A car spark plug firing. Spark plugs are used in petrol engines to ignite the compressed fuel/air mixture in the cylinders. The power from the car's battery, normally 12 volts, is stepped up to around 18,000 volts by an induction coil. This is then sufficient to discharge through the mixture. Although the spark gap used depends on the engine model, most use a gap of around 0.7mm. The spark plug seen here is a Champion N-9Y, compatible with most saloon cars. Close up version of T860/058.