The measurement of a kilogram is currently based on a hunk of metal kept under lock and key in France - but that's about to change.
This Way Up finds out why.
CGI image of the international prototype kilogram (IPK) Photo: Greg L / CC BY-SA 3.0
The kilogram is the last unit of measurement (as defined by the International System of Units) to still be based on a physical object.
From next year. The International Committee for Weights and Measures is redefining the standard kg unit.
No more physical objects - a kilo will be calculated by reference to a physical constant known as Planck's constant or 'h'.
Other units of measurement like the second, the metre, the ampere, the kelvin, the mole and the candela are all based on a physical constant - roughly speaking something that happens in nature and stays the same over time, whether you're in Timbuktu or Timaru.
The prototype kilogram held in Sevres in France Photo: AFP
