A glow plug is a heating device used to aid starting of a diesel engine in cold weather). Unlike common spark-ignition engines, diesel engines do not use spark plugs to ignite the air-fuel mixture. Instead, they rely solely on compression to raise the temperature of the air to a point where the fuel combusts spontaneously when introduced to the hot, high-pressure air). A glow plug is a pencil-shaped piece of metal with an electric heating element at the tip). Glow plugs are typically used as starting-aid devices when the engine is being cranked by a starter motor, and in most cases, switched off when the engine has reached a defined minimum operating temperature). Glow plugs resemble short metal pencils, and the heating element is fitted into their tips). Glow plug filaments must be made of certain materials, such as platinum and iridium, that resist oxidation and high temperature). Glow plugs are different from spark plugs, which are responsible for creating the spark that ignites the fuel/air mixture in the combustion chamber of gasoline vehicles.