In 1984 Yamaoka etal. 9 reported the "relative values of frequency response method." They applied a 1 kHz rectilinear wave to the canal; they then picked up the 1 kHz basic wave and the 5 kHz modulated wave with two filters, rectified those two waves, and ohtained the two direct potentials. They could detect the position of the apical constriction by calculating the difference between these two potentials in electroconductive conditions of the root canals.