Motor-car principles; the gasoline automobile

by Roger B. Whitman

Fig. 5.

When the armature is in this position, the neck is magnetized by the flow of lines of force through it and sets up a powerful magnetic field which is quite distinct from the field thrown out by the field magnets. If the armature is revolved, it takes the lines of force, or most of them, with it, and they continue to flow through the neck as long as one head of the armature is in contact with the north pole piece and the other head in contact with the south pole piece, for even this long and distorted path, as shown in Fig. 5, is of less resistance than the air gap. The lines of force, however, resist this lengthening of their path, and tend to hold the armature with the neck horizontal, when their passage is easiest. If the armature is turned by hand, it will be noticed that it becomes harder to turn as the neck approaches the vertical position, and if the magnets are sufficiently powerful, a great effort will be necessary. Once vertical, however, the armature hangs, and a still greater effort is required to continue the revolution, for the lines of force have found new paths of low resistance (Fig. 6). Each armature head now forms a bridge between the pole pieces, and the lines of force divide, some going through the upper head and some through the lower. The lines entirely abandon the neck, and in consequence its magnetic field dies out. When these paths are broken by continuing the revolution of the armature, the lines of force again flow through the neck, and its magnetic field is again established (Fig. 7). This action occurs twice during each complete revolution of the armature, and if the armature is revolved by hand, the two points when the neck is horizontal and the movement easy will be very distinct from the hard points when the neck is going over the vertical position.

Fig. 6.

Fig. 7.

In one revolution the neck is twice magnetized and demagnetized; or in other words, the magnetic field set up by the neck as the lines of force pass through it twice changes its strength, being strong when the neck is horizontal and weak when it is vertical. The winding on the armature is affected by this magnetic field, and electric currents are induced in it by these changes in its strength.

The greatest change in the strength of the field occurs as the armature moves into the vertical position, when its heads form bridges between the two pole pieces (Fig. 6). Up to this point the armature neck is strongly magnetized, but its magnetic field dies out as the neck becomes vertical. It is at this point then that the induced current is at its greatest intensity and becomes sufficient for ignition purposes.

In making an armature the channels are wound full of wire, and this is retained against the action of centrifugal force by two or three short lengths of wire bound around the armature and lying in grooves cut in the heads for that purpose. Disks of brass are also screwed to the ends of the armature core, and assist in making it dust and water proof. The magneto must have a base, and this must be of some nonmagnetic metal, like brass, for if it were made of iron it would provide a convenient path for the lines of force, and they would have no interest in passing through the armature. There must also be bearings for the armature shaft, and these are either plain or ball, set in brass plates screwed to the ends of the pole pieces. A zinc or aluminum plate covers the space over the armature and between the upper edges of the pole pieces, so that the armature revolves in a tunnel that is proof against the entrance of dust and water.

There are, of course, two ends to the wire wound on the armature, but the simplicity of a magneto is increased by grounding one end on the metal of the armature, the other end being brought to the single terminal (Fig. 2). The circuit is therefore complete when this terminal is connected to any metal part of the magneto; or, as the magneto is mounted directly on the metal of the engine, to any metal part of the engine or frame of the car. The live end of the armature winding is brought out by means of a metal rod passing lengthways through the shaft of the armature, the rod being insulated from the shaft by means of a hard rubber bushing or tube. The terminal of the winding is therefore found at one end of the armature shaft, and the current flows from this revolving part to the stationary binding post by means of a carbon or steel spring that is kept pressed against the end of this rod.