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

  • $\begingroup$ If you noticed, when they showed the IPK, it is kept under 3 jars. $\endgroup$
    – The Photon
    Commented Apr 5, 2018 at 23:31
  • 2
    $\begingroup$ What does IPK stand for? $\endgroup$
    – user4552
    Commented Apr 6, 2018 at 0:46
  • 2
    $\begingroup$ @BenCrowell International Prototype Kilogram $\endgroup$
    – Cort Ammon
    Commented Apr 6, 2018 at 0:54
  • 1
    $\begingroup$ To expand on @ThePhoton's comment, the IPK is kept under 3 jars. Its clones are all kept under 2 jars. While I don't know why they picked the specific numbers of jars, it's worth noting that the IPK has been drifting away from its clones for many years and nobody is entirely sure why. One of the hypotheses out there is that the IPK is experiencing different environmental conditions due to being under 3 jars. $\endgroup$
    – Cort Ammon
    Commented Apr 6, 2018 at 0:56
  • $\begingroup$ @CortAmmon - Yep, all that jarring will do that. $\endgroup$
    – Hot Licks
    Commented Apr 6, 2018 at 0:59

1 Answer 1


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.

enter image description here

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%.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.