CH. XII AKMATUEE KEACTION 281 



Suppose that the magnetisation in the gap due 

 to the magnets is 6,000 lines per square centimetre, 

 a=9'0 cm., S = 0'3 cm. Let the armature be designed to 

 carry such a load that the magnetisation at the pole-tip 

 due to reaction, given by Equation 115, is half that due 

 to the magnets, i.e. 3,000 ; the magnetisation under the 

 brush as given by Equation 116 will be 750. This will 

 be of the wrong sign for commutating, and sparkless 

 commutation will not be assisted, but, on the contrary, will 

 be hindered by this magnetisation. 



Suppose that the dynamo works satisfactorily under 

 these conditions owing to the use of carbon brushes. If 

 now we had to make the armature carry twice 

 the former load, we should have to double the width of 

 the gap to obtain a reaction at the pole-tip, as before, of 

 an intensity half that due to the magnets, supposing the 

 latter to remain unaltered. The magnetisation under the 

 brush would, however, be much increased, in fact, very 

 nearly doubled, the actual value being 1,410. 



Instead of doubling the width of the gap we might 

 increase the value of a, so as to have the same magnetisa- 

 tion under the brush with twice the armature load, S 

 being unaltered ; to do this we should have to rather more 

 than double the space between the pole-tips, the new value 

 a being 18'6 cm. It is true that the effect of the magnets 

 at the tip behind the brush would be reduced to nothing, 

 but this would not affect the sparking conditions, since the 

 magnetisation under the brush remains the same as before. 

 The value of a may be increased by cutting the pole pieces 

 at the tips without altering their relative positions, and 

 in fact anything tending to increase the magnetic reluc- 

 tance between the point where the brush is placed and 



