CONTINUOUS-CURRENT DYNAMOS. 309 



under the brush and the short circuit will be opened with- 

 out any sparking. 



The question then arises, how is sufficient difference of 

 E. M. F. between C and A to be obtained, so that the re- 

 quired reversing action may be provided ? The solution of 

 this is to be found in the adoption of a toothed armature 

 core, and a short air-space between the iron of the arma- 

 ture and the iron of the pole-pieces. Such a type of dynamo 

 has other merits which have led to its being frequently 

 tried, — primarily, of course, its great economy in the copper 

 of the field winding, and the electrical energy required for 

 its excitation. But in the long run, experience has so far 

 told against it owing to the difficulty of preventing sparking 

 at the brushes in dynamos of large output. Now, however, 

 the short air-space fits in admirably with the device by 

 which the sparkless reversal of the current is secured, and 

 in fact is essential to it. Let the armature be toothed, and 

 the winding be buried in the slots or tunnels between the 

 teeth, so that a small air-space of, say, one-eighth of an 

 inch may be obtained between the armature and the pole- 

 pieces. Then, owing to the closeness of the iron polar 

 surfaces, the strength of field will fall off quickly as we pass 

 outwards beyond the pole-tips, and the fringe of lines ex- 

 tending into the core from their edges will vary rapidly 

 from a great density to practically zero. If now the wind- 

 ing and position of brushes be as shown in Fig. 4, the coil 

 cba, being some distance from the trailing tip of the pole N, 

 will be inducing little or no E. M. F. and will therefore be 

 in a suitable state for commutation. Reverser bar C, being 

 well within the fringe of lines spreading from the trailing 

 pole-tip, will be generating considerable E. M. F., while 

 reverser bar A, being farther away from the pole- tip, will 

 be cutting an appreciably weaker field. Thus by experi- 

 mentally shifting the brushes, a position may be found for 

 them such that the difference in the E. M. F. produced 

 respectively by C and A is just so great as to reverse the 

 current in cba and to bring it up to its normal strength in 

 the new direction, within the time that the coil is short- 

 circuited by the brush. 



