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Give them an inch

Political struggles, war and economics all played a part in the making of the metre

Book review: World in the Balance: The historic quest for an absolute system of measurement by Robert P. Crease, W. W. Norton

PRECISION and fastidiousness – at first blush the quest for a precise system of measurement might seem a plodding pursuit. But as philosopher Robert P. Crease makes clear in World in the Balance, it was anything but.

From prehistory to the present, Crease ties humanity’s search for precision to the history of nations and of ideas. Any measurement must be based on a “standard” that embodies a unit, such as a foot, a finger or a hand. Eventually standards came to be inscribed in stone or metal and stored in protected areas such as the Acropolis for the Greeks, and
in later centuries in Paris and Washington DC. Ownership of the standard symbolised political and social power.

These standards had a financial value too: copies of them were used in commerce. Not only that, the new machinery that fed the industrial revolution’s boom relied on interchangeable parts, which required high degrees of accuracy and uniformity in measurement.

Turbulent political times had their impact too. Eager to establish a new national identity, in the late 1700s the leaders of the French Revolution moved to sweep away all vestiges of the old system, including its weights and measures. The metric system was born.

It spread slowly across Europe. The UK was the prominent holdout, stubbornly hanging onto imperial units: as the premier industrial nation it made no sense to replace perfectly functional machinery for the sake of a new measurement system. The US kept the British imperial system, although scientifically minded presidents such as Thomas Jefferson urged otherwise.

But one thing still missing was a “natural standard” that associated a length measurement with a property of nature. In the 1870s, US logician Charles Sanders Peirce made the key suggestion: link the metre to the wavelength of a spectral line. This way, the standard metre could be reproduced anywhere.

Crease takes us through the maze that led to today’s scientific measuring system, the SI system – Système International d’Unités – which extends the metric system to include electricity, magnetism and time, and whose standards are natural constants, such as the speed of light.

The metric system won out for science. Although for things like speed limits and consumer goods the UK and US still use their own systems, converting to the metric system is routine – if you remember to do it. I was surprised that Crease made no mention of the 1999 Mars rover mission, which failed when the craft burned up in the Martian atmosphere because American engineers neglected to make an essential conversion. And also that he left out the 1884 convention in Washington DC that standardised time zones. But these are quibbles with an otherwise fascinating book.

From issue 2841 of New Scientist magazine, 3 December 2011, page 55

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