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We present a simple method to determine the mutual inductance $M$ between twocoils in a coupled AC circuit by using a digital dual-phase lock-in amplifier.The frequency dependence of the real and imaginary parts is measured as thecoupling constant is changed. The mutual inductance $M$ decreases as thedistance $d$ between the centers of coils is increased. We show that thecoupling constant is proportional to $d^{-n}$ with an exponent $n$ ($\approx$3). This coupling is similar to that of two magnetic moments coupled through adipole-dipole interaction.Comment: 9 pages, 10 figures, Fig.1 is corrected, figures in png files, short  version is published in Am. J. Phys. 76, (2008) 12

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