Definition:Kilogram/Historical Note

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Historical Note on Kilogram

This itself was defined as the mass of one cubic centimetre of water at the melting point of ice.

  • 1799: The actual reference kilogram was manufactured as a prototype.

It had a mass equal to the mass of $1 \, \mathrm {dm}^3$ of water at its maximum density, approximately $4 \cels$.

This has a mass which is almost exactly equal to the mass of one litre of water.


The weight of the IPK has been known to vary, and so a more stable alternative was sought, based on a fundamental constant.

  • 2011: A decision was reached in principle that it should be redefined in terms of Planck's constant.

The actual decision was deferred to $2014$, and was once then deferred to the next meeting.


Sources