CH. XII ARMATURE REACTION 267 



only. We should in fact be simply taking away a certain 

 number of lines of force from one part of the pole and 

 adding them to another. 



We have already seen that the value of the induction 

 factor is unaltered so long as the area of the magnetisation 

 curve remains unchanged ; hence if the assumption made 

 above is correct, we ought to find that the torque for 

 a given current in the magnets increases in direct propor- 

 tion to the current in the armature. We know, however, 

 that this is not true, but that the torque observed 

 generally decreases as the current in the armature in- 

 creases. The explanation of this is that the magnetisa- 

 tion curve is not distorted symmetrically, but is reduced 

 at one pole-tip by a greater amount than it is increased 

 at the other. 



We have here in fact two magnetising forces, the one 

 that of the ampere-turns on the magnets ; the other that 

 of the ampere-turns on the armature. These both act 

 upon a magnetic circuit common to the magnets and the 

 armature. We assumed that we might neglect all the 

 reluctance of this common portion except that of the air 

 gap, whereas strictly the magnetisation produced by 

 adding the two magnetising forces in the common circuit 

 ought to be found in the usual way for a magnetic circuit 

 consisting partly of air and partly of iron. The result will 

 depend upon the degree of saturation of the iron in the 

 neighbourhood of the pole-tips. 



The pole-tip under which the effects are added will be 

 the one where the loss of area takes place, especially if the 

 pole is tapered and extended, while the result under 

 the other tip may generally be obtained by simply sub- 

 tracting the magnetising forces. 



