Apparatus comprises: (a) a longitudinal member (32), having a distal portion (34)； (b) a plurality of electrodes (38) disposed on the distal portion of the longitudinal member, such that a first electrode (38a) of the plurality of electrodes is disposed distally along the longitudinal member from a second electrode (38b) of the plurality of electrodes； and (c) a controller (40). The controller comprises an actuator, and circuitry (42) electrically connected to the electrodes via the longitudinal member. The actuator is configured to move the longitudinal member in discrete incremental movements such that for each incremental movement, (i) before the incremental movement the first electrode is disposed in a starting position, (ii) during each incremental movement the actuator moves second electrode toward the starting position, and (iii) at the end of each incremental movement the second electrode is stationary at the starting position.
