A method, consisting of providing a metal wire having a wire diameter and an end, and positioning a conductor at a distance from the end of the wire. The method further includes creating an electrical discharge between the conductor and the end, while setting the distance and an electrical potential of the discharge, so as to create a bead of a predefined size on the end. The method also includes assembling the wire with the created bead into an invasive probe, so that the bead is positioned at an outer surface of the probe.