Ampere in the 2019 redefinition of the SI base units

Starting in the 2019 redefinition of the SI base units, the elementary charge is assigned a fixed number, and the Ampere is based on it and on the second, which is beautiful.

This choice is not because we attempt to count individual electrons going through a wire, as it would be far too many to count!

Rather, it is because because there are two crazy quantum mechanical effects that give us macroscopic measures that are directly related to the electron charge. www.nist.gov/si-redefinition/ampere/ampere-quantum-metrology-triangle by the NIST explains that the two effects are:

quantum Hall effect, which has discrete resistances of type:
$R_{x y} = \frac{V _{Hall}}{I _{channel}} = \frac{h}{e ^{2} ν}$
(1)
for integer values of $ν$ .
Josephson effect, used in the Josephson voltage standard. With the Inverse AC Josephson effect we are able to produce:
$K_{J} = \frac{2 e}{h} V \cdot s$
(2)
per Josephson junction. This is about 2 microvolt / GHz, where GHz is a practical input frequency. Video "The evolution of voltage metrology to the latest generation of JVSs by Alain Rüfenacht" mentions that a typical operating frequency is 20 GHz.
Therefore to attain a good 10 V, we need something in the order of a million Josephson junctions.
But this is possible to implement in a single chip with existing micro fabrication techniques, and is exactly what the Josephson voltage standard does!

Those effect work because they also involve dividing by the Planck constant, the fundamental constant of quantum mechanics, which is also tiny, and thus brings values into a much more measurable order of size.

Ampere in the 2019 redefinition of the SI base units

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