76 DISCOVERMAGAZINE.COM
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TE^(^2 )Long Live the Kilogram
BY JONATHON KEATS41
i
In 1791, several prominent scholars from the French Academy
of Sciences began working to create a universal system of
measurement. They wanted it to be accessible to everyone on
the planet, so they decided it would be based on nature. The meter was
defined as a specific fraction of Earth’s circumference, and other units
were derived from it. For instance, the kilogram was defined as the
weight of a liter — one cubic decimeter — of water.
Because these measurements were difficult to replicate reliably, the
scholars ended up crafting physical objects out of platinum to use as
the official standards, which defined the values. These were stashed in
a vault in Paris. Scientists have been striving ever since to ditch these
artifacts, which are subject to wear-and-tear, and live up to the 1791
ideals by specifying metric units in terms of natural constants.
Redefining the meter was relatively easy. In 1983, it was tied to the
speed of light (as the distance traveled by a laser beam in a given
fraction of a second). But the kilogram stubbornly remained a lump
of metal — until recently, when representatives from many of the
world’s governments voted unanimously to redefine it in terms of an
unchanging physical value, based on the energy of a photon, called
Planck’s constant.
The resolution went into effect on May 20, and also grounded several
other metric units in nature.
“It’s a landmark in science and human endeavor,” says Terry Quinn,
director emeritus of the International Bureau of Weights and Measures,
a key figure in the effort. Quinn believes the 18th-century scientists
would be proud, even though Planck's constant wasn’t quite in their
lexicon yet. After 228 years, the whole International System of Units is
finally free of physical constraints.In May, the kilogram was
redefined using the energy
of a photon, as measured
by a device known as the
NIST-4 watt balance (left).
The new standard replaces
its platinum-based
predecessor (above),
in use for centuries.