The constants c and µ o were both defined in SI units to have exact numerical values until the 2019 redefinition of the SI base units. Therefore, until that date, ε o could be also stated exactly as a fraction, 1 c 2 μ μ 0 = 1 35 950 207 149.472 7056 π π F/m even if the result was irrational (because the fraction contained π). In contrast, the ampere was a measured quantity before 2019, but since then the ampere is now exactly defined and it is μ o that is an experimentally measured quantity (with consequent uncertainty) and therefore so is the new 2019 definition of ε o (c remains exactly defined before and since 2019).