66 ELEMENTS OF ELECTRICAL ENGINEERING. 



the places where the flux enters to the places where the flux 

 leaves the armature, and the shortest possible paths in the field 

 iron from the places where the flux leaves the armature to the 

 places where the flux enters the armature. Shortness of paths in 

 armature and field iron means a great saving in the amount of 

 iron required in the machine for a given amount of useful flux. 



The difference between the bipolar 

 field and the multipolar field in regard 

 to length of path of magnetic flux, is 

 shown in Figs. 57 and 58. These fig- 

 ures show a given sized armature with 

 seven-tenths of its surface covered by 

 pole faces and therefore with seven- 

 tenths of its windings effective at each 

 instant in the production of electromo- 

 tive force. The length of each mag- 

 netic circuit in Fig. 58 is very much 



less than the length of the magnetic circuit shown in Fig. 57. 

 The figures show in a striking way that a much greater amount 

 of iron is required for the bipolar than for the multipolar struc- 

 ture. 



($) Economy of field copper. Shortness of path in iron with 

 given flux density means a saving in the amount of field copper 

 for a given amount of power expended in field excitation (RI 2 

 loss), or a saving in the amount of power expended in field exci- 

 tation for a given amount of field copper. The dependence of 

 field excitation (ampere turns) upon the length and sectional area 

 of the magnetic circuit is discussed in Appendix A. 



(c) Reduction of sparking. The sparking of a dynamo at the 

 brushes depends largely upon the magnetizing action of the arma- 

 ture current. This magnetizing action of the armature current is 

 called armature reaction, and it is proportional to the product of 

 the current in the armature conductors and the number of arm- 

 ature conductors under a pole face. An increase in the number 

 of poles, therefore, reduces the armature reaction, and thereby 

 reduces the tendency to spark at the brushes. 



