Of the seven SI base units, the meter, the second, the ampere, the kelvin, the mole, and the candela are all defined based off universal constants, while the kilogram continues to be defined by a single physical artifact. The master International Prototype Kilogram is made of a platinum–iridium alloy and is stored under three bell jars in a vault at the International Bureau of Weights and Measures in Sèvres, France. Verification of the mass of the kilogram and its 40 original replicas world wide is a highly complicated task, to which entire careers have been devoted. By definition, the error in the measured value of the IPK’s mass is exactly zero; the IPK is the kilogram. However, it has been found that the IPK not only varies in mass over time, but the mass of the IPK and its copies spread around the world are drifting apart for known and unknown reasons. (The variations are generally in the range of 1 to 100 micrograms.) In 2005, it was officially proposed that the kilogram be redefined in accordance with some physical constant, as the meter was in 1960. At its 2011 meeting, the General Conference on Weights and Measures agreed in principle that the kilogram should be redefined in terms of the Planck constant, but deferred a final decision until its next meeting, scheduled for 2014.
Of the seven SI base units, the meter, the second, the ampere, the kelvin, the mole, and the candela are all defined based off universal constants, while the kilogram continues to be defined by a single physical artifact. The master International Prototype Kilogram is made of a platinum–iridium alloy and is stored under three bell jars in a vault at the International Bureau of Weights and Measures in Sèvres, France. Verification of the mass of the kilogram and its 40 original replicas world wide is a highly complicated task, to which entire careers have been devoted. By definition, the error in the measured value of the IPK’s mass is exactly zero; the IPK is the kilogram. However, it has been found that the IPK not only varies in mass over time, but the mass of the IPK and its copies spread around the world are drifting apart for known and unknown reasons. (The variations are generally in the range of 1 to 100 micrograms.) In 2005, it was officially proposed that the kilogram be redefined in accordance with some physical constant, as the meter was in 1960. At its 2011 meeting, the General Conference on Weights and Measures agreed in principle that the kilogram should be redefined in terms of the Planck constant, but deferred a final decision until its next meeting, scheduled for 2014.
 
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    I love this fact. I love that the kilogram is an actual thing. Like, literally. That thing doesn’t weigh a kilogram, it...
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