OH. XII AKMATUKE KE ACTION 273 



ing with magnets only, and none with armature only, 

 although under brush B the field is considerable, and of 

 the wrong sign for commutation. Using Equation 116 

 we can calculate the intensity of this field. We know 

 that A is 720, i is 12, < is 68, taking the larger of 

 the two poles, 8 is O358, and a is 9-0 cm. Hence, H is 

 850 lines per square cm. ; this is the calculated strength 

 of the field under the brush due to 24 amperes in the 

 armature only. The maximum ordmate of the magnet 

 curve in Fig. 68 represents 5,000 lines per square cm.; 

 using this scale, we see from the diagram that the mean 

 ordinate actually found by experiment under the brush, 

 with 24 amperes in the armature only, was 770, calcula- 

 tion giving 850. 



The calculated value of the intensity of the field due 

 to the armature under the tips of the tapered pole, is 

 2,850 from Equation 115, while the intensity observed, 

 taking the mean of the two, was 2,250. The difference 

 here is probably due to the tapering of the pole-tips. 

 Taking the square pole, with an angular width of 62 we 

 find that the calculated value for the armature reaction 

 under the tips is 2,600, while the observed value 

 is 2,500. 



Fig. 68 gives the results of experiments with 50 per 

 cent, overload, namely with 36 amperes in the magnets 

 and in the armature. There was no sparking when the 

 magnets only were excited. With the current in the 

 armature only there was a slight sparking under brush B, 

 none under brush A. With current in both magnets and 

 armature, brush B sparked badly, but there was no spark- 

 ing under A. 



We see here clearly the effect of the saturation of the 



T 



