Definition:Kilogram/Historical Note

Historical Note on Kilogram

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

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

Subsequently, the actual reference kilogram was manufactured as a prototype in $1799$.

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

The International Prototype Kilogram (IPK) was derived from this in $1875$.

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 is being sought, based on a fundamental constant.

In $2011$, 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.

As from $20$ 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.

Sources

• 2008: Ian Stewart: Taming the Infinite ... (previous) ... (next): Chapter $2$: The Logic of Shape: Problems for the Greeks