Old School Tools

RoundSlideRuleYes, these are slide rules, a round one and a straight one. Before computers, this was how engineers computed.

And before that, they used logarithms which, as it happens, are the basis for slide rules and, later, some of the parts in analog computers.

Today, of course, those have all been replaced by digital computers.

But sometimes the old stuff is, well, fun. I shoot target pistols one-handed at 50 yards because, well, it’s fun. And every now and then I come across one of these old slide rules in my desk and take it out to fiddle for a couple of minutes.

I won’t bore you with the details of operation other than to say that like the Chinese abacus, a skilled slide rule user can be faster than someone using a calculator. And while the calculator may tell them that 22 divided by 7 is 3.14285714, in most real-world engineer calculations, 3.14 — that’s three significant digits — is good enough. And that precision is well within reach of these old and relatively cheap tools.

I learned about slide rules in the electronics training I received in high school. Down the hall was the Radio and TV class that taught how to replace vacuum tubes and diagnose burned up resistors but in Electronics, we took things to a higher level. We did the math that was the heart of what was happening inside the circuitry. And with the answers from our slide rules, we could calculate what would happen if you did this and that, and then we would run an experiment with the live parts and see if we got the same answer.

When it worked as we had predicted, that was the magic, the power. We knew the secrets of the universe — at least the electronic portions, anyway — and could predict the future.


After high school I would get more education in electronics and electricity in general but, along the way, I would also take a couple of software classes and, Bingo!, I discovered what I liked even better.

Where electronics was fun, software was like sex!

And so, shortly before graduation, I took a job at American Express that led into a position writing software. I’ve previously written about some of that work earlier in this blog.

Eventually, after writing software for 25 years and working for several different companies, I would start teaching software as well as writing it, and I would discover a third professional passion: I love the classroom and helping people learn something new!

My present work includes teaching and writing software, and the kinds of software engineering I teach and do is often very close to the hardware. I still use that electronics background to this very day. And that job takes me on the road a couple of dozen times per year, mostly within the USA but with the occasional international trip.


In case you are curious, the slide rules are set to show 1.1 multiplied by different numbers. That’s why the “1” in the B (lower) scale is lined up with 1.1 in the A scale. The long black cursor (line) is set over 1.1 in the B scale to calculate 1.1 times 1.1 which, if you look closely (click the image to make it bigger) at where it lines up on the A scale, the answer is 1.21.

One of the harder parts of using a slide rule was keeping track of where the decimal point should be and, as you’ve probably noticed from time to time, engineers are sometimes off by 10X. In the “good old days”, this challenge when using slide rules may have been the reason for that error.

If you’re wondering if I still use these ancient calculating instruments the answer would be no. I don’t.

I use a spreadsheet. (Or guess.)

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