A system comprising: a catheter including an elongated carrier, a balloon about the carrier in sealed relation thereto, the balloon being arranged to receive a liquid therein that inflates the balloon, and first and second electrodes (22,24) within the balloon; a power source including a capacitor and a switch, and the power source being connected to the first and second electrodes (22,24); the power source being configured to: deliver voltage pulses to the first and second electrodes (22,24), each voltage pulse being generated by closing the switch that causes a charge stored on the capacitor to be delivered to the first and second electrodes (22,24), each voltage pulse having sufficient energy to generate an arc in the liquid within the balloon and allowing current to flow across the first and second electrodes (22,24) to produce a shock wave associated with the expansion of a steam bubble, and characterised by; the power source being configured to 2. sense the current flowing across the first and second electrodes (22,24) during each voltage pulse; and terminate the delivery of each of the voltage pulses across the first and second electrodes (22,24) after the sensed current reaches a predetermined limit, the predetermined limit being high enough to ensure the creation of the arc for each pulse, wherein the termination includes opening the switch in order to prevent the remaining charge on the capacitor from being delivered to the first and second electrodes (22,24) thereby conserving electrode material. a catheter including an elongated carrier, a balloon about the carrier in sealed relation thereto, the balloon being arranged to receive a liquid therein that inflates the balloon, and first and second electrodes (22,24) within the balloon; a power source including a capacitor and a switch, and the power source being connected to the first and second electrodes (22,24); the power source being configured to: deliver voltage pulses to the first and second electrodes (22,24), each voltage pulse being generated by closing the switch that causes a charge stored on the capacitor to be delivered to the first and second electrodes (22,24), each voltage pulse having sufficient energy to generate an arc in the liquid within the balloon and allowing current to flow across the first and second electrodes (22,24) to produce a shock wave associated with the expansion of a steam bubble, and characterised by; the power source being configured to 2. sense the current flowing across the first and second electrodes (22,24) during each voltage pulse; and terminate the delivery of each of the voltage pulses across the first and second electrodes (22,24) after the sensed current reaches a predetermined limit, the predetermined limit being high enough to ensure the creation of the arc for each pulse, wherein the termination includes opening the switch in order to prevent the remaining charge on the capacitor from being delivered to the first and second electrodes (22,24) thereby conserving electrode material. deliver voltage pulses to the first and second electrodes (22,24), each voltage pulse being generated by closing the switch that causes a charge stored on the capacitor to be delivered to the first and second electrodes (22,24), each voltage pulse having sufficient energy to generate an arc in the liquid within the balloon and allowing current to flow across the first and second electrodes (22,24) to produce a shock wave associated with the expansion of a steam bubble, and characterised by; the power source being configured to 2. sense the current flowing across the first and second electrodes (22,24) during each voltage pulse; and terminate the delivery of each of the voltage pulses across the first and second electrodes (22,24) after the sensed current reaches a predetermined limit, the predetermined limit being high enough to ensure the creation of the arc for each pulse, wherein the termination includes opening the switch in order to prevent the remaining charge on the capacitor from being delivered to the first and second electrodes (22,24) thereby conserving electrode material.