398 



GENERAL SCIENCE 



trolley-wire supplies current directly from the dynamo, and the track 

 acts as the return wire to the dynamo. 



Alternating (AC) currents and direct (DC) currents are gen- 

 erated on the same dynamo. The method by which the current is 

 obtained is through the arrangement of the commutator. Fig. 277 

 shows the method by which alternating current is obtained from a 

 dynamo. As the wire 1 passes by magnet S, current is generated 

 in the direction of the arrow, which is carried by the wire to the 

 brush on the armature at A. As wire 1 moves from the bottom of 

 the magnet N toward the top, current is generated in the direction 



FIG. 277. 



FIG. 278. 



shown by the arrow on wire 2, which is directly in the opposite direc- 

 tion from that in which the current was going when the wire was 

 passing S, causing the brush A first to send off current and then to 

 have current return into the dynamo from the supply wire X. In 

 the meantime, wire 2 generates the current in the direction of the 

 arrow while passing magnet N, and in the opposite direction when the 

 armature has turned so that wire 2 is passing magnet S, alternately 

 receiving and sending a current over the brush at B. Fig. 278 

 shows the method by which direct current is obtained from a dynamo. 

 As wire 3 passes by the magnet $, current is generated in the wire 

 in the direction of the arrow. The current is taken off at the com- 

 mutator by the brush X, which touches the sector P. It will be 

 seen that the commutator is divided into two sectors or parts, P and 

 T. Current will come off the wire H. As wire 4 passes magnet 

 N) current will be generated in the wire in the direction of the 



