made by a cast tape process that comprises casting a titanium dioxide slurry onto a plastic carrier tape evaporating the vehicle from the slurry stripping the plastic tape and punching discs from the remaining green ceramic layer the process is given in greater detail hereinbelow resistance wire 36 typically is made of platinum alloyed with about 13 rhodium and is about 0 008 inches in diameter electrodes 38 and 40 typically are made of platinum and are about 0 008 inches in diameter thermocouple 42 is a gold palladium platinum and gold palladium combination the green plates resistance wire electrodes and thermocouple are sandwiched together as shown and isostatically pressed after pressing the assembly is fired to form a unitary body the resulting disc is installed in the exhaust conduit of a reciprocating internal combustion engine where exhaust gases will heat the disc to about 700 u00b0 c when the engine is supplied with an air fuel mixture of about 13 1 the resistance across the electrodes is about 5 ohms changing the air fuel to 14 1 without changing any other engine parameters increases the resistance to about 10 ohms an air fuel ratio of 15 1 produces a resistance of 20 000 ohms it will be appreciated that these values are intended to be illustrative and that specific values will depend on the configuration of the ceramic body.