In B, the armature has revolved a little. Most of the magnetism is still flowing through the core, but some of it is finding an easier path by flowing through the heads and across the air space to the other pole. The magnetism of the core is, therefore, a little weaker than it is in A. In C, the heads alone form bridges between the poles, and none of the magnetism flows by the core because that no longer forms a path. The core is no longer producing magnetism; in moving from A to C there has thus been a complete change in the strength of the magnetism of the core, for from full strength it has died away to nothing. By a further movement, as in D, the core again acts as a bridge, and another change in strength occurs, this time from nothing to full strength again. In moving from D to B, there are slight changes in strength, but not enough to produce a sparking current; it is only in passing from B to D that a sparking current can be produced. In this type of magneto the space between the heads is wound full of wire, which of course revolves with the armature; the more turns of wire there are, the more intense will be the current, so very fine wire is used to get the greatest possible number of turns. In the Bosch magneto the first few layers are of coarse wire, and are the _primary winding_. The remainder, called the _secondary winding_, is very fine wire, and the two are connected so that one forms an extension of the other. It has been explained that it is most important to have the spark occur at exactly the right instant in the stroke. On a magneto the instant of sparking is controlled by a _timer_, or _circuit breaker_, which is a switch that is automatically operated at the time when the magneto is producing a current sufficiently intense to form a spark. Figure 44 illustrates one complete revolution of the armature, and it will be seen that it passes twice from position B to position D. This shows that it gives a sparking current twice during each revolution. The circuit breaker must therefore operate twice during each revolution. It is placed at the end of the magneto; in some makes it revolves with the armature and is operated by stationary cams, while in others it is stationary, and is operated by a cam on the armature shaft. In either case the effect is the same. [Illustration: FIG. 44.—ONE COMPLETE REVOLUTION OF THE ARMATURE] [Illustration: FIG. 45.—CONNECTIONS OF BOSCH MAGNETO] Figure 45 shows the way in which the winding on the armature of a Bosch magneto is connected with the circuit breaker and with the armature. The circuit breaker shown is not the kind used on the Bosch, and serves only to illustrate the principle. It consists of a lever pivoted at one end, with the other end resting against the tip of a screw. A cam bears against the lever and can move it to break the contact with the screw. The cam is so set that it moves the lever at the time when the current is most intense.