To facilitate assembly of a central glass rod in an optical sensor-spark plug combination, electrical energy is conducted to the spark end of the combination by a high temperature resistant tube or sleeve 11 surrounding a central glass rod 2 for only a limited portion of the entire length; electrical energy is conducted to the high temperature resistant sleeve 11, which forms the sparking end of the spark plug, from a terminal connector 1 by a packing 10 of electrically conductive powder, which may include radio noise suppression resistance characteristics, held in place by a compression spring 8, in electrical connection with the terminal connector 1; or by a separate tube 12 surrounding the glass rod 1, and of non-heat resistant material, the glass rod 1 being separated from the other tube 12 by an adhesive which, at the connecting end of the spark plug, need not be high temperature resistant.