I have maybe 100 slide rules in my collection, and each once belonged to a real scientist or engineer that made good use of it. Usually I have no idea who any of these owners were – but for the two slide rules in the photo below, I definitely know. I know, because that owner was me. These slide rules were mine!
Of course, this distinction is nothing like that of Einstein’s slide rule, or Von Braun’s, or Korolyov’s (you can see photos of the latter two in their museum exhibits here). Still – because they were mine, their modest stories deserve a page on my site.
(I am actually delighted that I was in time to use a slide rule as a real tool, not as a museum piece. Or does that make me a museum piece? Hmm…)
The first of these slide rules was an unexpected and very welcome gift that I received a year or two before finishing high school. Here it is:
Back then our Chemistry teacher, Dr. Shirizli, flooded us with homework exercises involving numerical calculations. These we did by using the Kallay-Tuchman tables of logarithms, the book you see under the slide rule in the photo. Perhaps the doctor considered it edifying, or even character-building, but we found it time-consuming and tediously repetitive work.
And then one day my uncle Shaul, who was a physicist, gave me a slide rule, and taught me how to use it. All of a sudden, I could zip through the calculations and leave my classmates in the dust. I had been given a secret weapon in the never-ending war on homework!
The weapon was an Acu-Math No. 400, an American-made entry level slide rule designed for high school students. As you can see in the images below, it was very basic, made of a plastic material with glued-on end braces. The back was left blank – the same company made other models that used both sides and no doubt sold at a premium. But for me it was of great benefit!
Had I been born a few years earlier, the next step would have been inevitable: soon after beginning my university physics studies, I’d upgrade to a “grown-up” slide rule. You can see this concept behind the Dietzgen ad shown here – it targets young engineers at the point in time where they start to spread their professional wings. You buy a quality slide rule at a young age and it can serve you for a lifetime. But by the time I reached that point the first HP scientific calculators – marketed as “Electronic Slide Rules” – were beginning to show up, and I got me a HP-21 that was way better for calculating. Still, I evidently felt a desire to own a better slide rule, because I did not skip that rite of passage – I eventually went and bought this Hemmi 130W from a friend who was happy to surrender it. I don’t remember whether I actually used it for real problems or not, but if I did it can’t have been for long.
This was definitely not an entry level rule. Made by Hemmi in Japan, it is a Log-Log slide rule in the so called Darmstadt configuration. It could easily have served me for the rest of my career, had electronics not made it obsolete. In fact, Einstein did quite well with a slightly less capable model…
It is interesting to compare the construction of these two slide rules. Check them out:
As you see, the Acu-Math is made of thin layers of plastic, with the end braces a likely point of failure, as seen in the cracked brace on the left. The Hemmi, meanwhile, is made of a bamboo core coated in celluloid, with a thin springy metal layer to maintain tension on the slide, whose core is laminated from three bamboo parts. This intricate cross section no doubt comes from decades of design experience, resulting in stable dimensions that ensure lifetime reliability.
The last photo shows the ends of the slide rules from above – again, we see the difference in construction quality. But it is the humble plastic rule that came to my rescue back then…
Exhibit provenance: As I said, an uncle and a friend!