10 ELECTROMOTIVE FORCE AND CURRENT. 



go through the cycle of changes simultaneously. If the 

 conductors are thus connected in series, the resultant voltage 

 due to the whole winding will vary in the same way as the 

 voltage in each single conductor. Consequently the value 

 of the total voltage will be the sum of the individual voltages 

 at any instant. 



In the case of a multipolar machine a period will be the 

 time during which a conductor moves from one pole to a 

 similar position under the next pole of the same polarity. 

 Thus in a 12-pole alternator there will be six periods during 

 each revolution of the armature. In general, if 



r revs, per minute of armature, or field, 



n = periodicity, 



P number of pairs of poles, 



n = r x.P-7-60. 



Hence large machines running at comparatively low speeds 

 invariably have a large number of poles in order to supply 

 current of the requisite frequency. 



In an electrical distribution system the periodicity is a 

 fixed quantity, and the number of poles and speed of the 

 generator have to be adjusted to suit this condition. 



Increase in Number of Conductors. In order to increase 

 the voltage generated in the armature, the number of con- 

 ductors actually employed is much greater than was assumed 

 above. The total voltage generated in an armature composed 

 of a great number of conductors will only be equal to the 

 sum of the voltages induced in the individual conductors if 

 the following conditions are fulfilled : (1) Conductors under 

 opposite poles must be joined together alternately at the 

 front and back of the armature, so that the electromotive 

 force which is induced in successive conductors in opposite 

 directions is made to act in the same sequence in the complete 

 winding. (2) The conductors passing under a single pole 

 at any time must be grouped together sufficiently closely to 

 enter and leave the pole nearly simultaneously.* 



The connections of such a winding are shown diagra- 

 matically in Fig. 5. 



Production of Current by an Alternator. If the armature 

 winding of an alternator is connected to an external circuit, 



* The effect of a distributed winding is discussed later. 



