Ushiyama presented an alternative method that could accurately measure the canal even in the presence of strong electrolytes. This method uses a concentric bipolar electrode that measures the current density evoked in a very limited area of the canal when a 400 Hz constant current is flowing through the canal. In this method when the bipotar electrode is located at the apical constriction, the maximum potential is obtained, because the current density is maximum at the apical constriction