282 THE DIRECT-CURRENT MOTOR CH. XII 



the tip of the pole, will diminish the intensity at the brush 

 and thus assist commutation. 



It is important that the point where the magnetisation 

 due to the magnets is nothing should not shift when the 

 load increases. Any dissimilarity in the form of adjacent 

 pole-tips, such as a straight radial face on one and a 

 tapered horn on the other as in Fig. 66 will cause this 

 point to shift with the load, owing to the unequal saturation 

 of the pole-tips, and this will cause the brushes to spark at 

 a load that might be carried easily if the magnetic field 

 were symmetrical. 



A motor with fixed brushes is always running with too 

 much forward lead, hence a current will flow in the coil 

 that is being short-circuited under the brush. This 

 current will be of the same sign as that which has to be 

 commutated, and the amount of the current may be very 

 large. There will then be a certain number of surface 

 conductors, corresponding to the coils that are being 

 short-circuited, carrying currents considerably greater than 

 the rest of the conductors ; these currents will give rise to 

 circular magnetic fields of their own, especially if the con- 

 ductors carrying them are imbedded in slots, and will 

 distort the curve of magnetisation under the brushes. The 

 curves given in Figs. 67, 68 and 69 show the hump in the 

 magnetisation curves under the brushes due to this 

 action. 



From what has been said it would appear possible to 

 reduce the amount of the magnetisation due to the magnets 

 when running with full armature current, without in- 

 creasing the tendency to spark. This is, in fact, the case, 

 provided the magnetisation due to the magnets is perfectly 

 symmetrical. The reason why a weakening of the magnets 



