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Last week I met a gentleman with one of these bad boys. He had no calculator on his desk, at least that I could see.

We had a brief, but fascinating conversation. First off, I learned that I need to point out, for the benefit of most readers, this is a picture of a slide rule. Before pocket calculators (which became popular in the 1970s), mathematicians, engineers, etc., used slide rules to multiply, divide, calculate exponentials and work trig functions.

In high school, I spent a couple of weeks learning how to use a slide rule at the end of the school year – sophomore, I think. By the fall, the school permitted calculators, so I never got used to using one.

The rulers are cumbersome and much harder to learn than a calculator. When you use one, for example, you have to keep track of the order of magnitude of a problem, because on a slide rule you calculate 30 times 40 pretty much the same way you calculate three times four. That one answer is 100 times greater than the other is a fact you need to keep in your head.

But the gentleman I was speaking to, a slender, white-haired statistician, continues to use his – long after its calculator-induced obsolescence. The ruler feels more comfortable to him than a calculator, the way in the 1980s some grizzled journalists preferred the feel of their old Royal typewriter to the fancy new word processor the young editor insisted upon.

He said he can use the ruler without thinking, as a skilled typist can ignore the selection of individual letters as they write.

So this habit must launch some interesting conversations, I said, memories like the meager ones I shared, or stories of classic miscalculations, or just questions about how the darn thing works. He said no, and I think he looked bemused and perhaps a bit sad. He gestured at me – “Most people younger than you” (I’m 52.) – don’t recognize it. It is such an anachronism that few people recognize it as one.

I confirmed this when I showed the picture to a friend who is in her 30s. She did not know what I had photographed.

The gentleman has used a slide rule since he was in his teens, and got extra reinforcement when he became a pilot around age 17. Back then, pilots used a circular slide rule, and at least according to this Forbes article, they still do, to calculate rate-time-distance or convert nautical miles to statutory miles. (Here’s a video tutorial.)

He never got fast enough, he said, to enter slide-rule contests. I had never heard of these, but they were speed-and-accuracy contests in calculating complex questions on the ruler – a Nerd Bowl if there ever was one.

The calculator killed off the slide rule as a practical tool. Thanks to the internet, though, nothing completely dies.

You can still buy them on ebay used for around $15. New ones are ridiculously expensive. On Amazon, one was $499; the other was $1,400.

Of course there’s a Wikipedia page. And here’s a link to the International Slide Rule Museum, which reported on the International Slide Rule Championships. (Typical problem: √(56/620)÷√(70/0.0082).) Winners are pictured. Some of them look disturbingly young.

I think he was happy to discuss his favored toy, and of course I was happy enough with the conversation to memorialize it. As I fumbled with my Android camera, he noodled with his own phone – checking messages, I assumed.

I held my photo out to him, and he offered his smartphone to me, saying, “I have two.”

And there, on the screen, was a slide rule app.

I downloaded my copy that afternoon.

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