The Slide Rule: A Computing Device That Put A Man On The Moon : NPR Ed Before the smartphone, the laptop and the pocket calculator, there was a powerful mechanical computer. Our new series, Tools of the Trade, begins with a look at the slide rule.
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The Slide Rule: A Computing Device That Put A Man On The Moon

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The Slide Rule: A Computing Device That Put A Man On The Moon

The Slide Rule: A Computing Device That Put A Man On The Moon

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  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
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The protractor and the Bunsen burner, playing the recorder in music class, drawing arcs and circles with a compass in geometry - these tools of the education trade become part of our lives for a semester or two and then we move on. Well, today our NPR Ed team begins a new series examining these icons of the classroom and we begin with a tool that once was essential to higher-level math in school and in the workplace, but has now all but disappeared. It is the slide rule, or if you're not of a certain age, the what? Here is a U.S. government training film on the use of a slide rule from 1944.


UNIDENTIFIED MAN #1: Careful practice is essential to using the slide rule with speed and accuracy, but makes possible the ready solution of problems in multiplication, division, proportion and percentage, squares and square roots, and these are only a few out of the wide variety of problems that can be worked with the standard slide rule.

SIEGEL: To talk about slide rules and how they work, we are joined by Deborah Douglas, director of collections and curator of science and technology at the MIT Museum in Cambridge, Massachusetts. Earlier this year, the museum had an exhibit of K and E Company slide rules.

Deborah Douglas, welcome to the program.

DEBORAH DOUGLAS: Thank you so much, Robert, I'm delighted.

SIEGEL: First of all, for people who are mystified by all this, give us a brief definition of a slide rule.

DOUGLAS: Imagine a ruler with a centerpiece that moves back and forth. In fact, I've got one here in the studio and I'm going to just make that sliding sound, if you will. (Sliding sound). Can you hear that?

SIEGEL: Not only can I hear it, I have one here and I'll make the sound too, hear? (Sliding sound). See?

DOUGLAS: That was the sound of anxiety for MIT students during the exam period. There'd be auditoriums filled with students taking tests. You could hear a pin drop, except for this sound (sliding sound) back and forth. But simply put, a slide rule is a device to remove the tedium from the countless calculations that you do in engineering, in other kinds of mathematics.

SIEGEL: Now, the slide rule model that I'm holding is K and E Deci-Lon 10. It was actually a gift from a friend of our family who had much higher hopes for me than I was able to materialize, in the way of mathematics and the sciences. I gather it's a historic model that I have.

DOUGLAS: It is. In fact, it was the very last slide rule designed by the company K and E, or Keuffel and Esser. K and E was the great American slide rule manufacturer. They probably made more slide rules than anyone else on the planet.

SIEGEL: I was told, some years ago, that K and E wrote a rather good forecast of future trends way back when and they got lots of things right, but one thing they missed was that the slide rule would soon be obsolete.

DOUGLAS: That's exactly right and I suppose that we all are blind in our own industries, in terms of making predictions, but no one really understood in the 1960s how transformative the digital computer was going to be and frankly, how quickly it was going to be possible to make an inexpensive handheld calculator.

SIEGEL: Well, for those three people out there who happen to have their slide rule handy by now - that they've heard us talking about it - why don't you talk me through here a simple problem that uses the C and the D scale and how we would do a multiplication problem, say?

DOUGLAS: Right. Robert, you're looking at your slide rule and on the left side - if you've got one of these at home - you'll see a series of letters. Mine is a very simple one, it says ABCD. This is the classic slide rule invented by Amedee Mannheim in the 1850s to do artillery and ballistics calculations. Take that C and the D and take the one on that C scale. If you notice, when they're lined up perfectly it looks like exactly the same scale, the one is over the one, the two over the two, so forth down the line. Take that one on the C, push that middle slider. Push it all the way so that the one is lined up over the two on your D scale.

SIEGEL: All right, I've got it right there.

DOUGLAS: Now, here's the magic - if you look at the two below and you say, well, if I were multiplying two times two, look now where the two is on your C scale.

SIEGEL: Oh yes, it's right over the four...

DOUGLAS: And it's right over the four.

SIEGEL: ...On the D. I guess the best way to explain this to someone is, if you can imagine just two ordinary 12 inch rulers and one slid over the other. If the one on one of them were over the five and then you added four, it would be nine. It would be at nine inches because you simply added.

DOUGLAS: That is correct.

SIEGEL: So if those scales are spread out logarithmically, rather than linearly, you're adding logarithms and therefore the result you get is a multiplication result.

DOUGLAS: That's correct. I think the most amazing thing about the slide rule is that you start to think about math in geographical terms. You begin to know where the right answer is supposed to be and that's what engineering students here at MIT or anywhere in the country began to understand is, when they looked at that initial equation, they had a sense of where they were supposed to find the answer. And as they did their calculations, if they found themselves looking at their slip stick if you will - that's the nickname for it - on the right side and they were expecting the answer on the left, well, that was a good clue that maybe they'd made a mistake somewhere along the way. So the engineers used to say to me, Debbie, we have a real feeling for numbers in our generation - and the young people don't seem to have that anymore.

SIEGEL: Well, you had an exhibit at the museum at MIT I gather, of K and E slide rules. So tell me about what young people, who would probably consider a desktop computer to be a kind of a clunky old machine, what did they make of the slide rules?

DOUGLAS: I was shocked, Robert. They were fascinated and they began to request an introductory session. So every weekend, one of our volunteers would teach young kids how to use a slide rule, the rudiments of multiplication and division and finding square roots and the like. My favorite story is of a kid who came in and he saw one of these giant teaching rules - these are six feet long and they would be up at the front of the classroom, teacher would use this rule demonstrating the various techniques - so the youngster asks me, he says, well, what is that and I said, oh, it's a green calculator. It's made of wood, renewable materials, doesn't use any electricity. His father came into the room. He said, dad, it's a green calculator. His father just laughed, scratched his head and said, I think you've just got to rebrand the slide rule.

SIEGEL: (Laughter). Well, Deborah Douglas, thanks for talking with us. Here, I'll give you a little bit of a send-off here with my slide rule for a moment and you can sign off as well, I guess.


DOUGLAS: Yes so again, I'm sending you off, I'm delighted to host people at the museum. We will have another slide rule display coming back in the new year, when we have still more from our collection.

SIEGEL: All right. For some reason I want to say be fruitful and multiply right now, but I guess that's what one says at the end of a discussion on slide rules. Thank you very much for talking with us.

DOUGLAS: Thank you, Robert.

SIEGEL: That's Deborah Douglas of the MIT Museum. If you have never seen a slide rule or could use a little remedial math lesson, take a look at our animated demonstration of the slide rule at

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