in individual cases it may be advantageous to carry out the introduction of current into the wire 71 under for example a protective gas atmosphere in order to keep inflammable gases in the hollow organs away from the region of action of the electrodes this is especially advantageous when arcs l are to be used cooling fluids rinsing liquids or other fluids may also be supplied via the lumen 14 this allows for example the electrode region or the distal end 11 of the instrument 10 to be cooled by means of the cooling fluid thereby avoiding overheating of the distal end of the instrument and resulting damage to tissue surrounding the stent 70 according to fig1 a control means 50 is provided for controlling the current and or arc by activating for example the power source the control means includes a current monitor and or an arc monitor the controller allows the current to be controlled or regulated such that the operator does not have to make decisions in this regard and instead the course of the machining is carried out in an optimized manner after detection of an arc it is possible to set a defined time period over.