The people who use them the most nowadays are amateur pilots who can't afford a fancy shmancy online computer or a copilot. My dad has a rather nifty circular one with all sorts of twiddly bits for calculating vectors and the like. It's one of the funkiest pieces of plastic I've ever seen. Especially the vector addition thing. That's one damn smart piece of kit.

One day when I walked into my high school physics classroom (mid 90's) early and went up to the teacher (retired army engineer)'s desk to speak to him, I saw what at first appeared to be a ruler. Upon looking carefully I asked him, "Is that a slide rule?" After being answered in the affirmative, the teacher then proceeded to give me a quick lesson in multiplication and division on it.

Other than watching two high school students "sword fight" with slide rules (I don't know where they got them) in a technical drawing class (the year before the class changed from being a manual drafting course to being a CAD course), I never saw a slide rule again until I found one in my parents' garage one day while helping to clean it out (It became mine). Unlike basically everyone else my age, I know how to use a slide rule, and do so occasionally.

    The following Everythingians own slide rules.
  • Eponymous ("Betsy", plastic Sterling, with orig. case). I keep in hanging on my wall next to the Tower of Power just to remind my computers that they are replaceble, if need be.
  • Anotherone just carries one to annoy math teachers
  • Ariels (I assume without concrete proof)
  • Zerth
  • Mr Option doesn't, but his uncle was once beaten up for owning a slide rule
  • Big Alba does, but it's "old and sucks"
  • Stavr0 owns one branded Northern Electric
  • ouroboros is reported to be an old-fashioned noder as well
  • fellowearthite has confessed, and on this very page!
  • BlueDragon calls hers "a little stick of magic"
  • Mr. Guilt says he owns "several, having one at the office, one in my home office (both see use), and, strangely, an old wooden one in my beadroom."
  • Rubyflame is the proud owner of a K&E 4095-3 slide rule, and planning to get a log-log rule.
  • xdjio has a Hughes-Owens and Mannheim 10-incher (whip out that big Ten inch!), bamboo with white plastic celluloid faces, and an engraved scale, vintage about 1960, and wouldn't mind learning how to use it.
  • sirnonya owns three slide rules: "a small pocket slide rule that I can't find, an Acu-Math ~10" slide rule, (both given to me by my grandpa), and a Teledyne Post Versalog II 25cm slide rule. This baby is sweet! Twenty-four scales front and back (duplication of C and D). My other grandpa picked it up at a garage sale really cheap 30 years ago, never used it, was going to frame it on his wall (he was an engineer), and gave it to me instead. A slide rule makes math a lot easier to understand."
  • dann says, "I've got one and actually use it! It's a simplex ACU-Math 400B, mannheim type (K, A/B, CI, C/D, L, S, and T)."
  • wertperch owns a pocket rule, which he occasionally takes out to amaze and astound people with. "Sometimes even by performing calculations."

Most good slide rules include a slide with a hairline, making it much easier to line up the numbers on the logarithmic scales. The actual way the rule works is simple.

Every number has a logarithm.
If you add two logarithms together they produce a third.
The number associated with that logarithm is the product of the first two numbers.

Any mathematical operation that can be reduced to addition and subtraction can be performed on a slide rule. That, really, is all they do. Logarithm tables you don't need to memorize. For basic operations, use the top two scales (often labeled A and B).

When civilization crumbles, after the EMP fries the computers and all the batteries wear out, the person who can use a slide rule will be king. OK, scientific advisor to the king.

Please /msg me to be added to the list of Luddite Everythingians.

Mechanical devices for multiplying and dividing real numbers. At its simplest, a slide rule is just 2 scales of positive numbers located at distances from 1 proportional to their logarithms. The 2 scales are arranged on opposite sides of a sliding rod, so they can be moved relative to each other.

1       2   3   4 5 6
        1       2   3   4 5 6
The diagram (distorted scale, to make everything line up nicely) shows how a slide rule is used. The bottom scale is moved so the `1' is lined up against the multiplier `2'. In this position, any number on the bottom scale lines up with itself multiplied by the multiplier. Of course, this also means we can divide by the multiplier by going from the top scale to the bottom.

Slide rules typically have another pair of scales with all distances halved. This not only gives greater range (at the cost of precision), but also enables you to square and take square roots, simply by transfering a number from the half-distance scale to the full-distance one.

More functions are often found on the back of the slide (you take it out, flip it over, and insert it like that) -- special scales can give you logarithms of sines, cosines, and tangents; secants are trivially there, since multiplication is easy on a slide rule.

Precision is typically a bit more than 2 significant digits, if you're careful -- more than enough for most engineering tasks.

Maintenance instructions: Keep slide rule in box, away from any moisture which might warp the scales. If slide sticks, remove it and apply talcum powder.

Everyone seems to be able to come up with a slide rule, but let's face it: not many people know how to use them anymore. With practice, though, using a slide rule can be faster than punching the same arithmetic or trigonometry into a TI-89. Unless you try to punch straight algebra into a slide rule. That just doesn't work. Thus, a good math grounding is essential for efficient slide rulings.

Here's what a slide rule can do for you: (this list also shows you the contents)

Neat, huh? However, there's beginner stuff to be gone over first.

How to Hold a Slide Rule

Hold the slide rule horizontally, and with both hands. Hold it overhand, i.e. with your hands on top, palms facing the slide rule (but don't let the palms touch it). Your thumbs, which are on the bottom, are on the runner, which is that clear plastic piece. This setup allows for easy, quick, and precise adjustments.

How to Read a Number from the Slide Rule

Slide rules are generally accurate to three significant digits. (i.e. they can multiply 13 and 243, but if you attempt 3 and 7664, there will be some rounding.) A scale on the slide rule is divided into three parts: divisions, sections, and spaces. A division is the area between two numbers, of the first digit. A section is the area between two numbers of the second digit, and a space is the area between the numbers of the third digit. For those who need a picture:

    4.0                                     5.0  
     |                   |                   |
     | i | i | i | i | i | i | i | i | i | i |

Between 4 and 5 is a division, between the double pipes is a section, and between the 'i's is a space. Thus, with 4 significant digits, estimation is a problem.

There are many scales on a slide rule, some more functional than others. There's the C (and CI) and D scales, which are used for multiplying and dividing, as well as a starting point for all the other functions. Also, there's the A and K scales, which are used for squaring and rooting respectively, the L, LL1, LL2, LL3 scales for logarithms, and finally the trig scales S, T1, T2, DF, ST, and P. Note that on each scale, decimal points are irrelevant. Thus, 23, 2.3, .0023, and 23000 are on the same spot. When working with numbers not directly indicated on the scale, remember to keep track of the decimal point.

Beginner stuff over. On to neat stuff.

  • Multiplication: Move the slide (the center piece with scales on it) so that the number 1 of the C scale is over the first number to be multiplied on the D scale. Then, move the runner to the second number to be multiplied on the C scale. Then, read the answer under the runner of the D scale. For example, to multiply 35 and 72, move the slide so that one end of the C scale is at 35 (or 3.5 on the scale), and the runner can be moved to 72(or 7.2). If 72 is off the slide rule, then you've picked the wrong end. Now, move the runner to 72 and read the number under the runner on the D scale. Adjust the decimal point, if necessary. This is your answer, which is 2520(or 2.52).
  • Division: Division works exactly backwards from multiplication. To divide, simply place the runner over the dividend on the D scale, then slide the divisor on the C scale so it lines up with the dividend. Then, read the answer (adjusting for decimals if necessary) off of the D scale, directly under one end of the C scale.
  • Squaring: Move the runner over the number to be squared on the D scale, then look under the runner on the A scale for the answer. Adjust decimals if necessary. To square root, do the opposite. Find the number on the A scale, answer is on the D scale.
  • Cubing and Cube rooting: Same as squaring, except the K scale should be used instead. Same goes for rooting.
  • Sines, Cosines, and their inverses: Using the runner, locate the angle in degrees on the S scale. The answer is under the runner on the D scale. Note: when using cosine, the angle values should be a different color and reverse of the sine values. To find the inverse, move the runner over the number on the D scale, and the sine or cosine is under the runner on the S scale.
  • Tangents and Co-tangents: Use the runner to line up the angle (on the T1 or T2 scale) for the calculation. Make sure that the slide is even, by comparing the C and D scales. Where one says 1, the other should say 1. Then, read the C scale (or D scale, if that's your thing) and find your answer. Do the opposite (by locating with C and reading with Tx) to find tan-1.
  • Logarithms: Use the runner to line up your number (C or D scale) to be logged. Look on the L scale to find the answer. Antilog in reverse.

Care for Your Slide Rule

Don't bend it. Don't break it. It won't work right if you do. </commonsense>

If you find that the slide has difficulty moving, detach it from the slide rule, and run a pencil over its edges. This provides lubricant as well as dirt removal. Also, make sure that the screws in the runner are tight. If they fall out, you'll have to put them back in before the slide rule will work again.

In time, you'll be faster than a calculator. Back up your slide ruling skills with some algebra and you'll be set for whatever comes your way!

ariels says re Slide rule : Maybe mention the fast way to compute fractions like (a*b*c)/(d*e*f)? Instead of computing (a*b*c) and then dividing by d, then e, then f, compute a, divide by d, multiply by b, divide by e, multiply by c, finally divide by f. This is much faster, because you already line up one scale for the next operation (try it and you'll see -- mine is a rotten explanation for a simple thing).

What ariels means is that it's faster to just do a/d*b/e*c/f, and just alternate between the multiplication terms and the division terms. And it is.

I didn't see anyone link to a picture of a slide rule, so let me start with that. The British Thornton slide rule was the recommended weapon of choice at my school. I say weapon as the slide rule was a little over a foot in length and came in a nice, and study, plastic carrying case (I'll leave it as an exercise to the reader to imagine a classroom of 30 boys all armed with one of these).

Of course, it was also an intellectual weapon and a whole lot faster, if less accurate, than a book of log tables. To make sense of this, imagine a time somewhere between when dinosaurs walked the earth and pocket calculators were invented. At that time, you either had to do multiplication and division long hand or use your book of log tables or, if you didn't need more than 3 digits of precision, you could use your slide rule. If you take the time to look at the picture (and as already observed), you'll see that the numbers are etched into the plastic on a logarithmic scale thus turning multiplication into division into addition and subtraction which, in a slide rule simply meant moving the center slide! The slide rule description of how to use a slide rule is really good and needs no repetition by me. The only annoyance being that if you estimated incorrectly, your first attempt would slide the central piece in the wrong direction. Huh? How can that be? The point is that you have to decide whether a number should be represented between 1 and 10 or between 10 and 100! They're just the same, except for an order of magnitude.

As an indication of its sturdy case, my slide rule has survived multiple moves and currently sits on my desk at work among a collection of other toys. There's slight damage to the slide (the clear plastic piece with the fine line in it for lining up numbers - though I called it the cursor, not the slide) has some damage on one corner from either dropping it or being incautious when putting the rule back in its case. It hasn't affected the rule significantly but is annoying! Every once in a while someone will ask what it is and it is fun to watch their eyes widen as I explain how calculations were performed.

I use it occasionally but have become lazy and the act of reaching past the computer monitors, getting it out of its case, and then performing the calculation is typically slower than a little mental arithmetic or, imagine me hanging my head in shame, bring up the calculator app on my computer.

I hereby resolve that, when I get back to my office, the slide rule will come to the desk near me, and sit, outside its case, waiting for me to use it!

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