148 SCIENCE PROGRESS. 



pole-pieces be far from saturation, the total number of lines 

 passing through the armature is not affected by the cross 

 magnetising force of its current. If, however, the pole- 

 pieces have insufficient area and are therefore near satura- 

 tion, the distortion of the lines may result in an actual dimi- 

 nution of their number owing to the trailing pole-corners 

 being unable to carry the increased number which is 

 crowded into them. This danger can, however, be obviated 

 by proper design. 



The real importance of the cross reaction lies in the 

 fact that it weakens the fringe of lines which extends from 

 the leading pole-tips. It is in this fringe that the section of 

 the winding short-circuited by the brushes must be reversed, 

 and the stronger the current which it is carrying, the 

 stronger must be the field to reverse it ; yet by the very 

 action of the armature current the reversing field is weak- 

 ened. It has thus been established that if the process of 

 commutation is to be carried out successfully without spark- 

 ing, the cross ampere-turns of the armature at full load 1 

 must not bear too great a proportion to the ampere-turns 

 of the field-winding which are expended over the air-gap. 

 Roughly it may be said that experience shows the advisa- 

 bility of not allowing the cross ampere-turns to exceed half 

 the ampere-turns required by the air-gap, or, to put it in 

 another way, the induction at the leading pole-tip must not 

 be reduced to less than about 2500 or 3000 C.G.S. lines per 

 sq. cm., i.e., one-half of the average density over the whole 

 of the polar air-space which usually ranges from about 5000 

 to 6000. Various methods have been suggested and tried 

 for neutralising the effect of the cross ampere-turns of the 

 armature, 2 but the only principle that can be said to be 

 used in practice consists in keeping the length of the air- 

 gap so large that the excitation which it requires is more 

 than double the cross ampere-turns of the armature. 



1 Journal Inst. El. Eng., vols, xix., part lxxxv., p. 95, and xx., part xciii., 

 p. 268. 



2 See "On the Reaction of the Armature Current on the Field" 

 (J. Fischer-Hitmen), Elektrotechnische Zeitschrift, 3rd Feb., 1893, and 

 Electrical World (Ryan), 19th Nov., 1892. 



