Impedance of free space
The impedance of free space, , is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, , where is the electric field strength and is the magnetic field strength. Its presently accepted value is 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 and the speed of light in vacuum . 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 retains an exactly defined value. Terminology. The analogous quantity for a plane wave travelling through a dielectric medium is called the "intrinsic impedance" of the medium, and designated (eta). Hence is sometimes referred to as the "intrinsic impedance of free space", and  given the symbol . It has numerous other synonyms, including: Relation to other constants. From the above definition, and the plane wave solution to Maxwell's equations, formula_1 where The reciprocal of is sometimes referred to as the "admittance of free space" and represented by the symbol . Historical exact value. Between 1948 and 2019, the SI unit the ampere was defined by "choosing" the numerical value of to be exactly . Similarly, since 1983 the SI metre has been defined relative to the second by "choosing" the value of to be . Consequently until the 2019 redefinition, or This chain of dependencies changed  when the ampere was redefined on 20 May 2019. Approximation as 120π ohms. It is very common in textbooks and papers written before about 1990 to substitute the approximate value 120 ohms for . This is equivalent to taking the speed of light to be precisely in conjunction with the then-current definition of as . For example, Cheng 1989 states that the radiation resistance of a Hertzian dipole is This practice may be recognized from the resulting discrepancy in the units of the given formula. Consideration of the units, or more formally dimensional analysis, may be used to restore the formula  to a more exact form, in this case to Further reading.