Lately I’ve been enjoying my new slide rule. It’s a circular slide rule, my first, and yes, it’s new. Concise still makes and sells different models of circular slide rules in Japan, and the instructions for this one are in Japanese. I found an English version online, but using it is practically the same as a traditional slide rule. Making a slide rule circular allows it to fit a smaller space. This one is 11 cm (about 4 5/16”) in diameter, which means the outer scale is about 34.5 cm (13 9/16”) long. Of course, the inner scales are shorter, but it’s still a remarkable compromise.

Slide rules are about compromise. The average slide rule has a accuracy of about three digits, but date from the era when exacting computations required books of logarithm tables. Using logarithms requires looking up numbers in a table to find the logarithm, preforming the calculations, then looking up the result in the table and converting it back. While more accurate, it also takes longer than using a slide rule. For quick calculations where great accuracy wasn’t needed, slide rules were a better choice.

Chances are you may have never held a slide rule. You may not even know what they are. They didn’t in a bookstore some thirty odd years ago, where they were marked “architect scales.” Slide rules are, well, rulers, with a center piece that slides through the middle and a cursor that slides over the entire thing to line up marks and to indicate results. Instead of being marked off in units of measure, slide rules are marked off according to the logarithm of the number shown on the ruler. In essence, slide rules are logarithmic tables on a stick, limited in accuracy by the readability of the markings.

I came along right at the end of the slide rule era. My first was a Pickett slide rule found at a drugstore, purchased to help with multiplication before I learned of trigonometric functions or the concept of logarithms. I would end up purchasing another before handheld scientific calculators became affordable. Even after handheld scientific calculators became common, I purchased a pocket-sized slide rule because it didn’t require batteries. Solar-powered scientific calculators made that obsolete, though you could argue it was obsolete when I bought it.

Affordable handheld scientific calculators were such an improvement over not only slide rules but logarithm tables that no one regretted exchanging slide rules for one. Handheld scientific calculators are faster, more accurate, keep up with the decimal point, and add and subtract. I did feel a bit of shock when a requiem for slide rules appeared in a magazine, with the realization that slide rules had become antiques in a short span of time. While I owned and knew how to use slide rules, an engineer just a year behind me in school didn’t. The change from slide rules to handheld scientific calculators happened that quickly.

Perhaps because there was such a rapid change, we had an odd mixture of technologies during those years. On the wall of one college classroom was a huge working slide rule that was likely used to demonstrate their use. On the work benches on the other side were desktop calculators with nixie tubes instead of LEDs. In the back of our textbooks were logarithmic tables. And by then we all had handheld scientific calculators. The technology in that room probably spanned less than a decade.

I admit to a fascination with slide rules. Years ago, I purchased a new old stock slide rule simply because they are relics of another time. I bought my new one for the same reason. And yes, I use them on occasion, to keep from forgetting how and because there’s a joy in it. I suspect that joy is mostly from the novelty; I won’t abandon my scientific calculators for it. Yet there’s also a feeling of nostalgia that likely wouldn’t exist if I’d never used them.

I have a similar feeling using my Reverse Polish Notation scientific calculator. With these, you enter the numbers in the calculator and then press the button to perform the operation. With RPN, you can perform complex calculations without the use of parenthesis. RPN solved some early issues technical issues with handheld calculators. When Hewlett Packard introduced the first handheld scientific calculator, the HP 35, it used RPN, and when handheld scientific calculators became affordable, some other companies introduced RPN based scientific calculators as well. My first handheld scientific calculator, not a Hewlett Packard, used RPN. RPN calculators were quickly lost in the wave of algebraic calculators. When I had to replace that calculator, the most affordable option was algebraic, as has been for practically every calculator I’ve owned since. Yet I still like to use RPN.

Alas, like slide rules, RPN scientific calculators seem to have become relics of the past. Hewlett Packard, one of the last bastions of RPN calculators, seems to have discontinued RPN scientific calculators with the end of the HP 35S. They still sell scientific calculators, but they seem to be algebraic.

It’s ironic: RPN handheld scientific calculators began the demise of the slide rule, but the Concise circular slide rules may have outlived them all.

**ADDENDUM**

It turns out that at least one company, Swiss Micros, continues to make RPN scientific calculators. Unfortunately, they are rather expensive, too expensive for me at this point in my career. Still, it means that RPN scientific calculators, like circular slide rules, are still chugging along.

There are also RPN scientific calculator emulators for computers and smart phones. Yes, I do have a couple for desktop computers, but find them as inconvenient as the Windows scientific calculator program. I tend to reach for the scientific calculator by my monitor at work or at home or use the one in my shirt pocket before I pull up the one on the desktop computer. That’s merely personal preference. That said, I’d be careful about the source of a scientific calculator app before I installed it.