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