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Weight lifted: Nations approve changes to units of measure

The “Grand K" kilogram, a mass of polished platinum-iridium alloy used as the world's sole true kilo since 1889, was retired Friday.

It is being replaced with a scientific formula.

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More than 50 nations approved an overhaul of the International System of Units, giving new definitions to four of seven categories; the kilogram for mass, kelvin for temperature, ampere for electrical current and mole for the amount of a substance. 

The other categories are length (meters), time (seconds), and luminous intensity (candela).

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The change will not visibly impact people but scientists lauded the decision, which will affect research and other industries. 

In 1967, time was redefined from tagging the second to the rotation of the Earth on its axis to the ticking of a caesium atomic clock, The Economist reported.

In 1979 the candela -- based on the brightness of a candle flame -- was redefined to the brightness of a source emitting light at a specific frequency in the green part of the spectrum, according to The Economist.

In 1983 the meter was altered to reflect the concept that light travels at a fixed speed through a vacuum, the magazine reported.

The Associated Press contributed to this report.

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