How Slide Rules Work - Hacker News

Posted by ColinWright 11/19/2025

203 points | 60 commentspage 2

fjfaase 11/20/2025|

I have one at home, which is the one we had to buy to use in highschool. In the math classrooms we had a 6 feet version that could be mounted on the blackboard such that the teacher could used for instruction. See for a picture on the Dutch page https://rekenlat.barneveld.com/rekenliniaal.htm

watersb 11/20/2025||

These TMSLs were also reasonably common in classrooms in the United States in the 1950s.

Two Meter Slide Rule

pinkmuffinere 11/20/2025||

Wow that's fascinating! Crazy how things change, no wonder my grandma thinks math is black magic!

gerdesj 11/20/2025||

It is worth keeping one around.

When the "cloud" is raining and your laptop and phone batteries are drained and you suddenly need to navigate your 4823 times table - its got you covered.

You will also need to work out how to write with a pen or pencil on paper or try and fix up your atrophied ability to remember arbitrary "facts" short term.

margalabargala 11/20/2025|

Honestly the scenarios where this becomes likely are dwindling with the advent of solar and batteries. Offline knowledgebases and the ability to use them long term are getting increasingly stable, and the likely low point in a societal collapse is probably getting high enough that a slide rule would not be necessary.

I have a Casio fx-991ES calculator, and twenty years later I have yet to need to replace the button cell in it thanks to the tiny solar cell.

eggy 11/20/2025||

And when the EMP washes over your home/office it will most likely be off and most likely survive. If you are doing your monthly finances at the time and it is on, it will be destroyed. The slide rule rules!!! I keep a pocket Pickett for fun...the window has a small crack, and it is missing a very tiny screw (1 of 8). I brought it to my engineering company one day and showed the 20-30-something group how it worked. I then did a full page of calcs they did in Excel, and even with the limits of visual resolution came within an acceptable percentage of their calc.

margalabargala 11/20/2025||

That's not quite how EMPs work. The wire traces act as antennas, and long wires like power transmission lines will have huge power surges, and small devices like calculators will have basically none. The miniscule increase in length of conductive material if the battery happens to be conducting at the moment won't impact the amount of current induced.

Solar EMPs won't be powerful enough to impact electronics. A nuclear EMP can impact electronics, but only over a small geographic area; close enough that if you are in the electronics-frying radius of a nuclear weapon explosion, you either have much larger problems to worry about, or nothing at all to worry about ever again.

Here's info from Los Alamos Lab on it: https://www.lanl.gov/media/publications/national-security-sc...

eggy 11/25/2025||

Thanks for the information and link. I was, however, being facetious and making puns (slide rules!)

entaloneralie 11/20/2025||

I have a little collection of them. I keep the small Hemmi bamboo on the navigation table at all time.

https://wiki.xxiivv.com/site/slide_rule

gcr 11/20/2025||

Last year I made my own rotary slide rule for playing Balatro, the poker roguelike!

My version has a couple interesting properties compared to ordinary linear slide rules:

1. It has three octaves, so it can scale from 1 to 1k or from 1k to 1m, or from 1m to 1b. This is great for calculating point values

2. It's rotary

3. It can be easily 3d-printed!

Source code and .STL files here: https://www.printables.com/model/1026662-jimbos-rotary-slide...

vjvjvjvjghv 11/20/2025||

When I was in 5th grade, calculators had become pretty affordable and there was this huge discussion whether kids should learn slide rules or use calculators. At the time it was decided we should learn slide rules and I am happy about this because it gives you some level of number intuition.

It reminds me a little of AI now. The question of whether students should use AI will probably soon go away and everybody will use AI. Not sure what the results will be.

ghaff 11/20/2025|

Calculators went from something that was really expensive to something that was relatively affordable over a very short period of time. In about 1974, a 5-function TI was about $100 (in 1975 currency). By the next year a full scientific TI calculator wasn't much more and no one, at least in engineering school, was still using a slide rule though I brought one to exams as a a backup. By a couple years later, I was able to buy a discontinued HP calculator for about the same.

jamesgill 11/20/2025||

Last week I donated several slide rules to Goodwill; a few were very nice. Meanwhile, I still have a pristine HP-41cx and HP-15c, and an HP-25 app on my iPhone.

incanus77 11/20/2025|

I have an HP-15c as well as a 16c and I've been using the latter on a daily basis while writing a byte-level network protocol client. I'm getting faster by the day and on the verge of writing some programs for shortcuts on the calculator. I still use the excellent PCalc as well, but seem to be faster on the physical calculator, which is kind of surprising.

ghaff 11/20/2025||

When I was writing a shareware program in x86 assembler and doing a fair amount of bit twiddling, the 16C was really handy. Presumably I still have it around somewhere.

dboreham 11/20/2025||

I came up on the cusp of the calculator take-over, so although slide rules were around, and we were taught how to use them in school, I never really used them for any practical purpose. However, since they were the buggy whips of that day, you could pick up nice fancy ones for cheap in stationary stores (also now gone). So somewhere I have a good collection of them.

watersb 11/20/2025||

A sort of non-logarithmic slide rule, the E6B Flight Computer, was still in use when I was a student pilot 20 years ago. I still carry one: they don't require electricity (although using one in the dark requires a light source).

https://en.wikipedia.org/wiki/E6B

KK7NIL 11/20/2025|

The main part of the E6B is a very standard logarithmic slide rule. Having it loop unto itself is a neat trick but very ancient.

But perhaps you were referring to one of the many other parts of the E6B which I am not familiar with.

agumonkey 11/20/2025||

These fascinates me, it's one of the highest simplicity / abstraction level ratio I can think of. A few marks well spaced on two bits of wood, and you get "linearized" multiplication. Nicest ~analog computer I can think of :)

egl2020 11/20/2025|

I still have the wooden 10" Keuffel and Esser that I inherited from my father and that I used in college. These days I use my HP15C unless I want to provoke glee and amusement in my younger colleagues by sporting my Pickett slide rule in my shirt pocket.

wyclif 11/20/2025|

K&E's are classic. What do you think was the most popular Pickett model?

ghaff 11/20/2025|||

I think MIT ended up with the K&E collection. I haven't had a chance to tour the MIT Museum in its new digs so I'm not sure what's on display.

VLM 11/20/2025|||

The microline series, antique stores are full of them. Every high school or lower undergrad boomer had one or a similar clone and they show up in antique stores and on ebay all the time. The 80 and 120 are about the same size and sell for about $20 and I don't bother buying them anymore when I see them. The 80 puts the T scale on top and the 120 more usefully puts it in the slider IIRC so you can chain calculations.

Grad students or undergrad STEM students would have something like a 900 series, I have several, very nice. This is a desk rule it will not fit in a pocket. Something like a 600 series is a short pocket model, anodized aluminum, very nice and desirable.

The microline series was definitely made to a price point and unless you find one in unusually good condition or its your first collector rule I would not bother picking it up. They stick very strongly and the cursor cracks after half a century and they are slippery in the hand and warp more than most rule and I don't think they're easy to read. They were cheap to make and cheap to buy.

Slide rules in the 2020s are an efficient market; something that barely works "the walmart solar calculator of its generation" like a microline series sells for around $20 today, a VERY desirable N600 series sells for like a hundred bucks and I think its a bargain at that price.

If you mean most popular as in most desired today not most sold back in the day, that's probably the 600 series or specialty rules like I have a N-16-ES with the electronics engineering scales. The latter sells for about as much as a working HP48 calculator, which is interesting. If you mean popular as in attractive that is surely the Faber-Castell short 83N series, I think that's a 62/83N. I would like one of those LOL. Unleash 1960s German graphics artists on industrial design and tell them to make the coolest looking slide rule possible under 60s industrial design rules, you get the 83N series, very very cool way to spend $300 or so, its the kind of thing you put in a lighted display case to admire.

wyclif 11/20/2025||

Wow, thanks. This is an incredible deep dive and I obviously came to the right place for that question. This kind of detailed comment is why I still appreciate HN so much...

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