= Josephson voltage standard
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The most practical/precise volt standard.
It motivated the definition of the <ampere in the 2019 redefinition of the SI base units>
Quick <NIST> article about it: https://www.nist.gov/news-events/news/2013/04/primary-voltage-standard-whole-world (https://web.archive.org/web/20190410011041/https://www.nist.gov/news-events/news/2013/04/primary-voltage-standard-whole-world[archive])
The wiki page https://en.wikipedia.org/wiki/Josephson_voltage_standard contains amazing schematics of the device, apparently made by the <US Government>.
\Image[https://upload.wikimedia.org/wikipedia/commons/2/2b/Layout_and_Schematic_of_JVS_Chip.jpg]