In practically all makes of magnetos the circuit breaker is at the end of the armature or inductor shaft, and is operated by it. The Bosch circuit breaker is illustrated in Figure 50, the parts being mounted on a plate attached to the shaft and revolving with it. The lever is L-shaped, pivoted at the angle, with one end resting on the tip of a screw. When the shaft revolves, the other end of the lever is dragged over a block of metal that acts as a cam; this makes it move on its pivot and separates it from the screw. By turning the screw the distance of separation may be adjusted. [Illustration: FIG. 50.—“BOSCH” CIRCUIT BREAKER] In the circuit breaker of the K-W magneto it is the cam that revolves, while the lever is stationary, as shown in Figure 51. It will be noticed that the cam will move the lever only twice during each revolution; the magneto can produce four sparks during a revolution, but with this arrangement of the cam only two of them are used. [Illustration: FIG. 51.—“K-W” CIRCUIT BREAKER] It has been said that an intense sparking current is produced when there is a great change in the strength of the magnetism, and when the change in strength occurs suddenly. There cannot be any alteration in the change in strength, for the greatest magnetic strength of the core is what is given it by the magnet, and changing from this to nothing is the greatest change possible. The suddenness with which the change takes place, however, depends on the speed at which the magneto runs. A 4-cylinder engine requires two sparks to each revolution of the crank shaft; the armature of a Bosch magneto for this engine will therefore run at the same speed as the crank shaft. The K-W magneto, giving four sparks to the revolution, could run at half of the speed of the crank shaft, but then the change in the strength of the magnetism would take place slowly, and the sparking current would not be sufficiently intense. By using only two of the sparks the magneto is run at the same speed as the crank shaft; the change in strength then takes place more suddenly, and a more intense sparking current is produced. The circuit breaker of a magneto for a 1-cylinder engine has only one cam, so that a single spark is produced during each revolution of the armature; the armature makes one revolution to every two revolutions of the crank shaft. However many cylinders an engine may have, the magneto must be revolved from one point of sparking to the next in the interval between ignition in one cylinder and ignition in the next cylinder to fire. A magneto is driven by the crank shaft through gears or by a chain, which are so proportioned and set that the magneto is at a point of sparking at the instant when a piston is in position for ignition.