 Impedance magnitude

The impedance magnitude of a linear electrical circuit in sinusoidal steady state is the ratio of voltage amplitude to electric current amplitude. For complex impedance $Z=R+iX$, it is $|Z|=\sqrt{R^2+X^2}$, whereas the dissipative electrical resistance is $\mathrm{Re}Z=R$. Measuring only amplitudes does not separately determine $R$ and $X$; a phase shift measurement supplies the missing information.