The main drawback of this "voltage gradient method" is that it uses a special bipolar electrode that will not fit into a narrow canal, and this electrode also makes it impossible to measure and enlarge the canal in a single procedure. Because this method detects a constriction of the canal, the meter identifies any constriction in the middle of the canal as an apical constriction. If the canal has no constriction, the meter cannot indicate anything at all.