Slide rules

[Johnboy]

I'm aware that some of the older forum members look at these threads to offer advice to beginners, so I felt this might be a good place to begin a discussion of this topic. Dave's recent mailbag video addressed a hybrid analog/digital calculator, made by the German company Faber-Castell in the 1970's. Those with an interest in following this thread might take a look at that video first.

The slide rule, a mechanical parallel computer, antedated the common use of the handheld electric calculator. Before the advent of the calculator, slide rules were used as a check on mental/pencil/paper calculations.

In my self-study of electronics, I started as many have, using the most recent available technology. Modern computer simulations of circuits and inexpensive chipsets in multimeters, as just a couple of examples, have mainly replaced the older tools used by previous generations of engineers to develop their understanding of circuit theory and operation.

After a lot of self searching about how I wanted to master the basics, I created a second "caveman" lab, mainly using older, used, analog equipment and supporting texts to try to recreate the electronics learning experience of the early 1970's in the United States. My main tools are an analog VOM, a CRO, a slide rule, compass, protractor, triangles, and prodigious amounts of graph paper to document my understanding as I go.

One of the most interesting aspects of the "old way" of learning the basics was the emphasis on estimation of mathematical solutions to real world problems. Before digital technology was everywhere, engineers often "overbuilt" their bridges, buildings and equipment as a result of having to make quick design decisions using approximations of formulae without the precision and accuracy that modern computing has made ubiquitous now. Erring on the side of caution produced thicker walls, safer distances, and larger workable tolerances. The relatively "hamfisted" approach also produced some gargantuan design mistakes, of course, but I'm not mainly interested in the discussion of the obvious, glaring limitations of these tools on their operators.

Cleveland Institute of Electronics (among many others) offered electronics courses designed around the use of the slide rule as a computing instrument. As the digital versions of calculators, multimeters, and even oscilloscopes replaced their analog counterparts in the workplace lab and field, the older tools were not-so-gradually phased out of the educational environment as well.

My questions to those of you with the background, education, and/or work experience in the use of the slide rule (worldwide, not just in the US) are these:

1. Which slide rule (manufacturer, model) did/do you use, and why? Did you select it yourself based on application, or was it chosen/required by the school, academy, or military branch?
2. Which texts did you use in concert with this tool during your education?
3. What things do you still occasionally use them for, if at all? If you use them regularly, is it merely out of habit or are there certain calculations for which you prefer them to having to punch buttons on a digital calculator?
4. What benefits do you feel you may have gained from learning how to use them?

Any and all replies would be greatly appreciated, as I'm trying to keep detailed notes on my own progress and would like to hear how others may have fared in similar, although less-self-imposed, surroundings.

[IanB]

Before the advent of the calculator, slide rules were used as a check on mental/pencil/paper calculations.

This is not quite correct. Before digital calculators were available, slide rules were used to do actual calculations, not just to check results. They were the equivalent of the modern pocket calculator. However, since digital calculators were superior in every way they rendered slide rules totally obsolete. Slide rules today are only a curiosity and have no practical use.

Interestingly, digital pocket calculators have since been rendered almost totally obsolete by personal computers. The software available for personal computers is far faster and more capable than anything calculators can do.

the older tools were not-so-gradually phased out of the educational environment

The "not so gradual" bit is rather true. Pocket calculators appeared when I was in high school. They wiped out slide rules overnight. One year slide rules were taught and used, the next year they had vanished.

[rstofer]

Well, shoot, I made it through undergrad with a slide rule.  Probably K&E Log-Log-DeciTrig.  I have a different 10" I got from eBay but I still have my original 6" K&E.  FWIW, the HP35 came out in my junior year of undergrad.  No way could I afford it.

We went to the Moon using slide rules.  Yes, there were many computers involved but EVERYBODY had a pocket slide rule.  When you absolutely, positively, needed an answer right now, you certainly didn't have time to write a Fortran program and submit a batch job.

Here's what I learned:  I automatically change to E notation with 1 digit to the left of the decimal point.  No problem multiplying such numbers in my head but the slide rule brings another digit or two.  But the real trick is keeping track of the powers (or decimal point) in your head.  I still do that today!  RC circuit 10k resistor, 100 nF capacitor:  1.0E+4 * 1.0E-7 so the result is 1.0E(-7+4) or 1.0E-3 so Tau = 1.0 ms.  Useful...

I provided a brief introduction to slide rules to my grandson, the BSME student.  I don't imagine he will use it for any course work but he did get some laughs out of it.

If calculators are obsolete, how come I have an HP41 emulation on my laptop/tablet(s) and an HP48GX on my desk.  The GX is about as far from my keyboard as is my mouse.  It's just over on the left and I use it constantly!

And in no way are any of these calculators the equivalent of Octave or MATLAB.

We've come a long way since I graduated in '73.

[MarkF]

None of that back-and-forth crap.
Round and round we go.

We got these in high school chemistry class, 1970s (teacher choose them).

[HighVoltage]

I grew up in Germany and went to the University in the 80s and everyone was using calculators and of course me as well. But I had always mby mini pocket slide rule with me, because it was much faster than any calculator, to get a good idea of the results.

I am still using it these days from time to time, just for fun.

Faber-Castell was most likely Germans best known brand for slide rules but they were large and they did not have a mini one at the time, when I was looking. So, a friend introduced me to the very well made USA slide rule in the pictures below.

[ArthurDent]

Johnboy - "3. What things do you still occasionally use them for, if at all?"

My Pickett slide rule still makes one hell of a back scratcher.

[Gyro]

I remember when we were allowed to take slide rules into exams but not calculators. I didn't bother - I was so inept at using the things, it wouldn't have done me any favours! 


P.S. I bought my maths graduating nephew an early '30s Japanese Sun Hemi Model 1 slide rule as a gift a while back. They had the unique feature of using the dimensional stability of Bamboo ply (what else). It had a really smooth action and zero play, even after all these years. A thing of mathematical beauty.  https://www.sliderulemuseum.com/Hemmi.htm

[rstofer]

There should be a "National Slide Rule Week" where participants agree to leave their computers and calculators dark and use only slide rules.  It would be fun!  If it ever becomes real, I'm all in!

Of course, I would need to learn how to use most of the scales all over again.  I remember C & D but I'd have to think about the others.

[Gregg]

From my experience, a slide rule was a solid representation of how the rules of math related to one another.  The emphasis on math estimation and the actual significant numbers necessary was a true revelation of real world uses for math. The linear layout of logarithms helps give a mental image of how logs work. The fact that a slide rule user has to mentally keep track of the decimal point placement certainly helped with learning how a calculation works.  An early revelation was a high school physics teacher that stated “You don’t have to memorize things verbatim if you know how to derive them”; a lesson I have found useful my entire 70+ year life.

1.  Kuffel & Esser Deci-lon 5 was and still is my favorite slide rule.  It has expanded log scales if more significate numbers are needed.  The sliding scale folded at pi is very handy.  I also have a 10 inch version, but found it unnecessarily bulky for the little bit better resolution; besides only the truly nerdy carried around 10 inch or longer slide rules.  The 10 inch did come with a nice hardbound instruction book, however.
There were many manufacturers of slide rules in the 60’s.  Post made a bamboo rule that many students bought on recommendations of instructors.  I don’t know if humidity changes affected the accuracy, but they certainly affected the sliding smoothness.  Pickett made aluminum ones that tended to gall on the sliding joints. I preferred the slide action and adjustability of the K&E.
2.  I don’t remember the text books at all.  I was financially strapped as a student and tried to share texts, buy used and sell at the end of the semester. 
3.  I confess, I really only use my slide rule(s) to keep a little bit in practice and to show them off to younger people.  It helps to know how to use one if you really want to impress someone.
4.  I feel I gained a better understanding of math from slide rules as stated above.

It is like the difference between an analog volt meter and a high impedance digital volt meter.  The digital will tell you that there is an AC voltage on a wire that runs parallel with powered wires when the analog will read zero; the voltage is there but it isn’t significant.

[basinstreetdesign]

I have had mine handy ever since I bought it in 1970 even though I never use it anymore.  It got me through the first two years at Waterloo.  I could be seen legging it across the campus with it swinging from my belt in its scabbard and build-in belt loop.  It was one certain way to tell an engineering student from all of the others.  It is a Dietzgen Cat # B-1734 and although it sounds German it has "MADE IN JAPAN" stamped on it.
Even it still works perfectly and even though I regard it with fondness I always rely on one silicon-based calculating instrument or the other to do the actual work.  I admit that I have gotten lazy about keeping track of the decimal point.
I have mentioned it a couple of times in one Reddit forum or the other to those just discovering electronics and it has never been met with insults but always with admiration.

[don.r]

My father worked for Keuffel and Esser in Montreal and gave me my first slide rule when I was 10. My math skill wasn't up to scratch to use it properly until I was about 15 and by then (1979) calculators started to become mainstream.

[CatalinaWOW]

1.  My favorite slide rule was my K&E log log dupli trig rule.  I had won a cheap plastic slide rule in middle school, chose this one among the styles available at the college bookstore my freshman year.  Reasons for selection.  Smooth action and wide range of functions.

2.  The slide rule was and remains faster than a pocket calculator for the things it can do.  The HP-35 came out when I was a senior in college and I couldn't afford one.  After a masters degree and entering employment I regularly had races with HP equipped colleagues.  They could only win by going for more significant figures than the data contained.  The speed occurs because digit entry is essentially serial on a calculator, but is largely parallel on the slide rule.

3.  Remember that for all the things that slide rules can do, the one thing they can't do meaningfully is add and subtract (few if any slide rules come with two linear scales.).  This is one of the reasons that calculators took over.   Another is bulk.  The pocket calculator is easier to carry around than a slide rule large enough for engineering precision.  Still another is the mystique.  I can't count the number of times in a group discussion that I ended an argument by holding up a calculator with a number on the screen.  In some cases I had merely typed the number in after doing the rough calculations in my head.

4.  I do agree about how rapidly the slide rule disappeared.  It was a little slow at first because HP's price tags were significant even on an engineers salary, but when TI came out with the TI-59 the game was over within a year.

5.  Slide rules hung on for a while longer in specialty applications.  I was still using my GE Infrared Slide Rule when I bought the programmable HP-65, my third scientific calculator.  But it wasn't long before I had the 65 programmed to do the job of the IR rule, but better.  I understand that other fields, perhaps including aviation navigation, had similar experiences.

[Tom45]

I was in college in the 60's and only used slide rules. Calculators didn't start to appear until several years after I graduated from college.

1. Which slide rule (manufacturer, model) did/do you use, and why? Did you select it yourself based on application, or was it chosen/required by the school, academy, or military branch?

I used K&E slide rules all through engineering school. Started with Log Log DeciTrig and then later the DeciLon came out and I switched to it. My father was a civil engineer and I had his old K&E Log Log Duplex slide rule when I was in high school. So I grew up with K&E and stuck with what I was used to using. I don't remember any guidance from the school on what slide rule to use. But that was over a half century ago, so they might of said something and I just don't remember.

I think K&E slide rules were in the majority at school, but second place was the metal Pickett slide rules.

2. Which texts did you use in concert with this tool during your education?

No texts. Engineering Fundamentals class my freshman year might have covered slide rule usage, but I don't remember. Same excuse, that was over a half century ago. In any case, I was fluent in slide rule usage before entering university. All self taught.


3. What things do you still occasionally use them for, if at all? If you use them regularly, is it merely out of habit or are there certain calculations for which you prefer them to having to punch buttons on a digital calculator?

Since the arrival of eBay I've acquired a considerable collection of slide rules from around the world. Back in the slide rule era, I really only knew about K&E, Pickett, and Post.

I don't use them now other than for the enjoyment of trying out different models from my collection now and then.

Using a slide rule to look at proportions is still hard to beat with any other method.


4. What benefits do you feel you may have gained from learning how to use them?

Before you do a calculation with a slide rule you need to understand the calculation well enough that you can predict the answer to better than an order of magnitude. The  slide rule gives an answer to a few places but the user has to supply the actual placement of the decimal point and exponent. That was a big deal in engineering classes that was drilled into us. We should have a good feeling for the likely result before picking up the slide rule.

Besides knowing about what to expect for the answer, we also were expected to know how many significant digits made sense in the result. A calculator can give a result to 12 places of a calculation that had measurements to 2 or 3 places for the inputs to the calculation.

My senior year in EE I attended a one term seminar course where the class designed a digital calculator. We didn't actually build anything, just did the logic design. It wasn't too many years later that the first pocket calculators arrived.

[vk6zgo]

Before the advent of the calculator, slide rules were used as a check on mental/pencil/paper calculations.

This is not quite correct. Before digital calculators were available, slide rules were used to do actual calculations, not just to check results. They were the equivalent of the modern pocket calculator. However, since digital calculators were superior in every way they rendered slide rules totally obsolete. Slide rules today are only a curiosity and have no practical use.

Interestingly, digital pocket calculators have since been rendered almost totally obsolete by personal computers. The software available for personal computers is far faster and more capable than anything calculators can do.

They are also huge, compared to a pocket calculator, & take time to set up for a quick calculation.
A pocket calculator "is ready to go" & you have the answer whilst your colleague is still finding the calculator "app"!

the older tools were not-so-gradually phased out of the educational environment

The "not so gradual" bit is rather true. Pocket calculators appeared when I was in high school. They wiped out slide rules overnight. One year slide rules were taught and used, the next year they had vanished.

PS, I had a slide rule, but never became competent in its use---I used Log Tables most of the time, instead.

[AG6QR]

I used a slide rule in high school, but basically because I was curious about how they worked and wanted to learn on my own.  I was in high school when the TI-30 scientific calculator was first introduced, and it, along with similar models from other manufacturers, pretty much made slide rules obsolete.  But there was still a huge slide rule, probably 8 feet long, on top of the blackboard in my chemistry/physics classroom.  It wasn't part of our curriculum, but the teacher demoed it once or twice, and that was enough to spark my curiosity and learn how to use one on my own.

In the early 1990s, I took flying lessons, and the most popular navigation computers in common use by pilots were still variations on the E6B circular slide rule.  in fact, you can still buy them. https://www.sportys.com/pilotshop/full-size-e6b-flight-computer.html  Using one of these made navigation and fuel consumption calculations easy.  And there are no batteries to wear out.

For a while, I wore a watch with a miniature version of the E6B circular slide rule around the bezel.  At the time, I was no longer flying, but it was still handy for calculating gas mileage or ETA during car trips, along with any general type of ratio problems.

When you only need two digits of precision, a slide rule is generally quicker to manipulate than a calculator or smartphone.

I have it on good authority that the E6B style circular slide rules will still be in widespread use in the 23rd century.

[WA1ICI]

The HP-45 calculator came out when I was a junior in engineering school. It appeared to be a magical device at the time!  I'm still hooked on RPN.

Before the calculator, I used an aluminum Pickett slide rule.  Since you had to manually figure out the exponent, you had to really think about the calculation, which, although slowing you down, keeps you focussed on what's going on.  With a calculator, it is garbage-in, garbage-out.

No one has mentioned log tables yet.

- John Atwood

[CatalinaWOW]

Log tables and trig tables.  I spent many hours in my early days interpolating five digit log and trig tables and doing the associated arithmetic on a desktop mechanical adding machine.  It was how the calculations that mattered were performed.  Slide rules were for the quick and dirty answers, preliminary designs and the like.  Thinking back on it I probably solved ten times as many problems using a slide rule, but spent ten times as much time on those relatively few occasions when precision was needed. 

Scientific calculators were slower for the quick problems, but not by much, only percentages difference and took the same time for as much precision as anyone could use.  That comment about log tables triggered that memory and is surely the real reason calculators took over so quickly.

[HighVoltage]

3.  Remember that for all the things that slide rules can do, the one thing they can't do meaningfully is add and subtract (few if any slide rules come with two linear scales.). 

For that we used the Sumax Addimult.
One side for addition and the back side for subtraction.
Made out of brass with a brass pen and worked very well before calculators.
The simplest mechanical calculator I own.

[SiliconWizard]

https://en.wikipedia.org/wiki/Pascal%27s_calculator

[ferdieCX]

I learned to use the slide rule when I was 15, and attended the basic level of technical High School. I used an introductory book that was written by the technical University.
One year later, I got this Aristo Studiolog 0969 that I used up to 1982. The obtainable precision was enough for the then usual 5% tolerance components.

[Canis Dirus Leidy]

Before the advent of the calculator, slide rules were used as a check on mental/pencil/paper calculations.

Not really. The slide rule was used for calculations where speed was important, with moderate accuracy requirements (the “standard” 25-cm ruler provides the accuracy of the result up to 3 digits). The "Iron Felix" was much more accurate, but it could only perform four basic arithmetic operations (no trigonometric functions, no logarithms, no arbitrary powers, etc.). As for electronic pocket calculators... The slide rule cost around 4 rubles. For comparison: the simplest (basic arithmetic only, some models have memory and few additional operations like square root and 1/x) calculators cost (in the mid eighties) 20-40 rubles (comparable to a hungry student scholarship), scientific calculators cost 50-70 rubles and price of programmable calculators was 80 and more rubles (almost the entire monthly pay of a freshly graduated engineer at first job).

P.S. Grandfather's slide rules:

I used them for a while (in Nineties), but it was mostly a show off.

[Circlotron]

As well as general purpose slide rules there are probably many specialist ones. Here is one used by the drag race crowd for estimating vehicle performance.

https://www.hotrod.com/articles/speed-rpm-gear-ratio-tire-size-formula/#moroso-analog-slide-rule-style-calculator-detail1

[edy]

I have a rotary slide rule on my Seiko watch... I actually use it sometimes. The bezel rotates. Besides having the normal multiplication capability, there are labels for certain ratios for conversions of units (KM/mile, liters/Gallons, Lb/Kg, etc):



To be honest, I'd love to get my hands on an old slide rule if anyone is planning to dump theirs. I think it is an art and skill that has been lost but still very interesting to learn and keeps your mind sharp, making sure you are keeping track of the order of magnitude/places when you are multiplying or dividing.

[chickenHeadKnob]

To be honest, I'd love to get my hands on an old slide rule if anyone is planning to dump theirs. I think it is an art and skill that has been lost but still very interesting to learn and keeps your mind sharp, making sure you are keeping track of the order of magnitude/places when you are multiplying or dividing.

Hello edy, I may have at least 1 and up to 3 standard slide rules. They would be in boxes stored at my parents house and it will take a few days to check, as I am visiting them tomorrow. I will PM you if I still have them. I think I still have 2 cheap plastic ones from 1976, about 200mm long which have slides that I've waxed so they run slick! They were intended for the edu-market so nothing special but  are very functional.

The story behind those is that in grade 10 and 11 I had a chem teacher who bemoaned the introduction of calculators, saying that students were losing the ability to self check and  recognize when a result was in the ballpark  correct magnitude. So he forced all his students to get slide rules.  In my other science classes and math I could use my first calculator, a TI SR-50,. I loved that thing, it started my interest in computers even though I could see my chem teachers point. He was an excellent instructor with a real degree in science, not education/teachers college. I may also have a third smaller pocket sized wooden core one my father used in his job (electrical engineer) he gifted me around the same time.

[thermistor-guy]

Log and trig tables. There were a few kids in my high school with slide rules, but most of us couldn't afford them. My Dad had one, but in his mind it was for adult use only.

...The story behind those is that in grade 10 and 11 I had a chem teacher who bemoaned the introduction of calculators, saying that students were losing the ability to self check and  recognize when a result was in the ballpark  correct magnitude. So he forced all his students to get slide rules...

Because my EE exams always had time pressure, I used to calculate answers twice: once with a calculator, and once in my head to check that the calculation was reasonable. That would pick up large errors caused by wrong key-presses. I'd estimate the answer, then estimate a correction to the first estimate - sounds involved, but after a while it became routine.

I still do that now when I help my kids with their Math homework. So now I pick up their keying mistakes.