169 



on]y an approximation for the purpose 

 of facilitating the calculations. It was 

 probably nearly true in the case of ma- 

 chines with no iron in the generative 

 portion, or armature, in which a copper 

 conductor was dragged through alter- 

 nating magnetic fields ; but, in the case 

 of machines in which the armature wires 



Fig. 36 



were coiled on iron cores, the electro- 

 motive force being due to the variation 

 of induction in those cores, he did not 

 think that law of variation could hold. 

 The maximum induction in the moving 

 iron was not reached until after the point 

 of nearest approach to the fixed magnet 

 had been passed. When once the lines 



