Fourth. That the current flows only while the change in strength is taking place, ceasing to exist when the field becomes uniformly strong or weak.
To make a practical application of the laws governing the production of an induced current, a conductor forming a closed circuit is placed in a magnetic field, and the strength of the field caused to change, alternately becoming strong and weak. In magnetos, the magnetic field is due to two or more powerful steel magnets, and the conductor, a length of insulated copper wire, is wound on a soft iron core and revolved between the poles of the magnets where the field is strongest. The magnets are known as the field of the magneto, and the wire on its core the armature. The shape of the iron armature core is shown in Fig. 2, the winding being indicated by heavy black lines. On the inside of the poles of the field are pole pieces, which are blocks of soft iron hollowed out to receive the armature. As the successful operation of the magneto requires the lines of force to have as easy a path as possible, the air space between the armature heads and pole pieces is very small, being in the neighborhood of 1/100 of an inch.
Fig. 2.—Armature.
Fig. 3.
When the armature is not in position, the lines of force will be required to pass from one pole piece to the other through the air, and as they will seek the path of lowest resistance, most of them will pass between the lower points of the pole pieces, where the air gap is short, and where the lines of force are present in the greatest number (Fig. 3). When the armature is placed between the pole pieces, however, it gives the lines of force a path of still lower resistance, and they will therefore follow it, whatever its position may be. If the armature is free to turn, it will take a horizontal position, as shown in Fig. 4, for then the heads are entirely in contact with the pole pieces, and the greatest possible number of lines of force can take the path offered by the neck of the armature.
Fig. 4.
