Based on this fact, Ushiyama (1983) proposed a method to measure the variation in impedance when a file was inserted into the root canal. Using bipolar electrodes and applying a 400-Hz alternating current, this device monitored variations in the impedance value. Ushiyama (1983) concluded that a sharp variation in the value, determined the position of the file at the apical constriction, the narrowest portion of the root canal system. Ushiyama (1983) also reported that in the presence of strong electrolytes the ‘voltage gradient method’ could accurately detect the apical constriction. However, use of a special bipolar electrode is one of the main disadvantages of this device, as the electrode will not fit into narrow canals. No commercial product of the experimental device developed by Ushiyama (1983) appears to be available.