At the moment a kilogram is still based on a single kilogram weight made of platinum and iridium and kept in a vault in France. This is known as the International Prototype of the Kilogram (IPK). There are lots of official copies of the IPK that countries use as their own national standards. There are also 6 copies held in France that can be used to check the accuracy of the IPK.
Over time some of these official copies seem to have lost or gained small amounts of mass. They are not large enough amounts that you would be able to feel it if you held them in your hand, but when very accurate measurements are needed for science and engineering tiny differences can create big problems.
For this reason, scientists have found a way to define the kilogram using some of the same universal constants used to define the other 6 SI units. This will have an impact on other SI units such as the mole, which use the kilogram as part of their definitions.
The definition of the Ampere and the Kelvin are also going to be changed to make them more accurate and easy to measure.
These new definitions are expected to come into force on 20th May 2019 (World Metrology Day). This will make the SI units of measurement even more accurate and standardised across the world. We’ve come a long way from everyone using the length of their own forearm!
A replica of the prototype of the kilogram at the Cité des Sciences et de l’Industrie, Paris, France. 15 February 2009 Credit: Japs 88