CONTINUOUS-CURRENT DYNAMOS. 307 



Modifying the words of Dr. Hopkinson, we may say 

 that experience has shown that we can not put up with the 

 sparking which would ensue if the brushes of an ordinary 

 dynamo were given a backward lead : but in the Sayers 

 dynamo which we are now to consider, the sparking is 

 annulled by the particular method of winding the connec- 

 tions between the loops on the armature and the segments 

 of the commutator. Thus even though the brushes be 

 given a negative lead in a generator of electricity, there is a 

 position in which a sparkless collection of the current is 

 possible. This achieved, the prediction of Dr. Hopkinson 

 has found its fulfilment, and a dynamo has been produced 

 in which, by utilisation of the reactions of the armature 

 itself, all the results of compound winding have been 

 obtained. 



Let Fig. 4 represent the development on a flat surface 

 of a portion of an armature, the short thick lines such as be 

 being the inductors on the surface of the armature connected 

 together by the slanting portions such as ab, which may be 

 either the internal inactive portions of the loops of a ring, 

 or the end connections of a drum winding, in which latter 

 case they will also include a second inductor on the 

 diametrically opposite surface of the core. Each commu- 

 tator connection, instead of being, as usual, taken straight 

 from the point a or c to the nearest segment, is turned back- 

 wards against the direction of rotation for a short distance 

 along the end of the core, and is then wound over the 

 surface, parallel to and above the ordinary winding : it is 

 then finally joined to the commutator segment in the usual 

 manner as at A or C. The portion of the winding such as 

 cC or aA is known as a reverser bar, or commutator coil. 

 The brushes are then so placed that the reverser bars 

 which at any moment lead to them are under or near to 

 the trailing pole-tips. 



An examination of Fig. 4 will show that of the three 

 portions connected with the brush M, viz., the short-cir- 

 cuited coil abc and the two reverser bars a A and cC, 

 reverser C is moving in a stronger field and is generating 

 a greater E. M. F. than reverser A, while the short- 



