Fig. 56. Magnets—Pole Blocks
The purpose of the magneto is to furnish electrical current at regular intervals, to jump the spark plug gaps and to ignite the gas which has been compressed in the combustion chambers. The discovery was made years ago that, by placing a coil of wire between two magnetic poles, current would be present at once. But it is only while the wire coil is in motion that the current will flow or circulate, and while there are many theories why this takes place only while the coil is in motion, none seem to explain the fact satisfactorily. The strength of the current depends on the size of the magnetic field, and the number of wraps of wire in the coil. Consequently the larger the coil the more intense the current. [Fig. 56] represents the magnets, of which there are from three to six. The U-shaped pieces are made of steel which has been case hardened and charged with electricity which causes them to become magnetized. Magnets have two poles or axes, one of which is positive from which the current flows, and one of which is negative to which the current flows or passes. [Fig. 56A] shows the pole pieces which are located on the inside of the lower or open end of the magnets. The pole pieces are channel ground, leaving a round space or tunnel in which the armature revolves.
[Fig. 57] shows the soft iron core which is shaped like the block letter H, and wound with fine wire, making up the coil shown in [Fig. 57A] of the wound armature.
Fig. 57. Armature Core—Wound Armature
Fig. 58. Primary and Secondary Winding and Current Direction
[Fig. 58] shows the primary and secondary winding. The primary or heavy wire is wound on the core lengthwise, each strand being separated from the other with rubber or tin foil insulation. The current passes from the top of the left pole piece to the top of the core until it passed out of range, crossing the upper gap between the two pole pieces. As the top of the core leaves or breaks the contact flow of current, the bottom of the core comes in contact range, leaving an open space which breaks the current and changes the direction of flowage as shown in [Fig. 58A and 58B]. This current is of a low tension nature, and will not jump the gap at the spark plugs when the engine is running slow. The secondary winding, shown in [Fig. 58], is made up of many more windings of a finer wire. The low tension or primary current is led through the armature shaft to a contact breaker at the rear of the magneto.
[Fig. 59] shows the contact breaker, which consists of a housing in which two platinum points are arranged, one point stationary, the other attached to an arm on a pivot. The points are held together by spring tension.