* Photo: Dieu Tan * Has the kilogram gone on a diet? Maybe. For some reason, the official kilo — a 118-year-old lump of metal stored in a vault at the International Bureau of Weights and Measures outside Paris — has slimmed down by as much as 50 micrograms in the past century. The solution? Build a better kilogram. Researchers at the Australian Commonwealth Scientific and Industrial Research Organization are cutting, grinding, and polishing a boule — a big crystal — of ultrapure silicon 28 into two baseball-sized spheres (one is for double-checking). Materials scientists are able to measure precisely how many atoms of that silicon isotope are in any given hunk (the completed orb should contain 215 x 1023 atoms). Creating "the roundest object in the world," says CSIRO engineer Katie Green, means technicians have to worry about only one dimension: diameter. Once finished, it will weigh a perfect kilogram. Give or take an atom.

Higher Standards

Except for the kilogram, every scientific unit administered by the International System of Units is based on an immutable property of the universe. * — C.E.*

Meter

Standardized: 1983

Measures: Length

Definition: The distance traveled by light in a vacuum in 1/299,792,458 second

Second

Standardized: 1967

Measures: Time

Definition: The time it takes for a cesium-133 atom to cycle 9,192,631,770 times between two specific quantum states

Ampere

Standardized: 1948

Measures: Electrical current

Definition: The current required to create a force of 2 x 10-7 newtons per meter between two parallel wires

Kelvin

Standardized: 1954

Measures: Temperature

Definition: 1/273.16 the temperature of the triple point of water — when it's simultaneously gas, liquid, and solid

Mole

Standardized: 1971

Measures: Amount of stuff

Definition: The number of atoms in 12 grams of carbon 12 (6.022 x 1023)

Candela

Standardized: 1979

Measures: Brightness

Definition: The intensity of a 1/683-watt yellow-green light spread over a square meter, seen from a meter away

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