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 International Prototype Kilogram (IPK) 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 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.

• 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

• 1964: Milton Abramowitz and Irene A. Stegun: Handbook of Mathematical Functions ... (previous) ... (next): $2$. Physical Constants and Conversion Factors
• 1989: Ephraim J. Borowski and Jonathan M. Borwein: Dictionary of Mathematics ... (previous) ... (next): kilogram
• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): kilogram
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): kilogram
• 2008: Ian Stewart: Taming the Infinite ... (previous) ... (next): Chapter $2$: The Logic of Shape: Problems for the Greeks
• 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): kilogram