<P> As the current of electrons surges across the gap, it raises the temperature of the spark channel to 60,000 K . The intense heat in the spark channel causes the ionized gas to expand very quickly, like a small explosion . This is the "click" heard when observing a spark, similar to lightning and thunder . </P> <P> The heat and pressure force the gases to react with each other, and at the end of the spark event there should be a small ball of fire in the spark gap as the gases burn on their own . The size of this fireball, or kernel, depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark . A small kernel will make the engine run as though the ignition timing was retarded, and a large one as though the timing was advanced . </P> <P> A spark plug is composed of a shell, insulator and the central conductor . It passes through the wall of the combustion chamber and therefore must also seal the combustion chamber against high pressures and temperatures without deteriorating over long periods of time and extended use . </P> <P> Spark plugs are specified by size, either thread or nut (often referred to as Euro), sealing type (taper or crush washer), and spark gap . Common thread (nut) sizes in Europe are 10 mm (16 mm), 14 mm (21 mm; sometimes, 16 mm), and 18 mm (24 mm, sometimes, 21 mm). In the United States, common thread (nut) sizes are 10mm (16mm), 12mm (14mm, 16mm or 17.5 mm), 14mm (16mm, 20.63 mm) and 18mm (20.63 mm). </P>

In si engine high voltage for spark plug is developed using