The U.S. Natioual Institute of Science and Tecbuology (NIST) maintains several accurate copies of the international stan-dard kilogram. Even after careful cleaning, these natioual standard kilograms are gaining mass at an average rate of about 1 JLg/y (1 Y = 1 year) when compared every ten years or so to the standard intematioual kilogram. Does this apparent change have any importance? Explain.?
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