day. Continue to PART 2 or view ALL POSTS in the series.

My new book, Empire of the Sum: The Rise and Reign of the Pocket Calculator, will be published in the US this coming Tuesday, the 22nd of August, and to mark the occasion I thought I’d start a series of posts about a few of my favourite calculators.

Today, it’s first things first. Well, not exactly, since I am rudely omitting hands, feet, genitals, pebbles, sticks, counting tokens and abacuses from this list of portable calculating devices. But today’s calculator is, at any rate, the oldest one that I’ll be writing about this week. Enter the slide rule.

Slide rule owned by Sally Ride, the first American woman in space.
Slide rule owned by Sally Ride, the first American woman in space. (CC0 image courtesy of the National Air and Space Museum.)

For the uninitiated, slide rules are analog mechanical calculators, like the one shown above. They’re similar to normal rulers but with two main distinctions: they have a number of specialised scales (usually four at a minimum, but often many more) that encode different mathematical operations; and they have a movable slide that allows their scales to be aligned as necessary to carry out those operations.

I had a lot of fun digging into the slide rule’s history. It was invented by a sixteenth-century mathematician and Anglican minister named William Oughtred, who, ironically, was avowedly against using anything other than good old fashioned brain power when it came to working through mathematical problems. He once proclaimed that devices that simplified mathematical operations turned their users into “doers of tricks, and as it were Iuglers [jugglers].”

Oughtred had the insight to line up a pair of what were called “Gunter scales”, or logarithmic rulers, to make it easy to multiply any two numbers together to at least a few significant figures of precision — a job that had been, until that time, a frustratingly manual process. In turn, Gunter scales were based on the concept of logarithms, as devised by John Napier of Edinburgh, an alchemist, astrologer, religious polemicist, and occasional mathematician. Edinburgh’s Napier University is named after him.

Courtesy of my father in law, who has indefatiguably trawled the antique stores of the American Midwest, I am now the proud owner of a number of vintage slide rules. Not the one shown above — that one belonged to Sally Ride, the first American woman in space — but a respectable selection nonetheless of rules covering the gamut from the classroom to the engineering laboratory. Slide rules are deceptively simple things, and they are intensely satisfying to get to grips with. If you aren’t familiar with them, head over to eBay to see what you can find. A piece of mathematical magic can be yours for as little as a few pounds.

If you’d like to pre-order a copy of Empire of the Sum, this post will point you in the right direction. And don’t forget that American readers can enter a competition to win one of two copies! Visit this post to find out more.

10 comments on “Calculator of the day: the slide rule

  1. Comment posted by Mary Ann Atwood on

    “A piece of math­em­at­ical ma­gic”… The slide rule, better than Harry Potter’s wand, if you have the patience and courage to use it. I shall attempt, yet again, to master that “ana­log mech­an­ical cal­cu­lat­or!

    1. Comment posted by Keith Houston on

      It really is like a magic trick, isn’t it? As someone who grew up with electronic calculators, I find slide rules to be quite enthralling.

  2. Comment posted by Steve Minniear on

    Among the things I’ve misplaced is one I really miss: a very small, compact slide rule my mother used in college. It was something she picked up so that it would fit in her purse, thereby avoiding the “nerdy college girl look” a full size slide rule would denote. It was especially cool to me because on the back it was marked “Made in Occupied Japan.” Sorry Mom.

    1. Comment posted by Keith Houston on

      Whereas I would have proudly rocked the “nerdy college boy” look, slide rule and all!

  3. Comment posted by Dale Ward on

    About 35 years ago a local charity shop was given a batch of new and completely unused slide rules. They may have been from a school storeroom or an educational supplies warehouse.

    I bought one for two pounds. It’s a British Thornton mark two. It’s still in absolutely pristine condition and makes an ideal talking point when shown to visitors.

    1. Comment posted by Keith Houston on

      Charity and antique shops can be goldmines for slide rules. Many seem never to have been used.

  4. Comment posted by Long Branch Mike on

    My father had one, he showed me it and I was amazed. I need to find it, as he has passed. That and the abacus he bought.

    As a teenager I had joined an artillery Canadian Army Reserve unit and trained as a command post technician, calculating gun bearing & elevation to hit targets. We used TI 41C programmable calculators to do this, taking into account windspeed, elevation difference, shell weight, and for high angle ‘lob’ trajectories, even rotation of the earth! On exercise in the States, we saw the comparable US Army equivalent – a truck containing a large fire control computer!

    But in case our calculators were damaged (or out of battery – pun intended), we were also taught how to use the artillery sliderules, nicknamed ‘sticks’, which was slower but still cool.

  5. Comment posted by Long Branch Mike on

    There are also, usually simpler, circular slide rules, sometimes with three or more discs to rotate to provide information. I found in a vintage shop a 1950s British military one that calculated radioactive fallout by distance, for land, and turning it over, for sea. Morbid but interesting. Must search for circular sliderule enthusiast website now.

  6. Comment posted by Walter Underwood on

    Ah, Sally Ride had good taste in slide rules, that is a Post Versalog, same model that I have. I’ve also collected the 5-inch version, plus a lovely little 4-inch simplex Post slide rule, just right for formal occasions.

    Here is that model at the National Museum of American History, though my leather case might be in slightly better shape.


    In my last year in university (1981, electrical engineering), my HP-25 calculator batteries were nearly dead, so I brought my slide rule to tests as a back-up and ended up using it most times. Worked just fine.

  7. Comment posted by Popup on

    Just like Steve Minniear, I had a miniature slide rule that used to belong to my mother. (But this was probably a gift/bribe/promotional material given to my grandfather, as it prominently displayed the name of a company providing raw material to the company he worked for.)

    I used it throughout university, and even managed to convince my quantum mechanics professor to explicitly allow slide rules for exams.

    But I have lost it in one of many moves since then.

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