An RF ablation system (10) includes a catheter (12) having a lumen therein and a first electrode (18). A second electrode (20) is disposed around the catheter (12) proximate the distal end (14) of the catheter (12) and spaced from the first electrode (18) by an insulating spacer which may be a part (36) of the catheter (12) or a sleeve (42) on the first electrode (18). The first electrode (18) is able to slide in and out of the catheter (12) in order to vary the exposed length of the first electrode (18) and as a result to alter the energy density at the first electrode (18) in order to exhibit different heating effects. A control unit (24) controls current through the electrodes (18, 20) and also the exposed length of the first electrode (18). The second electrode (20) has a surface area at least twice that of the first electrode (18) in order to act as a dispersive of electrode. As a consequence, negligible heat is generated at the second electrode (20) and therefore no blood clotting or other ablation material is formed over the catheter (12).