Monday, October 31, 2011

Kilogram is no longer a kg

The definition of the kilogram will be changed, as described in more detail here.
New recommendations of the Consultative Committee for Units of the International Bureau of Weights and Measures have been accepted, which define the kg in relation to the Planck constant.

The new definition is: 'the kilogram is such that the Planck constant is exactly 6.6260693 x 10-34 joule-seconds'. The kilogram can be defined in terms of the Planck constant given that the unit of the Planck constant is equal to the unit of action - J s = kg m2 s-1 - and the second and metre have fixed numerical values in terms of the numerical values of the caesium hyperfine splitting frequency and the speed of light in a vacuum, respectively. So fixing the magnitude of the unit kg m2 s-1 has the effect of defining the kilogram.

And just to cleanup the SI system, three other units were updated as well (mole, ampere, Kelvin).

The ancient unit of kg, a cylinder of platinum-iridium that was cast in 1879, can therefore be put to rest now.

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