In accumulator ignition the low-tension current from the accumulator passes through an induction coil, and is thus transformed to high-tension current. This current goes through a sparking plug, which is fixed in the head of the cylinder. The sparking plug contains two metal points separated by a tiny air gap of from about 1/30 to 1/50 inch. This gap provides the only possible path for the high-tension current, so that the latter leaps across it in the form of a spark. The spark is arranged to take place when the piston is at the top of its stroke, that is, when the explosive mixture is at its maximum compression, and the heat of the spark ignites the mixture, the resulting explosion forcing down the piston with great power. In practice it is found better as a rule to cause the spark to pass very slightly before the piston reaches the extreme limit of its stroke. The reason of this is that the process of igniting and exploding the charge occupies an appreciable, though of course exceedingly small amount of time. Immediately on reaching the top of its stroke the piston begins to descend again, and if the spark and the top of the stroke coincide in time the explosion does not take place until the piston has moved some little distance down the cylinder, and so a certain amount of power is lost. By having the spark a little in advance of the piston, the explosion occurs at the instant when the piston begins to return, and so the full force of the explosion is utilized. In magneto ignition the current is supplied by a small dynamo. This generates alternating current, and it is driven by the car engine. The current is at first at low pressure, and it has to be transformed to high-tension current in order to produce the spark. There are two methods of effecting this transformation. One is by turning the armature of the dynamo into a sort of induction coil, by giving it two separate windings, primary and secondary; so that the dynamo delivers high-tension current directly. The other method is to send the low-tension current through one or more transformer coils, just as in accumulator ignition. Accumulators can give current only for a certain limited period, and they are liable consequently to run down at inconvenient times and places. They also have the defect of undergoing a slight leakage of current even when they are not in use. Magneto ignition has neither of these drawbacks, and on account of its superior reliability it has come into universal use.