A catheter is adapted to exchange heat with a body fluid, such as blood, flowing in a body conduit, such as a blood vessel. The cradle arm is adjustably connecting the cradle arm support member, for selectively fixing the arm to the cradle arm support member in one of a range of vertical positions to accommodate changing water levels. The anode comprises first and second strips positioned on opposite sides of the cathode, which is also in the form of a strip, but one that is much longer than each of the anode strips. Further, there preferred embodiments do not rely on complex, pre-determined hunting sequences or expensive adjunct customer premises equipment. Light from a separate illumination source may pass through the transparent stepped waveguide optical valve to provide at least one further additional illumination function. By means of subsequent in-line or off-line method steps, the film, for example, by means of drawing with a drawing degree of 1.