A process for differentiating conductive and/or ferromagnetic objects (O) in a material stream (2) comprises generating an electromagnetic alternating field by exciting a coil (S) with a sinusoidal voltage (ue(t)) of a constant frequency (fM), detecting an impedance change in the coil, which has been caused by an object (O), by determining at least one pair of values from a peak value (ÎM) and a phase shift (Φm) of the coil current (iM(t)) toward the excitation potential (ue(t)) of the coil, and determining the material by comparing the peak values (ÎM) and phase shifts (Φm) with reference values, wherein the peak values (ÎM) of the coil current (iM(t)) are calculated at at least one measuring phase angle (Φm) with the aid of a window comparator having a constant window width (Δi), wherein the time (ΔtM) between the window inlet point (p4) and the window outlet point (p6) of the coil current (iM(t)) is measured and the gradient of the current profile (iM(t)) is calculated from the window width (Δi) and the measured time (ΔtM) and the peak value (ÎM) is calculated from the gradient of the sinusoidal current profile iM(t) and its frequency (fM).