Abstract: A method and device is described for removing concretions such as calculus stones within human ducts such as the ureter or kidney. The device includes a flexible probe insertable through the human duct so that a tip thereof is juxtaposed against the concretion. The probe includes a positive electrode extending coaxially within the conduit and embedded in a solid electrically insulative material. A negative electrode is coextensive with and outwardly encircles the positive electrode. The solid electrically insulative material stops short by a distance R from the end of the probe, thereby leaving a recess without solid insulation between negative and positive electrodes at the tip end of the probe. The recess achieves a more efficient fragmentation of target calculi at lower energy, with less human tissue damage and increase in useful probe life.