Motor-car principles; the gasoline automobile

by Roger B. Whitman

A magneto of this type is shown in diagram in Fig. 10. In this case, the inner ends of both the primary and secondary windings are grounded, the live end of the primary being connected with the insulated contact of the interruptor, and the live end of the secondary leading to the rotating part of a secondary distributor. For the greater part of the revolution of the armature, the cam on the armature shaft permits the interruptor to maintain a closed circuit, but as the current reaches a maximum the cam breaks the circuit, and the flow of the primary current ceases. During this time, the secondary circuit has been open, the moving part of the distributor being out of contact with the stationary points. Under the law of induction, a current will only be induced in a closed circuit, and there is therefore no action in the secondary winding. When the primary current is at its maximum and its circuit is broken, another condition exists, for then the secondary distributor is making contact, the only gap being in the spark plug, but the sudden demagnetization of the armature core as the primary circuit is broken, and the additional effect as the armature passes the vertical position, induces in the secondary winding a current of great intensity that passes to the spark plug by way of the distributor, returning to the winding by the ground.

The parts of the interruptor are carried on one of the end plates of the magneto, and the secondary distributor revolves in bearings that support it under the arch of the magnets.

In the diagram shown provisions are made for a double ignition system, the operator having his choice of ignition by high-tension magneto direct or by battery, timer and coil. In practice, the three switches shown operate together. When magneto ignition is desired, the switch between the battery and ground is opened, and the switch at the secondary distributor thrown into contact with the point connected with the secondary terminal of the magneto. The third switch is thrown to make contact between the primary terminal of the magneto and the interruptor. If battery ignition is desired, the switch on the magneto primary is thrown to the point through which it is grounded, and the primary and secondary switches of the battery circuit closed. The upper contact points of the interruptor are then utilized to close primary circuit of battery, and the secondary current induced in coil is led to distributor. This complication of switches is necessary to prevent possibility of the battery current flowing through the armature winding, and to provide a short circuit for the primary of the magneto. If the engine is running on magneto ignition, it is stopped by throwing the primary switch to the point by which the primary is grounded. If running on battery, the circuit is broken in the usual way.

The current from a high-tension magneto is of such intensity that the small points of an ordinary spark plug would quickly be burned, and it is necessary to provide plugs with heavy points, which will stand the work. The distance between the points should be about one sixty-fourth of an inch.

The Bosch magnetos, which are perhaps in more general use than any other, are built on similar lines to the magneto just described, except that the grounded end of the secondary winding, instead of being attached to the metal of the armature, is connected to the live end of the primary winding, as shown in Fig. 11. The live end of the primary is brought out through a conducting rod passing lengthways through one end of the armature shaft, from which the current flows to the stationary contact of the interruptor. While in other magnetos the interruptor parts are stationary and operated by a moving cam, in this type the interruptor parts revolve with the armature, and the moving arm is operated by two fiber wheels as it drags around the inside of a stationary ring. By rotating the ring and the two wheels the interruptor is caused to operate sooner or later in its revolution, this giving the advance and retard of the spark.

Fig. 11.—Wiring Diagram, Two-Spark Magneto.

The other end of the armature shaft carries the collecting ring for the secondary current, this consisting of a hard rubber disk around the surface of which is a metal ring to which the live end of the secondary winding is connected. A carbon brush is kept pressed against this metal ring by a light spring, and the current is thus led off to the revolving part of a secondary distributor.

The interruptor keeps the primary circuit closed until the current induced in the primary winding is at its maximum, when the opening of this circuit causes the rapid demagnetization of the core, and a powerful current is induced in the secondary, which flows to the spark plug with which the distributor is making contact.

A safety spark gap is applied between the brass strip that conducts the secondary current from the collecting brush to the distributor, and the metal of the magneto. The necessity for this device has already been explained, and it is essential that a magneto of this type should be so provided, because of the intensity of the current.