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).