Until 2019 the kilogram was defined by a metal cylinder kept in a vault near Paris. How is the kilogram defined today?
By fixing the exact value of the Planck constant✓الإجابة الصحيحة
By the mass of a fixed number of carbon 12 atoms
By the mass of one litre of pure water
By the same metal cylinder, now stored in a vacuum
الشرح
The kilogram is now fixed by giving the Planck constant the exact value 6.626 070 15 × 10⁻³⁴ when expressed in the unit joule second. No physical object is involved any more, so the unit cannot drift the way an object can.