CH. XII ARMATURE EEACTION 275 



tapered pole-tips. Under the tapered pole, where the two 

 fields are added, the intensity of magnetisation due to the 

 magnets is hardly increased at all by the superposition of 

 the armature field, at the same time the field under 

 the pole where the effects are subtracted is reduced by 

 the full amount of the armature magnetisation. Hence 

 the area of the resultant curve is considerably less 

 than that due to the magnets only, and the induction 

 factor with 36 amperes in the magnets is therefore 

 reduced by making this current also pass through the 

 armature. 



The mean value of the field under the square pole with 

 36 amperes in the armature is 5,900 lines per square cm. 

 The calculated value of the armature reaction under the 

 tip is 3,900, the mean of the values actually observed at 

 the two tips is 3,750. 



The intensity of the field under the brush due to the 

 armature is 1,100 by Equation 116; experiment shows 

 it to be actually rather over 1,300. The unsymmetrical 

 position of the brushes, and the fact of one pole-tip being 

 square and the other tapered would account for some 

 discrepancy between the calculated and observed values, 

 bearing in mind the assumptions that were made in 

 arriving at the equations used. 



Experiments were made with this dynamo to find by 

 actual measurement the values of the induction factor for 

 different currents in the magnets and armature. 



The machine was driven as a generator by a belt as before ; 

 the first experiment consisted in passing a current through 

 the magnets only, and observing the volts at the brushes 

 at a fixed speed. The results obtained are plotted as a 

 curve in Fig. 70. Horizontal ordinates give current in the 



T 2 



