In ignition coils, coil B is wound on top of coil A. Coil A, called the _primary winding_, consists of a few layers of coarse wire. The more turns of wire there are in coil B, called the _secondary winding_, the more intense will be the current that it produces, and the intensity is also increased by keeping the windings close to the iron core. The secondary winding is, therefore, made of exceedingly fine wire, and has a very great number of turns. To obtain a spark, a current is permitted to flow through the primary winding to create magnetism, and the flow is then stopped to cause the magnetism to die away. The secondary winding is affected by each of these changes in magnetic strength. The bar loses magnetism more rapidly than it gains it, however; it is therefore the dying out of the magnetism that has the greater effect on the secondary winding, and that causes it to produce a sparking current. To use this principle for ignition, the engine is fitted with a revolving switch, which closes the circuit as a piston is on the compression stroke, and then breaks the circuit at the instant when a spark is desired. Combined with the revolving switch, or _timer_, is a distributor like the distributor of a magneto, which passes the sparking current to the cylinder that is ready to receive it. [Illustration: FIG. 57.—“ATWATER-KENT” IGNITION SYSTEM] To produce an intense sparking current, it is necessary to break the circuit as abruptly as possible, in order to cause the magnetism to die away suddenly. Figure 57 shows how this is done in the Atwater-Kent system. The parts of the circuit breaker are carried on a plate, in the center of which revolves a shaft with a notch in it. Against the side of this shaft rests the hooked end of the sliding catch; as the notch comes under this hooked end, the sliding catch is drawn forward, only to be snapped back by its spring as the notch moves from under it. The lifter is a bit of metal, pivoted at one end, with its free end lying between the sliding catch and the flat steel spring that carries one of the contact points. A, Figure 57, is a diagram of the system. B shows the position of the parts as the notch carries the sliding catch forward, and C shows their positions as the spring snaps the sliding catch back to its place. It will be seen that in thus moving back it strikes the lifter, which in turn moves the contact spring, and so closes the circuit; but the circuit is instantly broken as the parts spring back to position. The movement of the parts is so rapid that to the eye they seem to be standing still. The circuit is closed only for an instant, but that is sufficient to magnetize and demagnetize the coil, and to produce a sparking current.