[Fig. 285] represents the magnetic flux through an armature at rest, where the field magnets are separately excited. If the armature be rotated clockwise, induced currents will flow upward through the two halves of the winding between the brushes, making the lower brush negative and the upper brush positive.
Ques. If, in [fig. 285], the current in the field magnet be shut off, and a current be passed through the armature entering at the lower brush, what is the effect?
Ans. The current will divide at the lower brush, flowing up each side to the top brush. These currents tend to produce north and south poles on each half of the core at the points where the current enters and leaves the armature. Hence, there will be two north poles at the top of the ring and two south poles at the bottom.
Ques. What effect is produced by the like poles at the top and bottom of the ring?
Ans. The external effect will be the same as though there were a single north and south pole situated respectively at the top and bottom of the ring.
Ques. In the operation of a dynamo, how do the poles induced in the armature affect the magnetic field of the machine?
Ans. They distort the lines of force into an oblique direction as shown exaggerated in the diagram [fig. 286].
Fig. 286.—Distortion of magnetic field due to cross magnetization. For clearness, the effect is shown somewhat exaggerated. A drag or resistance to the movement of the armature is caused by the attraction of the north and south poles on the armature and pole pieces respectively.
Ques. What effect has the presence of poles in the armature on the operation of the machine?