in order to obviate the necessity for design compromise of spark plugs with respect to their operating temperature , so that the ceramic insulator of the spark plug will rapidly reach the temperature at which deposits thereon will inherently burn off while not becoming so hot as to cause spurious glow ignition within the combustion chamber of an internal combustion engine , the end portion of the insulator is formed with a central opening within which a metal core is included which has a temperature coefficient of expansion such that , at temperatures below between 400 ° c .- 450 ° c . it is spaced by a narrow gap from the inner surface of the insulator but , as the spark plug becomes hot , the gap 25 closes , thereby providing good heat transmission from the insulator to the center electrode structure and thereby maintaining the insulator at a temperature below that at which it might glow . suitable materials for the center electrode are aluminum bronze , or other materials having a heat transmission characteristic of at least 90 w / mk . the insulator , preferably , has a higher - than - usual flux content so that , at low temperatures , its heat transmission characteristics are poor , to insure rapid heating to free combustion temperature of possible deposits .