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greater resistance to be overcome, for as soon as any extra work 

 is to be done, an extra current of Electricity is sent round the 

 fields, thus making them stronger magnets, able to induce a 

 stronger current round the armature, and so better able to 

 overcome the resistance. 



This, which is caused to revolve between the two poles of the 

 fields is called the armature. It consists of a piece or pieces of 

 soft iron also wound with wire. That of the machine on the table 

 is called Siemens' H armature ; it is all in one piece and the wire 

 is wound longitudinally over it. In the machines of Pacinotti, 

 Gramme, and others, the wire is in several pieces, the end of one' 

 coil of wire and the beginning of the next being connected with the 

 same part of the commutator. The way this is done can be seen 

 on referring to these diagrams. This is called the Pacinotti ring 

 armature, and is supposed to be wound with six coils of wire. 

 This number is taken only for the sake of simplicity. The com- 

 mutator is in the middle, and is divided into six parts corresponding 

 with the number of coils. The end of one coil and the beginning 

 of the next being connected with one of these parts. As the ring 

 revolves it is constantly cutting an unequal number of lines of force 

 which lie between the north and south poles, consequently through 

 each coil momentary currents will be induced. 



We must now bring back to our minds what we said at the 

 beginning ; a north pole always induces south magnetism in that 

 part of the iron near it, and north in the other part ; so it is with 

 the south pole. Therefore one half of this ring will have north 

 induced magnetism in it, and the other half south. This magnet- 

 ism of the ring will induce momentary currrents in the coils round 

 it, the currents in one half flowing round in the opposite direction 

 to those in the other half. As the ring is made to revolve rapidly, 

 momentary currents are so rapidly generated in the coils as to 

 make them apparently continuous currents. The currents which 

 flow in one direction are collected by one of the brushes, and those 

 which flow in the opposite direction by the other. 



Now let me explain the important part the commutator plays. 

 In the Siemens' H armature it is in two parts — each being quite 

 distinct from the other. First one end of the wire is connected 

 with one half of the commutator, then wound round the other 

 armature, and the other end connected with the other half. Now 

 although the fields are only temporary magnets, they always 

 contain sufi&cient residual magnetism to start the Electricity ; 

 therefore as soon as the armature is revolved the poles of the fields 

 act upon it and convert it into a magnet. 



Now the Electricity starts in the armature. As soon as this 

 becomes magnetised by the fields, a current of Electricity is started 



