The determination of the effective magnetic susceptibility of a conductor was read by a differential transformer using a compensation circuit with a lock-in amplifier as the null detector. The real and imaginary parts of the susceptibility were measured versus the frequency of the magnetic field.
At the equilibrium, the coil without a core and the electromotive force detected by the null detector in an alternating current bridge are equal to zero.
Wideband LCR meters and impedance analyzers employ the I-V converter consisting of a sophisticated null detector, phase detector, integrator (loop filter), and vector modulator to ensure a high accuracy for a broad frequency range." (1)
This technique is known as the I-V method in which both voltage and current are directly measured without the need for a null detector or feedback loop.