How Slide Rules Work

I used an HP-41CV for many years. I needed the financial calcs module which I used in place of the dedicated HP financial calculator in grad school. Eventually gav out on me but was a good calculator for a long time.

I did keep a slide rule as a backup for exams in college when calculators were still LED but never really used one after a couple of years in high school.

When using a slide rule, keeping track of the number of digits to the left of the decimal point (DLDP) in the result is fairly simple if you know the basic rule:

For multiplication, the DLDP in the result is:

- the sum of the DLDPs of the multiplicands MINUS 1 if the multiplication is done with the slide sticking out to the right of the ruler's body (for example 2.0 x 3.0 = 6.0).

- the sum of the DLDPs of the multiplicands if the multiplication is done with the slide sticking out to the left of the ruler's body (for example 5.0 x 4.0 = 20.0).

There's a similar rule for division, but that's left as an exercise for the student.

> You have to keep a parallel exponent calculation in your head, and that slows you down

We were taught to estimate and use the rule to refine. I date back to the early electronic calculator era and we still had textbooks referencing slide rules etc.

"I want a dropping resistor for a plain old 1980s LED in a car" (back in ye old red LED 20 mA days) "Well experience indicates that will be far more than 500 ohms and somewhat less than 1K and IRL you're probably going to install a 680 and call it good" If you want an actual calculation for engineering purposes you calculate the ideal value under worst case conditions as about 585-ish ohms or whatever using the slide rule, purchasing LOLs at the idea of buying 0.1% precision resistors for mere LEDs, installs cheap 680 ohms and ships it. Maybe 680s if you want it bright to see in daylight or 820 if you want better odds to survive an alternator field winding dump or open battery (about the same thing). You can at least use the slide rule to verify everyone rounded in the "safer" direction to handle the worst case scenario.

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.

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.

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.

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.

The all plastic ones are intrinsically safe in a potentially flammable environment, they don't require power, they're an education in how most engineering materials in the real world have surprisingly wide tolerances so they are far more than accurate enough for most work, for people who learn graphically/visually they are the logical next educational step after counting on fingers.

They're pretty useful for teaching amateur people how to implement algorithms. Multiple ways to solve problems, some easier than others, some more efficient than others, with immediate rewards of faster higher accuracy.

There were also all manner of specialty "slide rule" calculators of various kinds for special purposes. I used to have a bunch of them especially from the oil business. Don't know if I still have any at this point.