a spark jumps the gap between the spark-plug electrodes
1spark plug — A device containing two electrodes across which electricity jumps to produce a spark to fire the fuel charge. The center electrode is insulated from the spark plug shell by means of a molded insulator resembling porcelain. The side electrode… …
2Spark gap — A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air. When a suitable voltage is supplied, a spark forms, ionizing the gas and drastically reducing its electrical resistance …
