18 



chines perfectly smooth. The wire coils 

 on the armature, whether this contain 

 iron or not, must therefore occupy a con- 

 siderable space, and this circumstance 

 will, generally speaking, cause the elec- 

 tromotive force obtainable with the dif- 

 ferent forms of fields now considered to 

 be lower than the values given. The 

 reason for this reduction is that, with a 

 wide coil not all the turns are equally 

 and simultaneously influenced by the 

 field, and in certain positions the action 

 is differential. The calculation of the 

 effective mean electromotive force for any 

 configuration of field and armature coils 

 presents no special difficulty, being in 

 fact only an extension of the methods 

 already indicated, though for certain 

 complicated forms the operation is 

 laborious. The author only proposes to 

 deal with some simple cases, which may 

 be regarded as forming the limiting con- 

 ditions of practical designs. As regards 

 the field these cases are : 



(1) Rectangular field, width of poles 

 equal to the pitch, as shown in Fig. 4. 



