An ignition system consists of: First, the apparatus that produces the electric current, which is usually a _magneto_; second, a _timer_, which controls the instant at which the spark occurs; third, the _spark plugs_, which project into the cylinders, and at which the sparks take place; fourth, a _switch_, by which the sparking current can be turned on or off, and fifth, the wires, or _cables_, by which the parts are connected. The electric current that gives the spark is always produced by magnetism. In a magneto, magnetism is obtained from the heavy steel magnets that are part of it; there is a constant flow of magnetism from one end of these to the other. To obtain an electric current, a coil of wire is placed in the magnetism, and the strength of the magnetism is made to change; it alternately becomes weak and strong. Whenever a change in strength takes place, an electric current flows in the wire, and it continues to flow as long as the magnetism continues to change in strength. When the change in strength is very great, that is, when the magnetism changes from very weak to very strong, or from very strong to very weak, the electric current is more powerful than when there is only a little change in strength. A more powerful current is also produced by a change that takes place suddenly than by a change that takes place slowly. The electrical principle that produces a current in this manner is called _induction_; the current produced is known as an _induced_ current. A magneto has two or more magnets, and between their ends, or _poles_, there revolves a piece of iron called the _armature_. A piece of iron placed between the poles of a magnet becomes a magnet itself; the armature is so shaped that, as it revolves, its magnetism continually changes in strength, and it is the changes in the strength of the magnetism of the armature that produce the sparking current. [Illustration: FIG. 42.—ARMATURE] The iron armature of the Bosch magneto, which is the best known type, is shown in Figure 42. It has a central bar with two heads, the wire being wound around the central bar, or _core_. The shafts on which it revolves are attached to the ends of the heads. Figure 43 shows different positions of the armature between the poles of the magnet, and illustrates the changes in the magnetism of the central bar. There is a continual flow of magnetism from one pole of a magnet to the other; if a piece of iron lies between them the magnetism will use it as a bridge, but often its easiest path will be through the air. In A, Figure 43, the armature lies crossways, and its central bar or core forms a perfect bridge for the magnetism. Practically all of the magnetism flows through it, and it then becomes a powerful magnet itself. It sets up its own flow of magnetism, which flows through the core to one head, through the air to the other head, and so back to the core. [Illustration: FIG. 43.—FLOW OF MAGNETISM THROUGH ARMATURE CORE]