= Kibble balance
{c}
{wiki}
The <Kibble balance> is so precise and reproducible that it was responsible for the <2019 redefinition of the Kilogram>.
\Image[https://upload.wikimedia.org/wikipedia/commons/f/f8/NIST-4_Kibble_balance.jpg]
{title=NIST-4 Kibble balance}
{height=800}
It relies rely on not one, but three macroscopic <quantum mechanical> effects:
* <atomic spectra>: basis for the <caesium standard> which produces precise time and frequency
* <Josephson effect>: basis for the <Josephson voltage standard>, which produces precise <voltage>
* <quantum Hall effect>: basis for the <quantum Hall effect>, which produces precise <electrical resistance>
How cool is that! As usual, the advantage of those effects is that they are discrete, and have very fixed values that don't depend either:
* on the physical dimensions of any apparatus (otherwise fabrication precision would be an issue)
* small variations of temperature, magnetic field and so on
One downside of using some <quantum mechanical> effects is that you have to cool everything down to 5K. But that's OK, we've got <liquid helium>!
The operating principle is something along:
* generate a precise frequency with a <signal generator>, ultimately calibrated by the <Caesium standard>
* use that precise frequency to generate a precise <voltage> with a <Josephson voltage standard>
* convert that precise voltage into a precise <electric current> by using the <quantum Hall effect>, which produces a very precise <electrical resistance>
* use that precise current to generate a precise force on the object your weighing, pushing it against <gravity>
* then you precisely measure both:
* local <gravity> with a <gravimeter>
* the displacement acceleration of the object with a laser setup
Then, based on all this, you can determine how much the object weights.
\Video[https://www.youtube.com/watch?v=Oo0jm1PPRuo]
{title=How We're Redefining the kg by <Veritasium>}
\Video[https://www.youtube.com/watch?v=ZfNygYuuVAE]
{title=The Kibble Balance, realizing the Kilogram from fundamental constants of nature by Richard Green}
{description=
Presented in 2022 for a #CENAM seminar, the #Mexican <metrology institute>. The speaker is from the <Canadian> <metrology institute>
* https://youtu.be/ZfNygYuuVAE?t=854[]: they don't actually use the <Quantum Hall effect> device during operation, they only use it to calibrate other non-quantum resistors
}
\Video[https://www.youtube.com/watch?v=VlJSwb4i_uQ]
{title=The Watt balance and redefining the kilogram by <National Physical Laboratory>}
{description=Nothing much, but fun to hear Kibble talking about his balance in beautiful English before he passed.}
Bibliography:
* https://www.bipm.org/documents/20126/28432564/working-document-ID-11315/8532173e-8bae-2bdf-b74a-cb48296b4e67
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