Kern Aarau 360 Degrees

I spent a couple of hours this morning cleaning up a Kern Aarau slide rule I bought recently. Kern Aarau was primarily a manufacturer of surveying equipment, so almost certainly this slide rule is related to surveying.

The first thing you notice about this slide rule is its weight — it’s darned heavy compared to most slide rules. As well it should be: it’s solid steel! Every single part on this slide rule is beautifully machined from stainless steel. In terms of its construction, it looks exactly like what you’d get if you described how a slide rule works to a machinest, and then told him to go build one. It’s the kind of gadget that anyone who loves fine machinery would appreciate.

Some of the scales are familar. On the top stator, there is a scale just like an “A” scale, except that it is readable from both the top edge and the bottom edge (with the slide). The slide has two scales on it. One of them is a completely conventional tangent scale (marked “TG."); it has angles from about 35' to 45 degrees and the tangent is read from the A scale.

The other scale on the slide is marked “SIN.COS", and so far it has me mystified. It starts at just under 10' on the right hand side, and goes to 45 degrees on the left hand side. The 45 degrees lines up with the 2.0 mark on the left hand half of the A scale. I have not figured how to read sine and cosine values using this scale, though it seems clear that that’s the intent.

On the bottom stator, there is a scale marked “E-R” that looks like almost three decades of a log scale, starting at 0.01 and ending at 7.0. The 0.01 on this scale lines up with about 3.825 on the left hand decade of the A scale. I have absolutely no idea what this scale is for.

Finally, there is a mysterious scale on the cursor, arranged so that you can read it directly against the A scale on the top edge of the rule. This scale goes from 0 on the right to 45 on the left. If you set the 0 against the middle of the A scale, the 45 reads against 5 on the left hand decade of the A scale. The range of this scale (0 - 45) suggests to me that it has something to do with a function of an angle in degrees, presumably one that exhibits symmetry over the eight 45 degree segments of a circle (sine and cosine are like this). But I haven’t figured out what it’s for.

If you’d like to see this thing close-up, check out this 300 dpi scan. Anyone with any knowledge of these things, please drop me a line or leave a comment here. I’d like to solve this mystery!

Early Acu-Rule

I had a very nice thing happen over the past couple of weeks. There was an eBay auction, very poorly listed and photographed, that had what appeared to be a wooden Acu-Rule mixed in with a whole bunch of “vintage” drafting equipment that I cared nothing about. I bid on the collection, hoping nobody else wanted it, and thinking that the plastic drafting equipment would make great chew toys for my dogs. But…as fate would have it, someone else bid on that auction and won it. Figuring I had nothing to lose, I wrote the winner and told him that if he had no particular interest in the Acu-Rule, I’d be pleased to buy it from him. In response, he said that “it would be my pleasure” to send me the Acu-Rule, gratis, which he did — I just received it. This fellow is a doctor in Nevada; I don’t have permission to use his name — but what a nice thing to do!

And the rule turned out not only to be a wooden Acu-Rule, but it’s one that so far I haven’t been able to determine exactly what model it is. I’ve been told by one fellow who knows a lot about Acu-Rules that there were many variations on the early wooden rules. In addition, during World War II many materials were scarce, and substitutions were made as needed — resulting in even more variations. So perhaps we’ll never know exactly what this thing is!

When he sent me this slide rule, my doctor friend told me it was “scruffy"; afraid, I suppose, that I’d be disappointed. And indeed when I read that, I was expecting something that was scruffy by my standards — which I can now say are considerably lower than my doctor friend’s! I took one look at the slide rule and said “Wow! This thing is in great shape!” As indeed it was, for its age. The worst problem was some oily black dirt in a patch on the front, and some lightly penciled writing on the back. A gum eraser took care of the pencil marks and most of the dirt with hardly any effort — but a careful scrubbing with mild soapy water didn’t make much of a dent in the black goop. An expert in the slide rule world suggested that I try denatured alcohol on the black goop, and that worked like a magic wand — one swipe of an alcohol-wetted Kimwipe and the black goop was gone forever! Now the worst damage left on this rule is some light scratching on the bottom of the cursor’s plastic lens (not even visible in the scan); someday I may polish that out. Maybe…

The cursor on this rule has a “feature” I’ve not seen before: the spring is a wire (e.g., round in cross-section) instead of the usual flat strip. It works surprisingly well, running on sanded wood. Also surprising to me: the spring runs, apparently by design, in the same groove as the guide on the cursor runner.

The wood the slide rule is made from is not particularly hard, and it is very lightweight — feels like the density is less than pine and more than balsa wood, though that’s just my guess from hefting it. I can’t detect any odor to the wood. The Lawrence slide rules were made from a similar wood, though on most of mine the wood on the Lawrence rules is distinctly more yellow; this could simply be the finish used.