ALTEENATOES. 199 



consequently gives an indication of the nature of the material 

 employed. 



In the case of an alternator with rotating field, on account 

 of the short magnetic circuit and the high quality of the 

 material employed both for the poles and the whole magnet 

 system, which has to be made structurally very strong and 

 relatively light, the hysteresis and residual magnetism will be 

 low compared with what will usually be found in the case of a 

 machine with stationary field system formed of a massive 

 casting. 



Nature of Curve. From the curve may be obtained 

 directly the absolute value of the magnetic flux corresponding 

 to any voltage, and the vertical scale may be graduated in 

 terms of " lines " instead of volts, if desired. 



This is illustrated by the following example in connection 

 with the curve shown in Fig. 92. 



The machine did not give a perfect sine wave form, but 

 the virtual value of the curve only differed slightly from 

 that of a sine curve. The total magnetic flux cut by the 

 conductors at 100 volts was consequently approximately as 

 follows, 80 being the number of conductors in series, and 

 1,200 revs, per minute being the speed (see formula above). 



e x 10 s x 60 



Z = 



Nnk 

 100 



80 x 1,200 x 22-2 - 2 > 815 ' 00 HneS > 



or, since there are two pairs of poles, 



flux per pole = 1,407,500 lines. 



It will be seen on reference to Fig. 92, that the first part 

 of the curve is approximately straight. This is because the 

 permeability of the iron of the circuit and of the air gap is 

 fairly constant at low inductions, and the magnetic flux 

 consequently increases in the same proportion as the mag- 

 netising force. The slope is determined by the ratio between 

 magnetic flux produced and magnetising force applied. Hence, 

 the tangent of the angle which the straight part of the curve 

 makes with the horizontal will be inversely proportional to 

 the reluctance of the magnetic circuit, since 



-,, magneto-motive force 

 Flux = - 



reluctance 



