Slide Rules Are Still Amazing

The slide rule is extremely useful when the data itself is lousy. For example, it is tough to get an analog meter reading with more than 3 significant digits. Data from sources like that is ideal for slide rule use, and generally a waste of computer time (unless the data acquisition itself is automated). It is also important to know when not to trust overly-precise readings from inherently sloppy sensors, such as common thermocouples: Go with the first three significant digits and consider yourself lucky. Knowing your data provenance is key to knowing how best to process it. I was in high school when the TI SR-40 appeared. I was able to complete exams and check my answers before the calculator folks finished their first pass. A slide rule in trained hands is surprisingly fast and powerful. The key is knowing when and how to renormalize to avoid the inherent weaknesses of a slide rule, to preserve as many significant digits as possible. Also key is minimizing the number of steps in the calculation, even if it means using what may first appear to be more difficult math: If the slide rule has a scale for it, it's simple math. This in turn encourages expertise with doing symbolic calculation before numeric: Spending a moment to simplify or rearrange an equation can greatly improve the final result. Even if you use a computer. The most error-prone part of slide rule use is keeping track of magnitudes, the powers of 10. It is seldom practical to stay in engineering units where exponents leap by 3, which means careful dimensional analysis is needed at the end of any calculation of significant length. Most physical constants are given in units that are never present on a slide rule. The key in such cases is to first calculate/estimate (and sanity-check) the expected decade magnitude of the result, then calculate the significant digits. I still have my trusty 19-scale K&E (10 on the front, 9 on the back). I literally wore the cursor out multiple times, or simply broke it. None of my other 6 linear slide rules holds a candle to it. And certainly not any of my dozen or so circular slide rules, which mainly were for special purposes, and most often were promotional giveaways. Good table use greatly extends slide rule usefulness, as tables often provide scale change opportunities that allow slide rule precision to be retained. Some slide rules even came with long skinny paper tables, either in a pocket of the slide rule case, or in with the slide rule itself. Some specialized tables require slide rule use for non-linear interpolation, but when done correctly still manage to preserve accuracy. I once saw a German slide rule that included a Fresnel lens to read the miniscule fonts used on their dense tables. Alas, I don't use my slide rules much any more. My calipers are digital (with a serial link), and my GW121 multimeter reports its data over Bluetooth to my phone or PC, where a quick Python script makes short work of any calculations needed. I think my last real slide rule use was about 6 months ago, to calculate the impedance of an antenna. And that was only because I first learned that calculation on a slide rule nearly 50 years ago for my first FCC amateur radio exam, and my fingers still remember it. Everything else is either done on my phone's calculator or with a quick Python script.

BobC

Slide Rules: 1. Don't go down head-first. 2. Don't go down on your belly. 3. Don't jump from the top to half-way down.

BobC

OMG! I love my slide rules. I'm nuts over circular rules. I have my small Motorola one at work, a 6 inch rule at home and then my 10 rule with 5 digits of accuracy. My favorite use is when I'm knitting and need to be doing lots of scaling for different pieces.

Bert Nielson

Was given a slide rule in high school by my siblings for a birthday in the early 80s. I had a math teacher who was very excited when I asked him how to use it. I later made several specialty use cardboard slide charts that calculated A/D resolution and step size while I was in school. One of my instructors asked me to make one for him as well. Fun stuff.

Never had a slide rule, but use callipers sometimes, and always liked those conversion thingies, especially the round ones with several discs :)

Used a slide rule all the time when I was studying engineering in the 60's and 70's

Lawrence ORourke

Not half! Analogue Calculating instruments are the bomb. The batteries never go flat!

veritanuda

There’s a whole lot of analog calculation / scaling tools that tend to be forgotten these days, but are surprisingly effective on the right kinds of problems. I guess in another generation or so they’ll be pretty much completely forgotten. Thanks for the demo video. Ewen

Ewen McNeill

A well-calculated pun. But I'll let it slide.

Jac Goudsmit

They rule. #trytheveal

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