  We present a simple method to determine the mutual inductance $M$ between two
coils 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 the
coupling constant is changed. The mutual inductance $M$ decreases as the
distance $d$ between the centers of coils is increased. We show that the
coupling constant is proportional to $d^{-n}$ with an exponent $n$ ($\approx$
3). This coupling is similar to that of two magnetic moments coupled through a
dipole-dipole interaction.
