# How did scientists and engineers actually use slide rules, in the old days? **URL:** \ **Category:** Factual Questions\ **Created:** [September 3, 2015, 4:37am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784 "2015-09-03T04:37:48Z")\ **Posts on this page:** 20\ **Page:** 4
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [September 6, 2015, 8:37am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/61 "2015-09-06T08:37:19Z")
> [@eschereal](#): > > Yes, I am quite familiar with that thing (never heard “reticle”, we always just called it the hairline). Curious, we always called it the “cursor”. Neat trick was that it could have a number of lines on it - so could be constructed to provide direct readout of some useful ratios directly. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [September 6, 2015, 10:51am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/62 "2015-09-06T10:51:35Z")
> [@Chronos](#): > > Sometimes to an extreme degree: I’ve seen, for instance, a set of calipers capable of measuring to within 1/50 of a millimeter. This, on a device with only one moving part. Wow. That’s vernier than I’ve ever used, and is, in fact, the verniest I’ve ever heard of. KnowhutImean? ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [September 6, 2015, 11:15am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/63 "2015-09-06T11:15:32Z")
> [@cybersnark](#): > > Sounds like a Otis King Slide Rule > > [https://en.wikipedia.org/wiki/Otis\_King](https://en.wikipedia.org/wiki/Otis_King) From his description, it was larger and mounted on a frame. Possibly a bit like this one [http://osgalleries.org/os/photos/1122.jpg](http://osgalleries.org/os/photos/1122.jpg) although the illustration is of one used in currency transactions. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 6, 2015, 12:47pm UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/64 "2015-09-06T12:47:32Z")
For anybody reading along who can’t really picture the way one uses a slide rule, there’s a common situation in carpentry that might be a useful analogy. When you are using a ruler or tape measure and figuring out a distance, and you have to add a few things together and maybe subtract a few things too, you can imagine taking the first measurement, and then moving the ruler so that the point on it you just found now becomes the starting point for the next measurement. If you are adding a few things, adding up all the layers inside a wall perhaps, you keep jumping along the ruler like this. Maybe you need to subtract a few things, so you jump along leftward instead of rightward for these items. Since so many math problems involve chains of multiplication and division, you can do this on logarithmic scales held next to each other (because log(a\*b) = log(a) + log(b)). If it’s an actual logarithm that you need, you can use a linear scale, as someone pointed out above. For an exponent you can go the other way with the same scales. Then you can add other specialty scales, such as for trigonometry. Working your way up and down scales like this provides a framework that is handy for a huge range of practical problems. The idea that people should have some idea what the answer will be before they let a computer or calculator tell them is important. It is better to estimate the final answer as one works it out. It’s also better to get into habits like doing calculations starting with the most significant digit rather than the least, for this same reason (and also because many calculations give a useful result before you finish by this method). Occasionally I’ll hear a result that is obviously bizarre, though not to the person giving it. I remember one that involved a nozzle blowing compressed air onto a little part, all about the size of a Water-Pik squirting water onto teeth. The air consumption prediction called for thousands of horsepower for the compressor. A compressor of this sort is something you would have built, like a building, whereas what was really needed ought to come in a small box from a store. Without any reasonable estimation going along with the calculation proper, how would a person be able to immediately see this as ridiculous? OK, I admit to a bias for slide rules and logarithms… ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [September 6, 2015, 4:07pm UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/65 "2015-09-06T16:07:15Z")
In pre-calculator days a slide rule was good for fast ballpark multiplication because it gives you the most significant digits first. If you needed to know what 786 x 311 was, a slide rule would quickly tell you that it’s a little less than 245,000 (actual 244,446) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 6, 2015, 4:54pm UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/66 "2015-09-06T16:54:14Z")
Personally, I think that students would be better off if they were still taught to use slide rules. Not because of the usefulness of the tool itself (calculators do the same thing more easily and to greater precision), but because use of a slide rule leaves you intimately familiar with all sorts of things that are useful: The properties of logarithms, proper precision, order-of-magnitude estimates, and so on. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [September 6, 2015, 11:45pm UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/67 "2015-09-06T23:45:18Z")
> [@Chronos](#): > > Personally, I think that students would be better off if they were still taught to use slide rules. Not because of the usefulness of the tool itself (calculators do the same thing more easily and to greater precision), but because use of a slide rule leaves you intimately familiar with all sorts of things that are useful: The properties of logarithms, proper precision, order-of-magnitude estimates, and so on. Old Math! ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [September 7, 2015, 1:21am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/68 "2015-09-07T01:21:33Z")
> [@Senegoid](#): > > Old Math! Properly aged math. Like a fine Glenlivet. ---
**Author:** ![excavating\_for\_a\_mind](https://avatars.discourse-cdn.com/v4/letter/e/e8c25b/32.png) [@excavating\_for\_a\_mind](https://boards.straightdope.com/u/excavating_for_a_mind)\ **Post date:** [September 7, 2015, 1:37am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/69 "2015-09-07T01:37:12Z")
> [@eschereal](#): > > When using calipers, I will always take a straight Vernier scale model over one of those silly dial calipers, or a digital one that might need the batteries replaced. Mostly because calipers are not for the ultra-ultra precision work (like you use micrometers for), and the Vernier scale is just so damn elegant. Yes! Yes! A thousand times, Yes! I have a 7" Brown & Sharpe vernier caliper that I bought in a pawn shop for around $10 in '81, a 6" Mitutoyo (which, oddly enough is the same size as the 7" B&S) digital caliper that I bought back in the '90s when they cost $150-$200 each) and a 4" Pittsburgh digital caliper that I bought a couple of years ago at Harbor Freight for \<$10. The B&S is my Go-To caliper that lives on my workbench. The HF special is in the “house” tool chest, again with the battery removed. It is mainly for my wife to use. The Mitutoyo lives in my tool chest, stored without a battery in it, along with a new battery. The Mitutoyo is rarely used because it cost so much, although the B&S is probably worth more. The vernier is more than adequate for nearly any use and I can read it to within 0.005" with just a glance. The digital ones take longer to use if only because I have to take the time to put the battery in (and remove it once I am finished) and, while they will read to 0.0005", I would have a hard time trusting them to more than 0.005" without direct calibration in the range of interest. ---
**Author:** ![Ignatz](https://avatars.discourse-cdn.com/v4/letter/i/3be4f8/32.png) [@Ignatz](https://boards.straightdope.com/u/Ignatz)\ **Post date:** [September 7, 2015, 1:56am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/70 "2015-09-07T01:56:25Z")
> [@Amateur\_Barbarian](#): > > K+E (Keuffel and Esser) is pretty much considered the gold standard of slipsticks. > > I have a complete set of Log-Log Duplex Decitrigs that I.prize. Even use occasionally… That’s my latest model, too, from 1960. On the first week of classes, an itinerant engraver was allowed to set up shop in a corridor at W.I.T. and for, I think, $1.00, engrave the owner’s name into the face of the slide rule, and highlight it with red ink. We wore the thing in a hard leather sheath hanging from our belts. Nerd time. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [September 7, 2015, 3:17am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/71 "2015-09-07T03:17:20Z")
> [@CalMeacham](#): > > What it didn’t let you do was add or subtract. Why not? All that takes is a linear scale on the base and the slide. You’d think there’d be room for that. > [@](#): > > The venerable HP-35, which did logarithms and trig operations, cost $395. Are you sure? My recollection was the hp35 was 5-function. I remember a high school friend who had one as a freshman in 1971, way cool – but within a year, 5-function calcs were $80, and in another year, that TI came out, with lots of functions for under $100. > [@](#): > > I didn’t buy a calculator until the HP-25 came out. That was my 2nd. First was an off brand 5-function, needed for high school. When I started college in 1975 I needed something more, and picked the hp25c, after spending hours at the shop learning to program the hp65 (outside my budget!) The 25 was a great little tool. I later got a 34 iirc, and later still my brother gave me his 45 but I only fooled with it; by that time I’d use a computer. Yeah, I’m an RPN guy. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [September 7, 2015, 3:24am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/72 "2015-09-07T03:24:57Z")
> [@Learjeff](#): > > Are you sure? My recollection was the hp35 was 5-function. I should have checked my facts first. I do remember that 5-function hp, but can’t find what model it was. And it was expensive, like $300, about $1200 in today’s money. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [September 7, 2015, 3:36am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/73 "2015-09-07T03:36:04Z")
> [@Learjeff](#): > > I should have checked my facts first. I do remember that 5-function hp, but can’t find what model it was. And it was expensive, like $300, about $1200 in today’s money. Arrgh. Fail again. It had to have been the 35, must have been in 72, and had trig etc. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [September 7, 2015, 4:03am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/74 "2015-09-07T04:03:13Z")
Oh, the days when HP was a premium brand. TIs seemed like cheap junk compared to the solid trucks that HPs were. And they must have been better because the muggles had a hard time understanding them. ---
**Author:** ![Gunslinger](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@Gunslinger](https://boards.straightdope.com/u/Gunslinger)\ **Post date:** [September 7, 2015, 4:22am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/75 "2015-09-07T04:22:29Z")
> [@Learjeff](#): > > Why not? All that takes is a linear scale on the base and the slide. You’d think there’d be room for that. Because slide rule users were taught to add and subtract without assistance. We kids have it easy. 😛 ---
**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\ **Post date:** [September 7, 2015, 4:37am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/76 "2015-09-07T04:37:03Z")
Interesting fact: the Eugene Dietzgen Company, maker of engineering products including slide rules, once owned a building just west of the Fullerton Green Line “L” stop in Chicago. You can see the old nameplate on the building here: [Google Maps](https://www.google.com/maps/@41.9253763,-87.6534304,3a,75y,27.9h,91.38t/data=!3m6!1e1!3m4!1sJfRQp9VMMmHiFIi8z0JspQ!2e0!7i13312!8i6656) [http://www.chicagomag.com/Chicago-Magazine/The-312/April-2012/A-Chicago-Car-Elevator-Mystery-and-the-Man-Who-Invented-Dialectical-Materialism/](http://www.chicagomag.com/Chicago-Magazine/The-312/April-2012/A-Chicago-Car-Elevator-Mystery-and-the-Man-Who-Invented-Dialectical-Materialism/) > [@](#): > > The Eugene Dietzgen Company started in 1885 as an engineering supply house, and eight years later moved into manufacturing engineering and drafting supplies. Their second factory is a historical landmark, the Dietzgen Building, on the DePaul campus at 990 W. Fullerton. > > \ > > Eugene was the son of Joseph Dietzgen, a German tanner-turned-Communist who read Marx and joined the Revolution of 1848, which got him exiled from Germany. So he moved to the U.S. and traveled the antebellum south, writing about slavery and lynching. He returned to Europe, building tanneries for the Czar, writing philosophy, and corresponding with Karl Marx, who called him “the philosopher of socialism”; Joseph Dietzgen is credited with coining the phrase “dialectical materialism” in 1887, though the idea itself was sort of floating around in the socialist ether of the times… ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [September 7, 2015, 11:22am UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/77 "2015-09-07T11:22:29Z")
You can, of course, do the addition in your head if you have the log scales memorised – which was a requirment for my Dad’s NY state professional engineering licence. Limited accuracy, and he’s not around for me to ask him. I think he had to memorise the sine table at 5 degree intervals. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [September 7, 2015, 1:40pm UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/78 "2015-09-07T13:40:12Z")
> [@CalMeacham](#): > > What it didn’t let you do was add or subtract (although there were mechanical devices for that, if you wanted, including the abacus and various ingenious yet simple devices). > [@Learjeff](#): > > Why not? All that takes is a linear scale on the base and the slide. You’d think there’d be room for that. Usually when you’re adding numbers you want either the exact answer or at least more than three-digit accuracy. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 7, 2015, 2:25pm UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/79 "2015-09-07T14:25:54Z")
Have I ever mentioned my last grad school advisor, who wrote all of her Fortran programming for slide rules? As soon as she got any variable in her programs, if it wasn’t already, she’d take its log, and then do all of her multiplications and divisions by adding and subtracting. The only time she ever used the \* operator was for exponentiation, and she’d only ever work in linear space when she was doing additions or subtractions. Which made it a real joy to maintain the code, and to try to figure out whether every variable was logarithmic or linear. And I’m pretty sure that it wasn’t even consistent. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [September 7, 2015, 2:45pm UTC](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784/80 "2015-09-07T14:45:25Z")
> [@Chronos](#): > > Have I ever mentioned my last grad school advisor, who wrote all of her Fortran programming for slide rules? As soon as she got any variable in her programs, if it wasn’t already, she’d take its log, and then do all of her multiplications and divisions by adding and subtracting. The only time she ever used the \* operator was for exponentiation, and she’d only ever work in linear space when she was doing additions or subtractions. I can see a method to the madness if she was writing code that would run on a very old machine that only supported fixed point. Machines that were intended for business applications, and not scientific work, that were built back in the 50’s and 60’s. Such a tactic would have maintained precision for programs that would otherwise have probably been uselessly unstable. But to keep the habit up past about 1970? Odd. [Previous page](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784.md?page=3) [Next page](https://boards.straightdope.com/t/how-did-scientists-and-engineers-actually-use-slide-rules-in-the-old-days/729784.md?page=5)