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.
A magneto for an engine with more than one cylinder is provided with a distributor, which passes the sparking current to the particular cylinder that is ready for ignition. A distributor is a revolving switch built into the magneto, with as many points, or contacts, as the engine has cylinders. At the instant when the magneto produces a sparking current, the revolving distributor arm is in position to pass the current to one of the contacts, and the current flows to the spark plug with which it is connected.
An electric current must have a complete path, or circuit, in order to be able to flow. In a magneto ignition system this path is partly of wire and partly of the metal of the engine. The diagram in [Figure 45] indicates that the current returns to the magneto from the circuit breaker lever and the spark plug by wire, but in actual construction it returns by the metal of the engine. This is called a ground return; the circuit is said to be grounded.
Fig. 52.—“Bosch” Magneto in Section
[Figure 52] is a side view of a Bosch magneto, partly broken away to show the interior. As can be seen, one end of the primary winding is screwed to the armature, and is thereby connected with the metal of the magneto; as the magneto is attached to the engine the primary winding is thus in contact with that also. The other end of the primary winding leads to the insulated block of the circuit breaker, [Figure 50]. This block is insulated from the disk; that is, while it is attached to the disk, it is kept from touching it by means of pieces of hard rubber or mica. Through these an electric current cannot pass.
The lever is grounded; that is, it is in contact with the metal of the magneto. When the lever touches the screw of the insulated block, current can flow; when they are separated, the circuit is broken.
One end of the secondary winding, [Figure 52], is attached to the outer end of the primary. The other end leads to the slip ring, which is a metal rim on a hard rubber wheel attached to the armature and revolving with it. Sparking current flowing to the slip ring is led off by a carbon brush and passed to the distributor.
Should a spark plug wire fall off while the engine is running, the current would lose its path and would seek another; it is quite powerful enough to make a path for itself by breaking through the windings. As this would injure the magneto, such a thing is prevented by providing a safety spark gap, which acts like a safety valve in giving the current a path when the regular path is interrupted. It consists of two points of metal, one attached to the metal of the magneto and the other connected with the slip ring brush; it is a more difficult path than the one through the spark plug, but easier than breaking down the windings.