Alternatively, a layer of a conducting material such as SnO2 is deposited on an inert substrate, e.g., glass, and then followed by deposition of a layer of Ir onto a portion of the previously deposited SnO2. (Again, these depositions are done by conventional techniques such as electron beam evaporation for Ir and sputtering for SnO2.) After oxidation of the Ir, contact is made to the uncoated port