Editing Magnetic flux quantum (section)

== Measuring the magnetic flux ==
Prior to the [[2019 revision of the SI]], the magnetic flux quantum was measured with great precision by exploiting the [[Josephson effect]]. When coupled with the measurement of the [[von Klitzing constant]] {{math|1=''R''<sub>K</sub> = ''h''/''e''<sup>2</sup>}}, this provided the most accurate values of the [[Planck constant]] {{math|''h''}} obtained until 2019. This may be counterintuitive, since {{math|''h''}} is generally associated with the behaviour of microscopically small systems, whereas the quantization of magnetic flux in a superconductor and the [[quantum Hall effect]] are both [[emergent phenomena]] associated with [[thermodynamics|thermodynamically]] large numbers of particles.

As a result of the [[2019 revision of the SI]], the Planck constant {{math|''h''}} has a fixed value {{nowrap|1={{math|''h''}} = {{physconst|h|after=,}}}} which, together with the definitions of the [[second]] and the [[metre]], provides the official definition of the [[kilogram]]. Furthermore, the [[elementary charge]] also has a fixed value of {{nowrap|1={{math|''e''}} = {{physconst|e}}}} to define the [[ampere]]. Therefore, both the Josephson constant {{math|1=''K''<sub>J</sub> = 2''e''/''h''}} and the von Klitzing constant {{math|1=''R''<sub>K</sub> = ''h''/''e''<sup>2</sup>}} have fixed values, and the Josephson effect along with the von Klitzing quantum Hall effect becomes the primary ''mise en pratique''<ref>{{cite web |url=https://www.bipm.org/en/publications/mises-en-pratique/ |title=BIPM – mises en pratique|website=www.bipm.org|access-date=2020-01-21}}</ref> for the definition of the ampere and other electric units in the SI.