stiffening of an otherwise flexible shaft 16 typically during access the sheath 38 will be placed in a forwardly advanced position to provide a rigid structure which is more controllable during subcutaneous manipulation and advancement over a guidewire or through a cannula where flexibility is not required and can even be a disadvantage after positioning a distal end 18 of the shaft 16 at the desired treatment location the external sheath 38 can be partially or fully withdrawn to expose a distal length of the flexible shaft 16 to allow further advancement into the has or to simply remove the rigid structure during treatment or while external compression is used to manipulate the device tip into contact with the has wall the first and second electrodes 24 and 26 are illustrated as generally spherical or toroidal electrodes as defined above the flexible body 16 which is typically formed from a polymer or other electrically insulating material acts to isolate the electrodes and provide the desired axial spacing also as discussed above while the electrodes are illustrated as spherical or toroidal a variety of other specific designs may used under different circumstances as will be discussed below referring now to fig2 and 3 a first specific electrode design comprising a first electrode 44 in the form of a ring which is typically toroidal with a very flat surface and a second electrode 46 which is generally spherical or toroidal as defined above the first and second electrodes are disposed at the distal end of a polymeric shaft or.