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.
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Schauber et al. (2008) studied this question.