Measurement of mutual inductance from the frequency dependence of impedance of AC coupled circuits using a digital dual-phase lock-in amplifier

We present a simple method for determining the mutual inductance between two coils in a coupled AC circuit using a digital dual-phase lock-in amplifier. The frequency dependence of the real and imaginary parts is measured as the coupling is changed. The mutual inductance decreases as the distance d between the centers of coils is increased. We show that the coupling is proportional to d − n with n ≈ 3 . This coupling is similar to that of two magnetic dipoles.