3 o8 



SCIENCE PROGRESS. 



circuited coil (marked by the open circle) is moving in 

 the weakest field, being farthest from the pole-tip and 

 nearest to the neutral line of zero field. The difference 

 in the E. M. F. of the two reversers which at any time 

 may be under a brush is indicated by the heavy black 

 line of C, and the medium line of A : the other reversers, 

 as shown by the thin lines, are entirely inactive, since they 

 form no part of a closed circuit. Further, when a coil is 

 short-circuited, a circuit c-d-a-A-C-c is closed by the con- 

 tact of the brush surface ; hence though the direction of 

 the E. M. F. induced along the length of the two reversers 

 is the same, the one opposes the other, and the greater 

 E. M. F. of C as compared with that of A leaves an un- 

 balanced E. M. F. acting round the closed circuit in the 



< «* 



Fig. 4. — Sayers' method of winding Armatures. 



direction c-C-A-a-6-c. Now the self-induction of section 

 abc tends to keep the current flowing in its old direction, 

 viz., from c to a, this being the direction of the current in 

 the loops to its right, as shown by the arrowheads on the 

 inductors. But it has just been shown that the difference 

 of the E. M. F.'s of C and A tends to drive a current 

 through the short-circuited coil from a to c. If therefore 

 this unbalanced difference of E. M. F. is sufficiently great, 

 and is given sufficient time, it will stop the old current 

 flowing from c to a, and will start a reversed current in the 

 required new direction : at the moment when this new 

 current reaches the strength of that which is being carried 

 by the rest of the winding, segment A should pass from 



