In order to stop the magneto from producing sparks, when it is desired to shut down the motor, a switch is provided. One terminal of the switch is grounded to the engine or frame, while the other is connected to a binding post on the circuit-breaker housing. This binder post is, in turn, connected with the contact points of the circuit breaker. When the switch is closed, the current generated in the primary winding flows to the contact points and through the binding post and connecting wire to the switch, whence it passes through a wire to the frame work of the car and return to the beginning of the primary winding. In other words, the switch cuts out the circuit breaker and the primary winding is short circuited all the time, so that the opening and closing of the contact points has no effect.
Fig. 50 shows the path of the primary circuit originating in the primary winding of the armature. It flows through the contact breaker screw to the stationary contact point, thence across to the movable contact point, from where it is led through the contact brush in the framework of the magneto, whence it returns to the beginning of the primary winding, which is also connected or grounded to the frame. The beginning of the secondary winding is connected to one end of the primary winding, and since one end of the primary is grounded, the secondary is also grounded through the primary. The other end of the secondary winding leads to the insulated collector ring, from which the current is taken off by a carbon contact brush. From the brush holder the current is carried through a spring contact conductor to the distributor, from where it is distributed to the spark plugs.
Fig. 50. Path of Primary Circuit. Originating in the Armature Primary Winding.
So far high-tension magnetos of the independent type have been discussed. However, most all magneto makers also build high-tension magnetos, which provide a dual ignition system, using a battery and coil with one set of spark plugs. Some makers use the magneto breaker for the battery current, while others provide a separate circuit breaker to avoid the possibility of both systems being put out of commission by an accident affecting only one, but subsequently extended to the other on account of its close relationship. The same distributor is used for both systems.
The battery current is of a low tension and connected with a two-point switch on the coil. From the coil the current is led to the circuit breaked, and, as the circuit is broken at the proper moment, a very high voltage is induced in the secondary of the coil or transformer, and being delivered to a heavily insulated cable, is conducted to the central carbon brush of the distributor, whence it is delivered to the spark plugs in the different cylinders in correct sequence.
The coil has a primary and secondary winding, similar to that of the armature of the magneto, and perforins the same function in the buttery circuit, being provided with a separate condenser.
Fig. 51 is a wiring diagram of the Eiseman dual ignition system. The high tension is led from the collector ring of the magneto of HM and led to HM on the coil, thence to the switch H on the coil and to H on the magneto. When the coil handle is shoved over to the battery position, several operations take place in the switch of the coil. First, the primary current, emanating from the terminal MA of the magneto, is led to the ground or body of the magneto, and this prevents it from generating a high-tension current. Second, the battery current is allowed to flow in the coil at + on the coil, through the coil, thence to R on the coil to R on the magneto, where it is interrupted by a circuit breaker, thence it returns to the battery through the ground. When it is desired to stop the motor, the coil handle is moved to the off position, cutting off the battery current and the magneto current remains short circuited, thus eliminating all ignition.
Fig. 51. Wiring. Diagram of Eiseman Dual-Ignition System.
But, if instead of leaving the handle at "off," it is quickly shoved from "Bat," to "Mag," without arresting it between the two, the motor begins to run on the magneto current. In this case the battery current is left cut off as in the " off " position, while the connection, which in both the other positions has led the primary circuit of the magneto to be ground, is broken, with the result that this current is diverted to the breaker mechanism on the -armature shaft, thus generating in the secondary winding of the magneto the high-tension current led to the spark plug.
The same distributor is used for the battery high-tension current, for, when the switch is on "Bat," there is a connection made between the end of the coil's secondary winding and the terminal H on the coil, which sends the battery current over the same route as that of the magneto.
Before the motor is in motion, the interrupter R, still referring to Fig. 51, cannot operate, and this makes necessary interruptions by hand with a starter knob when starting from the seat on compression. This takes the place momentarily of the circuit breaker, being supplanted by the latter the moment the engine turns over.