The need to redefine two base units of the International System of Units (SI), the kilogram and ampere, has been discussed for several decades. There are many reasons to revise the present definitions of these units, most notably the inherent long-term instability of the artifact that serves to define the kilogram. Following the success of the 1983 definition of the meter, which is now based on the speed of light in vacuum, the International Committee on Weights and Measures has decided that this is also an opportune time to recast the definitions of all seven SI base units in terms of the most stable things known to scientists — physical constants.
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