I'm watching a YouTube video about the American standard kilogram by the Veritasium channel and noticed something that I'd seen before but never questioned. The standards are kept under two, nested bell jars.
What does two jars achieve that one does not? And then would three do that thing better again? Why stop there, why not four or five?
PS: I couldn't find a tag that I thought this question belonged under (mass doesn't fit the bill really) - suggestions welcome in the comments