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How and when did we build the first ruler ?

I have an hard time figuring how to build something "straight" without having first a "straight" tool.

Sure you could just stretch a thread/wire and then cut a wood block to have something ressembling a ruler. But is that it ?

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    $\begingroup$ This is why Wikipedia articles have history sections, see ruler:"The oldest preserved measuring rod is a copper-alloy bar that dates from c. 2650 BCE and was found by the German Assyriologist Eckhard Unger while excavating at Nippur. Rulers made of Ivory were in use by the Indus Valley Civilization period prior to 1500 BCE." Stretched ropes and light rays can be and were used to ensure "straightness". $\endgroup$
    – Conifold
    Commented Sep 12, 2017 at 19:41
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    $\begingroup$ The obvious answers to first getting a straight line are to a) pull a piece of string tight or b) make a shaft of light. $\endgroup$
    – Robert Frost
    Commented Sep 13, 2017 at 17:49

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The ruler is made by an expert joiner with an instrument similar to the modern plane: https://en.wikipedia.org/wiki/Plane_(tool). The straightness is checked by looking along the edge of the ruler (one end of the ruler is close to the eye). That is the edge is compared with the ray of light. In this way, the joiner detects the concavities, of the edge, and carefully removes them, until the edge looks straight. The surface of the instrument itself does not have to be very straight for this procedure.

A more difficult question is how the divided ruler is made, if you do not have a divided ruler for comparison. To divide the ruler, a construction with a compass is used, essentially consecutive dissections. This is how they divided precision instruments until the second half of 18th century. This was very labor consuming and thus expensive, until a "dividing engine" was invented. It uses essentially a comparison with already existing very finely divided scale.

EDIT. One of the most famous craftsmen of 18 century (John Bird, instrument maker) once said that there is no mechanical way to produce a straight line. One can only produce an arc of a circle (perhaps of a very great diamter), by compass. Since then, a mechanical mean of producing a straight line was actually discovered (it is called an Inversor, discovered by Paucellier, with rigorous proof by Lipkin (late 19 century)), https://en.wikipedia.org/wiki/Peaucellier%E2%80%93Lipkin_linkage)

But I've never heard of this device being used in practice. Its original purpose was to convert the circular motion to rectilinear motion (and back) in a steam engine. But the progress is lubricants made this invention redundant for its original purpose. It is easier to make a piston to move on APPROXIMATE straight line, then to implement an inversor.

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  • $\begingroup$ Ok thank you for your answer. So it's that it ?Mostly wood-crafting abilities, using muscles, sense of touch and eye-sight ? And we were able to build machine-tools, artillery cannons and spacecrafts able to land on the moon with nothing more formal than a ruler crafted by hand ? Or is there another decisive discovery later on that help having a "straighter" ruler ? $\endgroup$
    – Haiku
    Commented Sep 12, 2017 at 13:09
  • $\begingroup$ @Haiku: Are you talking of modern times? (Moon, etc.) In the modern times we have advanced technology (machine tools). But the basic CONTROL of the straightness of a ruler is still the same: comparison with the ray of light. $\endgroup$
    – Alexandre Eremenko
    Commented Sep 12, 2017 at 20:47
  • $\begingroup$ A great question and answer. This raises another question though: how does the human eye/brain determine if something is straight? say if one wants to program a computer to determine if a bunch of pixels are in a straight line like a machine learning problem what would the logic be? $\endgroup$
    – Manik
    Commented Aug 20, 2023 at 5:41
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    $\begingroup$ @Manik: Human eye or brain checks the straightness by comparing with a light ray: just look along the edge of the ruler, to see whether it is straight. Another common method is comparing with a stretched rope. In other words, the notion of straightness comes from physics. $\endgroup$
    – Alexandre Eremenko
    Commented Aug 20, 2023 at 12:09

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