266 TIIK DIRECT-CURRENT MOTOR CH. XII 



be the same in both motor and generator. This direction 

 we can find by an application of Fleming's rule ; pointing 

 the first finger of the right hand in the direction of the 

 lines of force ; the thumb in the direction of motion ; the 

 second finger gives the direction of the induced tension, 

 though not necessarily of the current ; now if the tension 

 impressed on the dynamo terminals is greater than the 

 induced tension, the current will flow against this tension, 

 that is towards us, as shown by the dots in the conductors, 

 under pole A in Fig. 64 and in the reverse direction 

 under pole B. 



Knowing the direction of the current in the armature, 

 we see that the lines of force caused by this current must 

 flow in a counter-clock-wise direction under pole A for the 

 motor, and must, therefore, be humped up at the back of 

 the pole ; this gives us the proper sign for the curve of 

 armature reaction, positive behind and negative in front of 

 each pole. 



If the tension impressed on the terminals of the 

 dynamo is less than the induced tension, as in the case 

 of a generator, then the current will flow in the direction 

 of the induced tension, or from us, as shown by the crosses 

 in the wire in Fig. 65 under pole A, and in the reverse 

 direction in the wires under pole B. 



When the dynamo is acting either as motor or generator 

 the magnetising effect of the armature is superposed upon 

 that due to the magnets. If the resultant effect can be ob- 

 tained by adding the ordinates of the armature curve to that 

 of the magnet curve when of the same sign, and subtract- 

 ing them when of different sign, we shall obtain curves like 

 those in Figs. 64 and 65 marked ' armature and magnets,' 

 whose areas would be equal to those due to the magnets 



