VARIOUS GENERATORS OF ELECTRIC LIGHT. 79 



studied. We shall thus have two currents superposed, and 

 to these will be united the currents resulting from the in- 

 versions of the polarities, for these constant polarities which 

 we have supposed in the ring do not exist, inasmuch as it 

 turns with the coils, and a stable distribution can be main- 

 tained in the system only by the continual inversion of the 

 polarities at the different points of the ring. It will be seen that 

 the resulting current must therefore be in the same direction 

 as those produced by the direct action of the magnetic poles, 

 for they are of the same nature as those which would result 

 from the displacement of the magnetic poles themselves, and 

 they must be direct for the advancing coils, and inverse for 

 the retiring ; but as these last are presented to the inducing 

 poles in the inverse manner, the definitive effect is produced 

 in the same direction. 



It has been supposed that, in the Gramme machine, the 

 iron ring performed other functions, perhaps more important 

 than those we have just analysed. Thus several physicists 

 state that it may prevent injurious inductions by acting as a 

 screen to the action of the inducing magnet on the portions 

 of the coils opposite to it. On the other hand, it is thought 

 that by acting as an armature to the magnetic poles, it con- 

 siderably heightens their energy and concentrates the action. 

 Although there may be some truth in these assertions, we 

 do not see that the advantages of the iron ring are so impor- 

 tant, since the Siemens machine, which is not provided with 

 it, gives considerable effects. 



As iron is susceptible of a magnetization much greater than 

 that which can be preserved by tempered steel, and as the 

 power of electro-magnets is infinitely more energetic than 

 that of permanent magnets, Gramme has combined his 

 schemes of annular induction coil with a Ladd's electro- 

 magnetic system, in which Siemens' armature is suppressed, 

 and in order to give more power to this arrangement he has 

 formed the electro-magnetic system with a double set of 

 electro-magnets, the branches of which are opposed to each 



