THE DIRECT-CURRENT MOTOR 



CH. 1 



Fig. 1 is a section of the armature and pole pieces of 

 a dynamo at right angles to the shaft. A and B are two 

 surface conductors, one on each side of the plane through 

 the centre containing the brushes. The currents in these 

 conductors are in opposite directions, but the lines of force 

 due to the magnets are in the same direction. Hence 

 the forces acting on the conductors constitute a couple. If 



any number of conductors are arranged evenly round the 

 armature, the action of the field on the current in the con- 

 ductors on one side of the brush plane, combined with that 

 on the other side, results in a couple tending to turn the 

 armature about its axis. 



Let there be A surface conductors, and let their axes 

 lie on a cylindrical surface of radius r and length I centi- 

 metres, as shown in Fig. 2. The distance between any 



