An internal combustion engine includes at least one cylinder, at least one feed conduit for combustion gas, and at least one exhaust gas conduit for carrying exhaust gas away from the internal combustion engine. The substance fluoresces in response to ambient light and the flouresced light is subject to internal reflection within the rob, with the flouresced light being substantial emitted at one or both ends of the rod. The layers of insulator 25 prevent the sensor electrodes 29, 31 from being electrically connected, by splashes of conductive ink, to the deflection electrode provided by the conductive layer 21. The resistance changes of the device are retained after applied electric pulses.