The amount of energy in the spark required to ignite the air-fuel mixture varies depending on the pressure and composition of the mixture, and on the speed of the engine. Under laboratory conditions as little as 1 millijoule is required in each spark, but practical coils must deliver much more energy than this to allow for higher pressure, rich or lean mixtures, losses in ignition wiring, and plug fouling and leakage. When gas velocity is high in the spark gap, the arc between the terminals is blown away from the terminals, making the arc longer and requiring more energy in each spark. Between 30 and 70 milli-joules are delivered in each spark.