How the Metric System Was Meant to Work

[A] The metre was not chosen for convenience. In the 1790s a French commission decided that the new unit should be derived from the Earth itself, and fixed it as one ten-millionth of the distance from the North Pole to the equator along the meridian passing through Paris. [B] Two surveyors were sent out to measure the relevant arc, a task that took them seven years and included being arrested more than once as suspected spies.

[C] The reasoning behind that choice was political as much as scientific. Existing units belonged to particular trades and particular lords, who set them and profited from them; a unit taken from the planet belonged to nobody and could be checked by anyone with the right instruments. [D] It was intended, in the phrase of the time, for all people, for all time. The irony is that the measurement was slightly wrong. The surveyors had not fully accounted for the flattening of the Earth, and the metre they produced is about a fifth of a millimetre short of the definition it was meant to satisfy. The error was discovered, and then ignored, because by then too many instruments had been built to the existing standard. The metre is now defined by the speed of light, and the number chosen was the one that keeps that slightly imperfect bar unchanged.