Definition:Kilogram/Historical Note

Historical Note on Kilogram

 * 1795: The kilogram was defined as $1000$ times one gram.

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 $4 \cels$.


 * 1875: The was derived from this.

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

The weight of the 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.


 * 20 May 2019: The kilogram is no longer defined by a physical artefact, being the last of the fundamental units of physics which was so defined.