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Weighty matters

Sept. 1, 2008
Just when you think you can count on a unit of measure, it changes. That's exactly what's happening to the kilogram, the official unit of mass from the

Just when you think you can count on a unit of measure, it changes. That's exactly what's happening to the kilogram, the official unit of mass from the International System of Units (SI).

In the late 1700's, a kilogram was defined as the mass of a cubic decimeter of water. Then, in 1889, the first CGPM (General Conference on Weights and Measures) sanctioned the international prototype of the kilogram — “Le Grand K” — a bar of platinum-iridium alloy (90% platinum,10% iridium) tucked away in a vault near Paris at the Bureau International des Poids et Mesures (BIPM). By definition, “The kilogram is the unit of mass equal to the mass of the international prototype of the kilogram.”

For various reasons, there's now a push in the scientific community to redefine the kilogram, basing it on standards of universal constants rather than a physical artifact that loses weight each time it's handled. Scientists at Sandia National Laboratories are among those offering ideas for a new standard. One concept involves the experimental measurement of force on a watt balance scale, while another approach tallies the number of atoms in a silicon crystal.

According to Sandia physicist Harold Parks, the watt balance method is based on an idea that compares electrical and mechanical power with a high degree of accuracy. “The NIST (National Institute of Standards and Technology) watt balance experiment has achieved the accuracy needed to redefine the kilogram, but the experiment will need to be confirmed by other groups for the results to be fully accepted,” says Parks. The kilogram could be redefined as early as 2011, but don't count on it.

Six Sigma approach refines other six SI Units

Defining — and redefining — the SI base units is an ongoing task for scientists. Besides the kilogram, following are up-to-date definitions of each SI unit and the year of its most recent iteration.

Unit Definition Last update Meter
(length) The length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second. 1993 Second
(time) The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. 1967 Ampere
(electric current) Constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per meter of length. 1948 Kelvin
(thermodynamic temperature) The fraction 1/273.16 of the thermo-dynamic temperature of the triple point of water. 1967 Mole
(amount of substance) The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. 1971 Candela
(luminous intensity) The luminous intensity, in a given direction, of a source that emits mono-chromatic radiation of frequency 540 × 1,012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. 1979

Source: NIST Special Publication 330 (SP 330), The International System of Units (SI).

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