A new definition of the International System of Units

Since the earliest civilizations, people have traded foodstuffs, goods and livestock. Trade means exchange, and exchange requires evaluation and measurement. Metrology (the science of measurement) therefore became essential to life in society. People devised various systems of measurement to quantify commercial transactions. But to measure something is to determine a number by comparing it with a standard. Extended to the base units of physics, the standards for quantities such as mass, length and time formed a system of units that evolved considerably as scientific knowledge advanced. From the earliest progress in understanding nature—written in the language of mathematics, as Galileo maintained—the need for consistent, universal standards for the base units became clear.

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The meter brings order! -----------------------

In 18^th-century France, the system of weights and measures was riddled with disorder and inconsistencies. Depending on the region, the foot, pound, toise, ell, bushel, pint and arpent had different values; moreover, different numeral systems were used. The ell and the toise were incommensurable. Measures of volume bore no relation to measures of length. Yet trade, industry and science were expanding, making it essential for everyone to speak the same language. Scientists therefore investigated a system of units based on objective, universal features of the planet, which were increasingly well understood and measured. By the end of the 17^th century, natural physical laws provided the first standards of measurement. For distances, opinion was divided between the length of an arc on the Earth and that of a pendulum with a given frequency at a precisely defined location (scientists had known since 1673 that the Earth was not perfectly spherical). As early as 1669, Abbé Picard proposed a natural reference for units of length and suggested using the length of a pendulum beating once per second. The length of the toise had to be tied to nature and therefore regarded as universal and invariable. That same year, the Académie des sciences commissioned the abbé and astronomer to measure an arc of the Earth's meridian by triangulation, which he did in 1671. Jean-Felix Picard chose the length of a simple pendulum beating once per second (equivalent to 36 inches and 8.5 lines, one-third of which was the universal foot).

Two years later, Tito Livio Burattini named this length the metro cattolico. During the 18^th century, several expeditions by scholars from the academies—some of them decidedly adventurous—used this pendulum reference to measure the length of a meridian arc by triangulation: in Lapland, with Pierre Louis Moreau de Maupertuis's expedition, and in Peru, with Charles Marie de La Condamine, Pierre Bouguer and Louis Godin.