Abstract: In a process for etching aluminum foil to increase its actual surface area to apparent surface area for use as an electrode in an electrolytic capacitor, a wire coil is placed adjacent the foil as it exits the etch station. The coil is energized to produce eddy currents in the foil. The voltage induced by theses eddy currents is sensed at the same coil or another coil. The foil drawing speed through the etch station is altered in response to that sensed voltage, or another process parameter is changed, to maintain at a constant value the ratio of the actual to apparent foil surface area.