![IPK (international prototype kilogram)housed in nested bell jars](http://static.sciencelearn.org.nz/images/images/000/002/300/full/BellJar.jpg?1674167957)
The IPK (international prototype kilogram) is housed under three nested glass bell jars with a carefully controlled atmosphere. The IPK was retired on 20 May 2019.
The IPK (international prototype kilogram) is housed under three nested glass bell jars with a carefully controlled atmosphere. The IPK was retired on 20 May 2019.