{tag=2019 redefinition of the SI base units}
{wiki}

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. https://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> <electrical resistance>[resistances] of type:
  $$
  R_{xy} = \frac{V_\text{Hall}}{I_\text{channel}} = \frac{h}{e^2\nu}
  $$
  for integer values of $\nu$.
* <Josephson effect>, used in the <Josephson voltage standard>. With the <Inverse AC Josephson effect> we are able to produce:
  $$
  K_{J} = \frac{2e}{h} V \cdot s
  $$
  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

ID: ampere-in-the-2019-redefinition-of-the-si-base-units