However, Meredith & Gulabivala (1997) reported there is neither constant reference impedance nor constant resistance for all root canals. The reason that, for example, resistance-based devices work in a reasonable number of cases is that there is a substantial difference between the resistance (or the impedance) value at the pulp and periodontal junction compared with intracanal positions. Indeed, this is the property Custer (1918) described many years ago.