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Its my understanding that before metric units of length were based on the speed of light, there were reference bars that were used to calibrate other (mostly calibration) equipment.

How were these units accurately subdivided into centimeters, millimeters, ect?

In other words, given a bar of length X, how do you derive sub-lengths of that bar that are precisely 1/100 or 1/1000 of that bar?

Sawyer Schumacher
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  • It seems much detail about copying procedures was codified early on by the Comité International des Poids et Mesures (International Committee of Weights and Measures). The minutes of their meetings, going back to 1874, are available at https://www.bipm.org. For example, the 1888 minutes include a detailed cross section of the X-shaped meter reference, other parts appear to deal with temperature control and the microscopes used for comparison. It's all in French so very slow going for me with my meager knowledge of the language. It seems later (1927, p. 49) they even controlled for air pressure – njuffa Dec 16 '22 at 03:04
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    Other potentially useful literature: "Travaux et mémoires du Bureau international des poids et mesures", partially available at Gallica BNF. For example, the volume for 1885 shows a drawing of a machine called Brunner's comparator, which looks like a machine for comparing meter bars (?) – njuffa Dec 16 '22 at 03:20

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