where Ω is the ohm, the SI unit of electrical resistance. The impedance of free space (that is, the wave impedance of a plane wave in free space) is equal to the product of the vacuum permeability μ 0 and the speed of light in vacuum c 0. Before 2019, the values of both these constants were taken to be exact (they were given in the definitions of the ampere and the metre respectively), and the value of the impedance of free space was therefore likewise taken to be exact. However, with the redefinition of the SI base units that came into force on 20 May 2019, the impedance of free space is subject to experimental measurement because only the speed of light in vacuum c 0 retains an exactly defined value.