Resonant methods for the measurement of electrical impedance were quite common in the past.
A once common instrument that allows resonant impedance measurements is the $Q$-meter. A $Q$-meter is an instrument capable of measuring the quality factor $Q$ of a resonant circuit.
The basic schematic of a $Q$-meter is the following (from ):
The instrument is composed of: i) a voltage source with adjustable frequency and very low output resistance $R$; ii) a calibrated variable capacitor $C_v$; and iii) a voltmeter with with very high input impedance whose scale is calibrated in terms of $Q$.
The measurement is carried out by substitution. Suppose, for instance, that you want to measure an unknown capacitance $C_x$ at a certain frequency $f$. You proceed as follows:
- Adjust the source frequency to $f$.
- Connect an inductor with suitable value to the instrument so that, together with the variable capacitor, it forms a resonant circuit.
- Adjust $C_v$ until the voltmeter reads the maximum value (resonance). Record the value of the calibrated capacitance ($C_1$).
- Connect the unknown capacitor $C_x$ to the instrument in parallel to the variable capacitance (parallel insertion).
- Adjust $C_v$ until you attain resonance again. Record the value of the calibrated capacitance ($C_2$).
Since you didn't change the source frequency and the inductance, the total capacitance is not changed between points 3 and 5. Thus, the unknown capacitance is given by
$$C_x = C_1-C_2.$$
Furthermore, from the $Q$ readings, you can obtain the capacitor's loss.
A dual procedure, with series insertion, can be used for inductance measurements.
An example of $Q$-meter: Boonton Q-Meter Type 160-A.
 E. Rubiola, Laboratorio di misure elettroniche, CLUT, Torino, 1993.