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Amid the ideological and religious upheavals of the last 200 years, the metric system has spread around the world as an exemplar of science and rationality. But in both its champions and detractors, it has evoked as much passion as reason.

Created beginning in 1790 by the French Academy of Sciences at the behest of the revolutionary National Assembly, the metric system reflected a century of measurement reform proposals. The meter was defined by a law of the National Convention in 1793 as one ten-millionth of a quarter-meridian, the distance from the earth’s equator to one of its poles. Ken Alder of Northwestern University, studying records in Paris, found that the attempt to measure the meridian mixed painstaking detail with high adventure. It took two French astronomers seven years to measure the distance between Dunkirk, France, and Barcelona, Spain, and Alder’s memorable account, The Measure of All Things, reveals that one of the men covered up for the other’s fudged work. The astronomers knew the earth was slightly flat at the poles–Pierre-Louis Moreau de Maupertuis had proved Newton’s prediction in 1736–but thought it otherwise uniform. Survey one meridian, they thought, and you’ve surveyed them all. They soon learned the lumpiness of reality.

Not only was the earth-based metric system difficult to devise, and flawed: it was also unnecessary. Two Italian scientists, Paolo Agnoli and Giulio D’Agostini, recently noted in a paper that well before the French Revolution, scientists proposed a new unit that was less than a half-centimeter shorter than the current meter. They defined it not by measuring the earth, but by timing a pendulum. The Italian polymath Tito Livio Burattini proposed such a “catholic [i.e., universal] meter” as early as 1675. Burattini noted that a weight swinging on a string the length of his proposed meter returned to its original position in two seconds. The amount of time the weight took to travel from one maximum elevation to the other was one second: a unit that corresponded with the approximate duration of a human heartbeat. Timing a pendulum, even in a vacuum at a controlled temperature, was easier than surveying a meridian, and its deeply human rhythm was satisfying.

Why did the Academy of Sciences take the more difficult course? Because time itself was in play. The Academy was considering the establishment of a decimal-based ten-hour day, with 100 seconds to a minute and 100 minutes to an hour. Besides, France’s scientists believed the project would unite humanity in the thrill of common proprietorship of the newly measured globe. After the shortcomings of the meridian-based meter became apparent, a platinum meter bar was presented to the French legislature in June 1799 as an arbitrary basis for the new measurement.

Since 1983, to establish greater precision than any material object allows even in controlled conditions, the 51-nation General Conference on Weights and Measures has defined the meter as the distance traveled by light in a vacuum during a time interval of 1/299,792,458 of a second. The Système International d’Unités (SI) starts with seven basic units – the meter, kilogram, second, ampere, kelvin, mole, and candela – and defines 22 others that include every unit science, technology, and commerce need but money (and even that is now universally decimal).

Elegant as it is, the metric system has provoked spirited resistance. Some countries, including France in the 19th century and the U.K. today, have imposed draconic penalties on recalcitrant vendors; a northeast English grocer was sentenced to probation in 2001 for selling bananas by the pound rather than the kilogram. In the U.S., the government has yielded to opposition even to dual systems of measurement. In 1978, the Federal Highway Administration’s policy of adding distances in kilometers to highway signs was reversed following protests of antimetric folk who saw the thin edge of a wedge.

The conflicts over metrication tell a messy truth: no single system of measurements is ideal for all uses. Like any object of human design, a measurement system trades one advantage for another. In its avoidance of thirds, for example, the metric system has no colloquial equivalent of the foot. Decimeters are seldom used; the system skips an order of magnitude from the centimeter to the meter. And liters exceed normal individual human thirst.

On the other hand, the millimeter has its own advantages. Around the thickness of two fingernails, it’s the smallest unit we find useful for measuring common objects; a dime is 1.35 mm thick. It avoids the contortions of arithmetic involving sixteenths and thirty-seconds of an inch. Only where objects are regularly divided in half, as in carpentry and the building trades, does the inch come into its own. What is being measured dictates the appeal of the system used to measure it. The metric system has become the world’s lingua franca, but traditional measures, rooted in the body and its crafts, are its tenacious vernacular.

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