According to equation 3, the magnitude of the capacitance of the model is proportional to the distance between the two nodes shown in Fig. 6. That means,of the capacitance sharply increases probably because of change in the morphology of the apical portion of the root. On the other hand, the fH frequency used in this device is five times the fL value. Therefore, according to
equation 4, the change in Z(fL) will be five times larger than Z(fH), i.e. the difference between two Z(fL) and Z(fH) impedances rapidly increases at the ‘apical foramen’. This method has been used in the Apit device (Osada, Tokyo, Japan).