Comment by clickety_clack
3 months ago
Slide rules are super cool. Such an easy gift to give the engineer in your life.
I never spent the time to get quick with it, but I could absolutely see it being quicker than a calculator. You’d just have to be aware of the limits to its precision if you were in a field that required it.
Quicker than an algebraic calculator, maybe, but very few people could get. faster with a slide rule than an ergonomic RPN calculator. like the HP 41 series. And I say that as an enthusiastic and experienced slide rule user, before I switched to a calculator.
One problem with a slide rule is that it only performs operations on normalized mantissas. You have to keep a parallel exponent calculation in your head, and that slows you down. Also, maintaining best precision slows you down.
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.
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.
The financial people I know all own 12Cs and they've been in continuous production since '81 although the innards are just a very boring ARM processor now.
They do what people want, the keyboard feel is infinitely smoother than tapping on a phone, etc.
1 reply →
You still have to be aware of the difference between precision and accuracy, and how to propagate precision through calculations to maintain accuracy. It's a forgotten skill that lets us now create data out of whole cloth and call it actionable information but back when slide rules and log tables ruled the day the difference was stressed over and over in math and science classes and you would fail an assignment or a test question if you had the wrong precision in a result.
We have speed electronic calculators now instead of slide rules, but they give a wronger answer and people aren't even aware of it or know why.