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