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. 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.

A cam on the armature shaft comes into contact with the arm on which the second point is located, forcing it from the stationary point, thus breaking the low tension current which returns to the secondary coil, the magnetizing and demagnetizing caused by the break in the low tension current, and sets up a rapid alternating current. One end of the secondary is led to a collector ring on the front of the magneto. Fig. 59A shows the collector ring. A carbon brush collects the current from the ring and conducts it to the distributor’s centrally located arm. Fig. 59B shows the distributor. The centrally located arm is timed to deliver the current, or comes into contact with one of the segments or brushes and allows the current to flow from the segment to the gap at the spark plug, where it jumps the gaps and ignites the gas in the cylinders at the proper time. Then it returns through the ground (the engine and the frame) to the magneto, where it passes back into the secondary coil, passing through an insulated condenser consisting of small plates of steel insulated from one another. This regulates the flowage of the returning current, by reducing it through resistance, and prevents the armature from heating.

A safety spark gap is provided on some magnetos which causes the spark to jump and lose some of its force through resistance when the plugs become shorted. This also restricts the current and greatly aids the condenser in performing its purpose.