Recently I got some old drawing and writing tools. Among them was this unusual scale/ruler.
I have no idea what ‘at.’ stands for (yes, 100 at. really correspond to 40 mm). Does anyone have any idea what it was used for?
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I couldn’t find anything while searching for it, that implies it might be specific for a certain country or field (Architecture maybe? They had some niche units in the past…)
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Maybe “at.” stands for “actual” and means 0.4 mm on the ruler equals 1 real-world unit, like a meter or something. It’s probably used for scaling stuff in technical drawings to keep everything proportional.
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Both could be possible! This ruler most probably comes from Germany, but I’m not sure. I think it’s too short for a scaling ruler, but who knows – there are the strangest drawing instruments.
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the ruler itself seems a bit small for architecture, unless it’s for details or a specific material — e.g. a 1:20 scale that comes with a specific product or equipment that is represented 4mm per unit.
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My first thoughts were:
- Tiny architectural scale for checking cadaster maps (“Katasterplan”), as some of those can have very unusual proportional ratios.
- Typography ruler, like a small typometer, with some unusual scale.
- Specialised nomograph for some dedicated applications
I’ve tried to identify the “at” unit, but nothing meaningful popped out so far. Knowing the provenance and the age could help: seems something not so far away in the past.
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I thought “At.” May be atmosphere because it is measured in mmHg, but 1atm= 760mmHg
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An “at” is the unit for “technical atmosphere”.
Even though it’s not for this scale, I did find this interesting tidbit:
In the United States, leather thickness is typically measured in ounces, where one ounce is equal to 0.4 mm.
Interesting that it stated the value in the metric system.
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Thank you all for sharing your thoughts! I’ll keep looking for an explanation, and if I find out anything new, I’ll post it here.
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The things that you can learn here are fantastic!
Today I remembered my visit to the “Pfunds-Museum” (https://www.pfunds-museum.de/, roughly translated “pounds museum”) in the Rhön Mountains in Germany about twelve years ago. This museum is home to an incredible number of exhibits on measuring and weighing from all over the world and many centuries ago. I have just sent them an email and asked about that strange ruler.
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I’d also drop a line to the guy beyond the “Graphography” weblog, maybe there are things to be learnt there as well — the guy surely has connections, and maybe, within the famous six degrees of separations, it will become possible to settle this mystery.
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That’s a great suggestion – I’ll do that!
Solved!
This scale was part of the so-called Maihak indicator, a device for measuring pressure, which was developed about 120 years ago.
James Watt, who significantly improved the steam engine, was looking for a way to measure fluctuating cylinder pressures. Around 1790, he devised a device with low inertia that could precisely follow the pressure fluctuations. It consisted of a spring-loaded piston with a pointer on the piston rod that indicated the steam pressure on a fixed scale. In 1796, John Southern, one of Watt’s employees, improved this device by replacing the pointer with a pen and the scale with a tablet to which a sheet of paper was attached. The board could be moved perpendicular to the movement of the pen, turning Watt’s indicator into a recording device. Later, Southern had the idea of having the board moved by an oscillating machine part, creating a closed diagram that could also be used to determine the machine’s performance.
One of the many clever minds who came up with variants and improvements to the indicator was the engineer Hugo Maihak, born in Upper Silesia in 1858. He invented the pressure chart recorder known as the “Maihak indicator”, which he produced in his company in Hamburg, Germany, from around 1900 and sold all over the world.
To read the pressure from the recording, the appropriate spring scale was selected depending on the piston size and spring (the set with the Maihak indicator contained several).
The scale in question with 0.4 mm pen travel per 1 at (kg/cm²) can be found at the bottom, approximately in the middle. – Some of the details as well as the illustration and the table are taken from the book “Der Indikator. Seine Theorie und seine mechanischen optischen und elektrischen Ausführungsarten” (The indicator. Its theory and its mechanical, optical and electrical designs) by Prof. Kalman J. de Juhasz and Dr. Jos. Geiger, published by Springer in 1938.
Incidentally, the color and grain of the scale indicate that it is made of boxwood, which used to be popular for rulers and scales because its hardness and short fibers prevented it from tearing when scored.
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The wealth of information on this site boggles the mind.
Thank you all!
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