376 ELEMENTARY LESSONS ON [CHAP. x. 



each strip of the commutator being united to one end 

 of each of two adjacent coils. Against the split -tube 

 commutator press metallic brushes to receive the current. 

 When this ring is rotated the action is as follows : 

 Suppose (in Fig. 152) the ring to rotate in the opposite 

 direction to the hands of a clock in the magnetic field 

 between the N and S- poles of a magnet (or electro- 



Fig. 152. 



magnet), and that the positive direction of the lines -of- 

 force is from N to S. As a matter of fact the lines will 

 not be straight across from N to S, because the greater 

 part of them will pass into the ring near N and traverse 

 the iron of the ring to near S, where they emerge ; the 

 space within the ring, being almost entirely destitute of 

 them.. Consider one single coil of the wire wrapped 

 round the ring at E" which is ascending toward S ; 

 the greatest number of lines -of- force will pass through 

 its plane when it lies near E", at right angles to the 

 line NS. As it rises toward S and comes to E the 

 number of lines -of- force that traverse it will be steadily 

 diminishing, and will reach zero when it comes close to 

 S and lies in the line NS, edgeways to the lines-of-force. 

 As it moves on toward E' it will again enclose lines-of- 

 force, which will, however, pass in the negative direction 

 through its plane, and at E' the number of such negative 



