## Know How To Use a Slide Rule? 388

Posted
by
kdawson

from the try-your-hand dept.

from the try-your-hand dept.

high_rolla writes

*"How many of you have actually used a slide rule? The slide rule was a simple yet powerful and important tool for engineers and scientists before the days of calculators (let alone PCs). In fact, several people I know still prefer to use them. In the interest of preserving this icon we have created a virtual slide rule for you to play with."*Wikipedia lists seven other online simulations.
## E6-B (Score:1, Informative)

## Stupid virtual rule slide! (Score:2, Informative)

At least, this one is usable:

http://www.antiquark.com/sliderule/sim/n909es/virtual-n909-es.html [antiquark.com]

## Re:At least (Score:3, Informative)

Try this one [sliderulemuseum.com], it's much better and actually correctly laid out

## I have (Score:4, Informative)

I soon got the hang of using it (and it can be quicker than a calculator sometimes), but I knew the general principle from before anyway. The main thing you have to remember is the slide rule only ever gives you the mantissa; you have to work out the exponent yourself. This means you have to do a rough mental calculation. People often put too much trust in calculators. When I was filling in order forms by hand in a previous job, I never used a calculator -- and I never got called out on a wrong total.

## Pilots know how to use slide rules. (Score:5, Informative)

It's preferred over digital devices because they still work when the batteries go flat, they are easy to use with one hand, and some models are actually smaller.

http://en.wikipedia.org/wiki/E6B [wikipedia.org]

## Re:Um No. (Score:4, Informative)

## For dumb Americans: (Score:3, Informative)

## Re:No, and what the hell is the index line? (Score:2, Informative)

http://www.hpmuseum.org/srinst.htm [hpmuseum.org]

## Re:No, and what the hell is the index line? (Score:5, Informative)

First, the term "index" has the old-school meaning of the number "1" and it appears at either end of the C scale. On the left side of C it means 1, and the right side it means 10, but there are no actual decimal points involved (you're on your own for order of magnitude with this device) so they're equivalent at either end. Also, for multiply and divide, you don't need that hairline slider that covers all the bars; that's only useful if you need to align two values on non-adjacent rules. Just slide the center bar (the one that holds scale C) back and forth.

The other important point to note is that you'll see numerals 1-9 between 1 and 2; those are just convenience markers for 1.1 through 1.9. That first (smaller) 2 you see, reading from left to right, is really 1.2 not 2.0.

So to multiply 6x2, we can go either direction, starting at 2 and multiplying by 6, or starting at 6 and multiplying by 2. To start at 2, slide the center part of the bar so that the right-hand "index" (1) of scale C is directly above the 2.0 on scale D. Now to "multiply" you don't do anything; you just read the result, which is found on scale D, directly under the 6 you wanted to multiply. Here you'll see 1.2 is directly under the 6.

Wait, though, we used the right-hand index, which is 10 not 1, so we need to multiply the result by 10. So 1.2 becomes 12 (which is why I said you have to do your own decimal point management). To start at 6 instead, slide the right-hand "index" (1) of scale C directly above the 6 on scale D; your answer will on D again, directly under the 2.0 of slide C. Again, we used the right-hand index of 10, not 1, so we multiply the 1.2 by 10 to get 12.

How did I know to use the right-hand index rather than the left-hand index? Well, if you slide the left-hand index of C all the way to 2.0 on D, you'll notice that the 6 you need to multiply is off the edge of the device--an overflow, if you will--so you must essentially work with 10 rather than 1 and move the decimal at the end.

With this extremely trivial example, you should be able to follow the rest of the terribly-written instructions FTFA for divide (although you can do significantly more with a slide rule than just multiply and divide).

## Re:Mildot Master (Score:3, Informative)

## Re:Of course (Score:3, Informative)

If anyone is interested, here are several links to downloadable ebooks and manuals for using slide rules:

My only experience with using a slide rule was back in the 1960s in an 8th grade math class where we spent two weeks learning to use slide rules. We were just 8th graders, but were able to use a few basic features of something that was normally used mostly by scientists and engineers. Mr. Turner, our math instructor, even wore a small slide rule as a tie clasp. I suspect that the use of slide rules was something that probably was not normally taught to 8th graders.

Later on in Junior College, I once thought about possibly taking a 1 credit slide rule class, but didn't. That was in the days back before pocket calculators. In the College Algebra class our textbook had Log tables, a square root table and various other tables in the appendixes in the back which we used to get answers without a pocket calculator (or a slide rule).

I still have my dad's old Ivory and wood slide rule that he bought back in the 1950s and also a more modern plastic slide rule which I later purchased. I am plan to briefly brush up on how to use them just for the heck of it.

## Re:Of course (Score:4, Informative)

A slide rule enforces estimating a reasonable answer before hand, and encourges arranging computations for economy of calculation. I think there is a big benefit to critical thinking skills in praticing basic computation with a slide rule.

That said, computers have made it possible to do what was formerly impossible due to computational expense. Integrated circuits would not be where they are if you couldn't burn many flops running spice. Cars would weigh more and get less gas mileage without mechanical simulations because they would have to be over-built in order to simplify strength calculations. Pre-computer-simulation camera optics suck when compared to modern computer optimized lens, ditto for antennas.

I once met a guy whose mother was a computer.... that was her job title: "computer". She worked for a university research department, where row upon row of "computers", mostly women, sat in front of mechanical calculators all day long, 40 hours per week, cranking through tablets of computations for various numerical models. Modern electronic computers enable solutions to problems there were too expensive to attack before, and life *is* better as a result.

## Re:Where Can You Get One? (Score:2, Informative)

Grab a Microline 120 or 140 for about US$10.00.

Yes, it's plastic, but it's a damn fine slipstick for a beginner, and there's several "How to use a Slide Rule" books on the Gutenberg site.