It has been found by experiment that there is an attractive and repulsive force between magnets, and that this magnetic force pervades the surrounding space. The fact that a magnet when suspended near another magnet will take a certain direction, depending upon the relations of the poles, has led to the conception of the magnetic lines of force, which emanate from the north pole of the magnet and pass through the surrounding atmosphere to the south pole.
In a magneto the magnetic lines of force flow from the pole piece of the north pole to the pole piece of the south pole through the armature and the air gap between the armature core and the pole pieces. The space between the pole pieces is termed the magnetic field.
The magnetic lines of force which pass through the armature are variously distributed according to the position of the armature in rotation. This is true because the magnetic lines of force pass more readily through the sides of the H than through the wiring laid between them.
Any movement of the armature on its shaft, either in making a complete revolution or in oscillating backward and forward, must operate to deflect and distort these lines of force in such a manner as to set up powerful induced currents in the armature winding.
Four positions of the magneto armature are shown in Fig. 48. At A the magnetism is represented as passing through the soft iron core, the heads of which are in close proximity to the pole pieces and threading the armature winding. At B the armature has passed to a point where its heads are just passing out of proximity of the pole pieces, and thus breaking the magnetic path between the pole pieces. At this point a sudden generation of electrical pressure is taking place in the armature winding, this causing a current to flow in it. When the armature reaches position C its core again forms a path for the passage of magnetism from pole piece to pole piece and the current becomes zero again. At D the same condition exists as in B, although in the opposite direction.
Thus, in each revolution two electrical impulses in opposite directions are induced in the primary winding of the armature. These impulses last only for a small fraction of the time of a revolution and are equally spaced. The electromotive force, or tension, of these is very low and entirely insufficient to cause a spark to jump between the spark plug electrodes, separated by even the shortest air gap.
Fig. 48. Magneto Armature Positions. A, Magnet. R, Pole Piece. C, Armature.
The next step consists in transforming these impulses, or multiplying their pressure several thousand times. It is for this purpose that the fine wire winding is provided on the armature. When the armature is being rotated between the pole pieces an electromotive force is being induced in the secondary or fine wire winding, the same as in the primary or course wire winding and many times greater, but still not sufficiently great to bridge the spark plug electrodes.
The primary winding is ordinarily closed upon itself. This causes a current to flow in it more or less proportional to the electromotive force. That is, the current is at a maximum when the armature is in a vertical position. At that time there are practically no lines of force from the permanent magnets passing through the central part of the core. But the heavy current flowing in the primary winding makes of the core an electromagnet, setting up a magnetic field at right angles to that of the permanent magnets. Now, if at this moment the primary circuit be suddenly opened, the current in it will almost instantly cease flowing and the magnetism set up by this current will vanish. These lines of force are also included by the turns of the secondary winding, and as they are withdrawn so exceedingly rapidly, and since there are such a large number of turns in the secondary winding, the result is that an enormous electromotive force is induced in the secondary winding, which will cause the spark to bridge a gap in the atmosphere of from one half to three fourths inch long.
A device known as the circuit breaker, or interrupter, is used to open and close the primary circuit. This is carried on the armature shaft opposite the driving end of the magneto.
It is represented in Fig. 49 and consists of a stationary contact A and a movable contact B on the arm C. Both of these parts are mounted on a brass disc D, which is securely fastened to the armature shaft and rotates with it. The stationary contact A is insulated from the disc D, while the movable contact B is in metallic contact with it, and the disc D is grounded to the frame of the magneto by a grounding brush. The circuit breaker is surrounded by a cylindrical housing F, to the interior of which at diametrically opposite points are secured steel cam blocks, G-G. Ordinarily these two contact points are kept in contact by a spring. As the disc D rotates, the outer end of the arm C comes in contact with the cam blocks G, whereby A and B are separated momentarily. As soon as the cam block G has been passed, a spring brings the two contact points together again. Stationary contact A is connected with one end of the primary winding of the armature, while the other end has a metallic connection with the armature core; or in other words, is grounded.
When these two contact points are suddenly separated, there is a tendency for the current to continue to flow across the gap, it possessing a property similar to the inertia of matter. This would result in a hot spark being formed between the two contact points, which would not only burn the points away rapidly, but would also prevent a rapid cessation of the current.
Fig. 49. Diagram of Circuit Breaker.