264 THE DIRECT-CURRENT MOTOR CH. XII 



only, so that the intensity at this point may be expressed 

 * 1 iAd> 



We are thus able to complete the curve of armature 

 magnetisation, and find that it cuts the axis at points 

 on the armature at the centre of each pole, and that it 

 rises to a maximum at the tips of the poles, and bends 

 down again to a minimum, but does not cut the axis, at 

 points midway between the tips of the poles. 



Figs. 64 and 65 show the curves of armature magnetisa- 

 tion for a motor and for a generator. In each case the 

 motion is in the same direction, also the induced tension 

 and the magnetisation due to the magnets. The only 

 difference is in the direction of the current in the armature ; 

 this depends upon whether the tension impressed on the 

 terminals of the armature is greater or less than the 

 induced tension. 



The student should reason out for himself the signs of 

 each curve of magnetisation in these figures. The 

 direction of the current in the wires is shown by dots and 

 crosses, the dots representing currents coming towards us, 

 and the crosses representing currents going from us. 

 Looking at the pole A across the air gap, the lines of force 

 will go from us ; this is shown as positive by the uniform 

 curve above the axis. In the next pole this direction 

 is reversed, the magnetisation at b being nothing ; b and a 

 will be midway between the pole-tips if, as we here 

 suppose it to be, the field is symmetrical. 



If we assume the direction of motion, we are able to 

 determine the direction of the induced tension, which will 



