Definition:Impedance of Free Space/Historical Note

Historical Note on Impedance of Free Space

Prior to the redefinition of the SI units in $2019$, the impedance of free space was defined as $\mu_0 c$, where:

$\mu_0$ is the vacuum permeability, defined as (exactly) $4 \pi \times 10^{-7}$ henries per metre

$c$ is the speed of light, defined as (exactly) $299 \, 792 \, 458$ metres per second

leading to a value of:

$Z_0 := 4 \pi \times 29 \cdotp 97924 \, 58 \, \Omega$

that is:

$Z_0 := \pi \times 119 \cdotp 91698 \, 32 \, \Omega$

exactly.

This works out as:

$Z_0 \approx 376 \cdotp 73031 \, 34617 \, 7 \ldots \Omega$

Also, some older books define $Z_0$ to be:

$Z_0 = 120 \pi \, \Omega$

based on:

the above value of $\mu_0$ of $4 \pi \times 10^{-7}$ henries per metre

the popular approximation to $c$ of $3 \times 10^8$ metres per second.