placed. To prevent the wires from being thrown out by the action of centrifugal force the groove is closed by the insertion of dove-tailed wooden wedges. The reason for using thin plates instead of a solid body for the core of the armature is to prevent the creation of parasitic currents in the mass of the iron. This point has already been discussed earlier in this chapter when dealing with alternating current machines, and the reasoning then applied remains valid also for D C machines.
When explaining the action of an armature with closed winding, we assumed the existence of a magnetic field emanating from the poles N S without specifying how this field is produced. We might produce it by using some source of electricity such as a primary or a storage battery to excite the field magnets, but this would be a cumbersome method, since it would make the action of the machine dependent on some other source of electric supply. It is possible to dispense with this extraneous original source of exciting current by letting the machine excite itself. Imagine the current coming out of the brush marked positive in Fig. 17 not led straight away into the external circuit, but passed first through the magnetising coils of the field system.
