* 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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