CH. XII ARMATUKE EEACTION 283 



\g supposed to increase the liability to spark is because 

 such weakening usually takes place in some way tending 

 to disturb the symmetry of the field, as, for instance, when 

 a portion of the winding on one magnet is short-circuited. 

 If a well-designed motor is driven at full speed by some 

 external means, there should be no sparking when the 

 magnets are cut out and the armature is carrying its 

 maximum current. On the other hand, the magnetisa- 

 tion due to the magnets is often so unevenly distributed 

 that sparking is actually produced when the magnets 

 are excited, although the brushes may run sparklessly 

 with no current in the magnets and full current in the 

 armature. 



When a coil as ab in Fig. 71 has its segments equally 

 covered by the brush, the resistance in the two circuits acf 

 and bdf will be equal, and if the currents flow according 

 to Ohm's law, equal portions of the main current pass 

 through each segment, and there is no current in the 

 coil itself, so that when the coil is half-way across the 

 brush the current in the coil is reduced to nothing. As 

 the coil moves, the current in l)d increases, and that 

 in ac decreases, until when segment 1 is about to leave 

 the brush, the whole of the main current is flowing 

 down bd, and commutation will be effected without 

 sparking. 



In practice, however, there is a tendency of the 

 current in the coil to persist in flowing, and this ten- 

 dency is directly proportional to the rate at which the cur- 

 rent is changing in the coil. Now if the current is changing 

 at the rate we want it to change, that is, if it is completely 

 reversed during the time that the coil is under the brush, 

 which is the condition of sparkless commutation, then we 



