Units of measurement emerged at the end of the 18th century to put an end to the arbitrary method that existed until then.
The history of measurement systems has been anarchy throughout the History of Science, where each town, each region, or each kingdom had its own. To complicate it even more, some regions took criteria from the human anatomy as a reference (foot, elbow, palm…). To all this, it was necessary to add that, on many occasions, the unit also depended on the type of object.
This hodgepodge explains why, in pre-revolutionary France alone, there were eight hundred different units of measurement, behind which around 250,000 different values were hidden. Thus, for example, charcoal was sold by baskets, wood by beams, fruit for cider in barrels, firewood by ropes, beer by pints…
Between Dunkirk and Barcelona
Faced with this chaos, in 1790 Charles Maurice de Talleyrand (1754-1838), a prestigious politician and diplomat, called for a new system of units. According to him, a process of harmonization and generalization was necessary so that the transmission chain of industry and commerce would not be slowed down.
Just one year later, the Weights and Measures Commission was established, made up of scientists of the intellectual stature of Borda, Condorcet, Laplace, Lagrange, and Monge. It was the beginning of a utopian project, which was intended to unite the world through an international system that would allow the exchange of goods and information.
His first task, not an easy one, was to resolve the definition of a meter (from the Greek metron, measure), which had to be extracted from nature so that “it would be common to all peoples, at all times”. Finally, he settled on fixing it as one ten-millionth of the distance between the North Pole and the Equator, on the meridian passing through Paris.
As it was impossible to travel just to the north pole and to the equator to carry out the measurement in its entirety, the scientists in charge of the project decided to take as a reference the imaginary line that passes through the French capital and measures the distance between Dunkirk and Barcelona.
To get larger or smaller units – multiples and sub-multiples – of the meter, it was established that it would be enough to multiply or divide by ten, simply moving the comma would be enough. You would not have to go through the torment of multiplying by 12, 14, 16, or even 112, as in earlier systems.
The system devised by the Commission on Weights and Measures was simple, useful, and elegant. Lavoisier would refer to him by stating: “Nothing greater or more sublime has come out of the hands of men.”
With a ‘g’ for grave
Next, it was the turn of the unit of mass, for which the commission chose the ‘grave’, which would be equal to the mass of a cubic decimeter of pure water at the temperature of maximum density. Water was chosen and not another fluid because they understood that it was easier to obtain and distill.
They soon realized that the definition was not without complications, since the density of water depends on pressure, which in turn depends on the height, which requires a stable definition of the meter, which at that time had been defined according to a terrestrial meridian. A concatenation of assumptions too intertwined.
On the other hand, the French National Assembly disliked the name ‘grave’, since at that time it was a polysemic word with which the noble title of ‘count’ was also called, something that was incompatible with the principles of the revolution. For this reason, on April 7, 1795, it was decided to use the term ‘gram’ (from the Greek gramma, meaning letter) instead. Also, scientists redefined the value, one gram would not weigh one grave, but one milligrave.
Due to its small size, it was very difficult to have a reliable standard for the gram, so, in consensus, it was decided to use one of its multiples, the kilogram, which corresponded to the original ‘grave’. This helps explain why the meter and kilogram are used in the metric system instead of the gram.
In 1799 the standards for the meter and the kilogram -both in platinum- were deposited in the Archives of the Republic. The same year that Napoleon presented to the world the new system “for all peoples and for all times.”
