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Comment Re:Why isn't there a precise atomic standard? (Score 5, Informative) 177 177

The mass of X number of molecules of element Y = 1 gram.

Like there is for the second:

"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 caesium-133 atom."

Because we don't yet have an accurate-enough measure of the Avogadro or Planck constants.

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