The Kilogram was for many years the only one of the seven SI base standards that was not defined in terms of a fundamental attribute of nature.
It was specified as the weight of a platinum-iridium cylinder stored in a lab outside of Paris under inert conditions.
The problem with that is that it needs to be managed in order to use such a standard for calibrating exact copies. No matter how meticulously you do it, surface molecules must experience some minimal impact.
Therefore, efforts have continued to find a replacement standard that is not a tangible thing. With the suggestion for a new Standard Kilogram utilizing a gadget called the Kibble Balance, it now seems that the quest is over.
