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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.
Why?
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
I think it would be reasonable to assume that the purpose of the jars is to prevent contaminants getting in contact with the standard.
Judging by the pictures, the jars of the american copies, K4 and K20, are pressed against some sort of gaskets, presumably, to provide a better seal.
Why two jars?
Because the effectiveness of two layers of protection or filtering could be on the order of the product, not the sum, of the individual contributions, i.e., it could be very cost effective.
As a hypothetical example, if the leakage in the outer jar, due to some sort of selective diffusion of a contaminant, causes, over some period of time, the concentration of the contaminant on the inside of the outer jar to reach 10% of the concentration on the outside, the concentration inside the inner jar, may reach just 1%.
