RATINGS AND GUARANTEES. 159 



the whole of the magnetomotive force around the circuit CC is 

 brought into action at the two points a and d where the circuit 

 crosses the gap space. Therefore one half of the whole mag- 

 netomotive force is brought into action at a, and one half at d, 

 Fig. 107, so that, dividing the magnetomotive force across the 

 gap at a or d by the distance / in centimeters across the gap, we 

 have the desired value of 34 ', namely, 



(36) 



The various devices and schemes for reducing the effect of 

 armature reaction are chiefly of interest to the, dynamo builder, 

 but they are of some importance to the operating engineer also, 

 and they are briefly as follows : 



In the first place the demagnetizing action of the armature can 

 be reduced to a very small value by making the brush lead 

 small, a condition which is realized, without excessive sparking, 

 in most modern machines. In the second place, the disturbing 

 effect of cross-magnetizing action depends upon the ratio c^'/c^, 

 where 3f f is the field intensity under the pole tips due to the arma- 

 ture alone, and 3C is the field intensity in the gap space due to 

 the field winding alone. The disturbing effect of armature reac- 

 tion is never troublesome when the value of this ratio is small. 

 (d) A small value of the ratio 3{ f / 3{ may be obtained by using 

 a very large value of &C or, in other words, by using a very 

 strong field excitation. (<) A small value of the ratio ctf f / '&C 

 may be realized by keeping 3f f small ; and to keep &C' small for 

 a given current in each path in the armature and for a given 

 number of armature conductors, requires, according to equation 

 (36), the use of narrow pole faces (small value of 0), and of a 

 wide air gap (large value of /). 



The angular breadth of the pole faces is never reduced below 

 about 65 per cent, of 360 jp degrees, where p is the number of 

 field magnet poles.* 



*The ratio /3-$-36o// (=pole span/pole pitch) is usually made about 0.7 in 

 multipolar dynamos. 



