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

TODO how does basing it on the <elementary charge> help at all? Can we count individual electrons going through a wire? https://www.nist.gov/si-redefinition/ampere/ampere-quantum-metrology-triangle by the <NIST> explains that is it basically due to the following two <quantized (physics)> <solid-state physics> phenomena/experiments that allows for extremely precise measurements of the <elementary charge>:
* <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>, which provides the <Josephson constant> which equals:
  $$
  K_{J} = \frac{2e}{h}
  $$


Ampere in the 2019 redefinition of the SI base units