THE CONTINUOUS CURRENT GENERATOR 99 



tions of e.m.f. also increase in rapidity until on commercial 

 generators they are in the neighborhood of 1000 or more 

 per second. 



We see, then, that a multiple-coil armature, equipped 

 with a multiple-part commutator, will produce a uhi-direc- 

 tional line e.m.f. of practically constant magnitude; the 

 machine is therefore called a continuous current or direct- 

 current generator. The e.m.f. in the individual coils is 

 alternating in direction; the commutator cannot change 

 this, but it does so change the connection of the coil to the 

 line that what is, in the coil, an alternating e.m.f. becomes 

 uni-directional on the line. 



Method for Calculating the E.M.F. of a Generator. In 

 determining the e.m.f. of a generator it is only necessary 

 to calculate at what rate flux is being cut by all the con- 

 ductors connected in series in one path of the winding. 

 We shall use the term active inductors to indicate those 

 conductors on the armature which are cutting flux at the 

 time considered. Evidently all the inductors on an arma- 

 ture are not active because while some of them lie under 

 the pole face, generating an e.m.f., others must be situated 

 in the interpolar space where there is no flux to be cut 

 and hence they can generate no e.m.f. Generally 60% 

 to 70% of the inductors of a machine are active. 



Also we have to consider the fact that some of the 

 active inductors lie in a weaker field than others. The 

 field may be considered in two parts; that directly under 

 the pole face where the field has normal density, and that 

 near the edge of the pole or pole shoe, where the density 

 of the field is less than normal. This part of the field 

 is called the pole fringe. To calculate the e.m.f. of the 

 machine it is necessary to find the voltage generated, (a) 

 by inductors under the pole face and, (6) by inductors in 

 the pole fringe, and then add the two voltages so obtained. 

 Example of E.M.F. Calculation. We will first consider a 

 bipolar machine wound with 44 coils of 8 turns each of 



