The preferred embodiment provides apparatus for ablating a vessel by generating heat within a vessel to cause blood clotting to occlude the vessel. The apparatus includes an electrically resistive element at the distal end of the apparatus, the resistive element being uncoated and therefore in bare form. An alternating current supply feeds electrical current through the resistive element to cause heating of the element and surrounding blood or tissue. The alternating current power supply preferably is balanced about a centre point so as to create a net sum of alternating pulses of zero. It has been found that the supply of alternating current to the resistive element reduces or eliminates corrosion of the resistive element in blood as the resistive element is heated. Use of conductive coatings for a sacrificial anode can be avoided.