2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/170267We 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.9 pages, 10 figures, Fig.1 is corrected, figures in png files, short version is published in Am. J. Phys. 76, (2008) 129Physics Educationtext