Home > Geeky > I got your ‘obsolete’ right here, buddy…

I got your ‘obsolete’ right here, buddy…

April 14, 2006

One of the curiosities you’ll find in my briefcase is a slide rule.  It isn’t a conversation-piece; I still use it.  I love slide rules, though some people think I’m a little odd because of it.

So I was just delighted to see this wonderful article by Cliff Stoll in the May ‘06 Scientific American on the history and use of the slide rule.  It even includes a ‘make-your-own’ page you can photocopy and cut up to produce a demonstration slide rule.  Stoll writes:

Yet slide rules had an Achilles’ heel; standard models could typically handle only three digits of precision… worse yet: you have to keep track of the decimal place.  A hairline pointing to 3.46 might also represent 34.5, 3,460 or 0.00346.

That slippery decimal place reminded every competent engineer to double-check the slide rule’s results.  First you would estimate an approximate answer and then compare it with the number under the cursor.  One effect was that users felt close to the numbers, aware of rounding-off errors and systematic inaccuracies, unlike users of today’s computer-design programs.

Chat with an engineer from the 1950’s and you will most likely hear a lament for the days when calculation went hand-in-hand with deeper comprehension.  Instead of plugging numbers into a computer program, an engineer would understand the fine points of loads and stresses, voltages and currents, angles and distances.  Numeric answers, crafted by hand, meant problem solving through knowledge and analysis rather than sheer number-crunching.

All true, but that isn’t why I use a slide rule.  Fact is, I am dyslexic and have a terrible time reading the numbers on a calculator. My dyslexia went undiagnosed until I was in my thirties, but I am forever grateful to a math teacher named Frank Marvin, who took his lunch hours to teach me how to use a slide rule back in about 1970.  Since the slide rule is an analog device, and the ‘numbers’ on it are just positions on an engraved logarithmic scale, it’s easier for me to track them accurately.  And it doesn’t hurt that I am a sucker for cleverly designed, elegantly functional things.

I am not steering spacecraft in the trackless vacuum between here and Mars.  One part of accuracy in a thousand is plenty for most of my purposes, and the fact is, no matter how many decimal places appear on the display of your calculator, most likely the incoming data isn’t any more exact than that.  Or, operational variables will devour that accuracy long before it reaches the output of whatever real-world process you are reckoning.

When you are using a slide rule, the concrete relationship between the numbers and a physical reality needs to be grounded in visualization.  Scientific notation helps a lot, and you have to concentrate on ‘what it all means’. This is a lot easier than it sounds, and it works just fine on either side of the decimal point.

The slide rule played a major role in the industrial revolution right up to the space age, and even helped develop its own replacement, the “electronic slide rule” that we now know as a pocket calculator.  It is one of those little-known inventions whose economic importance is all out of proportion to its modest function.

I would love to see someone manufacture new slide rules – with laser engraving and Chinese manufacture a very good one could be made quite inexpensively.  They could be used for a good high-school math course: “math and physical reality”, if you could find anyone to teach it.  I understand Frank Marvin has retired.

NOTES:

Categories: Geeky
  1. April 15, 2006 at 09:38 | #1

    The three significant digit limitation was never a problem given the accuracy of the data **I** used.  I used to do bridge classification (for weight carrying capability) in the Army with my slide rule.  It was perfect for that.  And jst about everything else.  People today don’t understand that just because you divide one three-decimal place number by another three-decimal place number and the CALCULATOR shows SIX decimal places, that those SIX decimal places are NOT significant.  But it impresses the ignorant…

    MC
    another old fart…

  2. April 15, 2006 at 13:43 | #2

    “the fact is, no matter how many decimal places appear on the display of your calculator, most likely the incoming data isn’t any more exact than that.”

    I wish more people understood the concept of significant digits. I still own my slide rule, but I must admit that it has been 30 years since I’ve used it, and I have forgotten how. Alas, these days a hero is just a sandwich, and a log is just an output file….

  3. Lucas Wiman
    April 16, 2006 at 04:35 | #3

    Significant digits are usually introduced to people in high school chemistry of physics class.  One of my teachers in HS didn’t really get the reason behind them, insisting that we round at each stage in a calculation.  With modern calculators, it is easier and more precise round at the end, but points would be deducted for giving a more correct answer.

  4. zilch
    April 19, 2006 at 05:37 | #4

    I still have a circular slide rule I use occasionally.  I agree that three significant digits are nearly always enough, and that using a calculator doesn’t make you think the problem through so that you understand it- my kids often make the most ridiculous errors because of that.

    Here’s a significant digit anecdote my father told me:  a co-worker of his requested from the machine shop a block of steel, a “1.000 inch cube”.  The machinist delivered it a couple of days later, commenting that it was a real bitch making a cube accurate to a thousandth of an inch on each side.  The guy who ordered it said “I just need it to prop up a motor housing” and was surprised when the machinist swore at him.  The guy had just thrown in the zeros more or less at random…

  5. Newbie
    September 3, 2006 at 18:52 | #5

    If you’re worried about the third significant digit of precision, I don’t want you designing my bridge! (Though I acknowledge that space flight is another matter entirely.)

Comments are closed.