Sunday, November 26, 2006

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!

1 comment:

  1. In the old blog, Maptack said:
    Hi SlighlyLoony,This sliderule was for for use with a Tachymeter Theodolite also known as a Tacheometer Theodolite but principally with a Tacheometric Plane Table Telescopic Alidade. The tacheometer/tachymeter/alidade gradually became a “self-reducing” instrument from the last quarter of the 19th Cent. The stadia slide rule calculated the horizontal distance & the height difference component from optically measured distances using fixed stadia lines in the telescope field of view. So the slide rule competed with the evolving self-reducing (simplified the calculation so you could do it in your head) aspect of improved surveying telescopes. The Stadia or tacheometric slide rule was at its most useful with a telescopic alidade which permitted the drafting of the map in the field on the plane table drawing board on which the alidade rested. The telescope fixed direction & distance and an attached ruler permitted the plotting of the position of the surveyed object. This “real-time” requirement demanded field calculation hence the Stadia slide rule. This was generally for small scale mapping 1/2500, 1/10000 etc particularly for exploratory and archeological map making. The Kern stadia rule was still on sale during the 1920’s and probaly into the early Thirties. The slide rule was made by a company well used to metal as a basic component which of course prevented warping and damage during the gruel of fieldwork. Tachymetry simply means quick measure from the Greek. I have a copy of the manual for the Kern slide rule but it is in French or German if you are interested.