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Introduction to the Study of Science 



rent in the coils of the armature is reversed in direction. You 

 recall that as one part of the coil of wire moves across one pole 

 of the magnet, the current is pushed in one direction, and as 

 it moves across the opposite pole the current is pushed in the 

 opposite direction. The commutator is the important item in 

 this kind of generator. 



The alternating current generator has no commutator. The 

 ends of the wires of the outside circuit are directly connected 

 to rings on the shaft of the armature. With this generator the 



electric current in the conducting 

 wires follows first one wire and then 

 the other, or it has first one direction 

 and then the other. This is different 

 from the D. C. generator, where the 

 current leaves by one wire and returns 

 by the other throughout. In the A. C. 

 generator the current is taken off just 

 as it is generated in the coil of the 

 armature in moving across the op- 

 posite poles of the magnet (Fig. 107) . 

 The alternating current is generally 

 used for ordinary lighting and heating. 

 It may be used in specially con- 

 structed motors, but is not generally in the motors of street cars 

 or electric locomotives. These make use of the direct current. 

 In this case if the alternating current is generated, it must be 

 rectified, or converted into the direct current (page 320). 

 The alternating current cannot be used for charging secondary 

 cells (page 333) ; the current for this purpose must be in one 

 direction only. 



145. The transformer. The alternating current is best 

 adapted for transmission over long distance, because it is easily 

 changed from a moderate electrical pressure at which it is 

 generated, say 2500 volts, to a very high potential, as 150,000 

 volts, at which it can be transmitted ; and then back to a low 



FIG. 107. Diagrams illus- 

 trating the production of the 

 direct and the alternating cur- 

 rents. A, the armature ; B, 

 brushes ; C, commutator ; R, 

 rings ; arrows showing the 

 direction of the generated 

 current. 



