The Slide Rule: A Computing Device That Put A Man On The Moon
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.
Today, NPR Ed begins a new series examining these icons of the classroom. We start off with a device that once was essential to higher-level math, in school and in the workplace, but now has all but disappeared:
The slide rule.
"Take your batteries out," Jim Hus says, watching his pre-calculus students remove the AA batteries that power their calculators. "Let's do those multiplication problems again."
For the next calculations, Hus's juniors and seniors at Highland High School in Highland, Ind., will use a different tool: A tool that dates back 400 years.
Before the smartphone, the laptop and the graphing calculator, there was the slide rule. It's a powerful mechanical computing device, often no larger than a 12-inch ruler, marked with numbers — but part of it slides in an out to to show relationships between different sets of numbers.
That seemingly simple tool has a serious resume. NASA engineers used slide rules to build the rockets and plan the mission that landed Apollo 11 on the moon. It's said that Buzz Aldrin needed his pocket slide rule for last-minute calculations before landing.
"The slide rule is an instrument that was used to design virtually everything," says Deborah Douglas, the director of collections and curator of science and technology at the MIT Museum in Cambridge, Mass. The museum just ended a three-year exhibit on slide rules. "The size of a sewer pipe, the weight-bearing ability of a cardboard box, even rocket ships and cars."
So, What Is A Slide Rule?
Slide rules are typically rectangular and about the size of a ruler. They are divided into thirds, the top and bottom are fixed in place, but the middle section slides back and forth. Each section has scales — numbers and line marks for calculations.
The first one was built by William Oughtred, a cleric teaching math in England in the 1600s. It was based on John Napier's discovery of logarithms.
In its simplest form, the slide rule adds and subtracts lengths in order to calculate a total distance. But slide rules can also handle multiplication and division, find square roots, and do other sophisticated calculations.
For generations of engineers, technicians and scientists, the slide rule was an essential part of their daily lives. Until, all of a sudden, it wasn't.
In 1972 Hewlett-Packard came out with the first handheld electronic calculator. Practically overnight, the slide rule had become obsolete.
"The death of the slide rule was pretty instantaneous," says Bob De Cesaris, who oversees chip manufacturing at Intel and has one of the largest collections of slide rules in the country. De Cesaris estimates his collection has nearly 4,000 slide rules.
He is also the president of the Oughtred Society — a group of 400 slide rule collectors and enthusiasts seeking to preserve the device's history (and where you can find out all sorts of information about them).
And yet, despite the calculating power these days in even your handheld phone, the slide rule isn't quite dead.
Here and there, teachers like Jim Hus still use them in the classroom. Sister Paula Irving, a nun at the Community of Jesus in Orleans, Mass., teaches a computer programming course to homeschooled high school students. And when she covers the history of the computer, she teaches students how to use a tool she remembers watching her father use as he calculated their family's finances.
More than a thousand miles west, Laurie Emery, a math teacher at South Winneshiek High School in Calmar, Iowa, will teach her junior pre-calculus class how to do intricate calculations without a calculator.
There's even a freshman seminar about the slide rule at the University of California, San Diego, that Professor Joe Pasquale has been teaching since 2003.
"We live in this age when computing is getting exponentially more powerful but we often don't even think about the calculations being made," says Pasquale. "We just let our computer do all the work."
In the seminar, Pasquale says his students are often amazed that the slide rule's answers make sense.
Thinking About The Math
"The nice thing about a calculator is you don't have to think – but it's also a bad thing," he adds. "When you're using a slide rule you have to be engaged. You have to be thinking about math."
And that's one of the main reasons some teachers still hang on to the old "slipsticks."
MIT's Debbie Douglas says that even though the slide rule isn't as precise as a calculator, students can understand the idea of what it's doing. "It really makes one engaged with the process."
Jim Hus still remembers deciding to invest $400 in a calculator and abandoning his slide rule during his freshman year at Purdue, back in 1974.
But it's a device he'll never forget and he hopes his students at Highland High School in Indiana won't either. He's planning to have his students build a classroom slide rule.
"I joke it will be the largest slide rule in the world," he says, laughing. "But this way, I'm not just handing the students a tool, we're learning how it operates so we can access higher math concepts."
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Transcript
ROBERT SIEGEL, HOST:
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.
(SOUNDBITE OF GOVERNMENT TRAINING FILM)
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.
(SLIDING SOUNDS)
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 npr.org. Transcript provided by NPR, Copyright NPR.
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