i6 DIRECT CURRENT GENERATORS. fExp. 



generated voltage. Likewise, the armature current is so small 

 that armature reactions are negligible, and the curve is practically 

 the same as a separately-excited magnetization curve. There is 

 no necessity, therefore, for taking curves both self- and sepa- 

 rately-excited. By separately exciting a generator, it is pos- 

 sible to obtain a higher magnetization and consequently a higher 

 generated voltage than can be obtained by self-excitation. 



In design work and in manufacturing tests, the saturation curve 

 is commonly plotted with field ampere-turns, instead of amperes, 

 as abscissae. However plotted, the abscissae are proportional to 

 magneto-motive force and the ordinates to magnetic flux.* 



10. Saturation Factor and Percentage of Saturation. There 

 are two ways for expressingf numerically the amount of satura- 

 tion for any point P on the working part of the curve. ( i ) The 

 saturation factor, f, is the ratio of a small percentage increase in 

 field excitation to the corresponding percentage increase in volt- 

 age thereby produced. (2) The percentage of saturation, p, is 

 the ratio OA -f- OB, when in Fig. 2 a tangent to the curve at P 

 is extended to A. 



Compute these two for some one point on the curve, corre- 



* (Qa). Magnetic Units. For electrical quantities there are three sys- 

 tems of units in use the C.G.S. electromagnetic, the C.G.S. electro- 

 static and the practical or volt-ohm-ampere system. For magnetic quanti- 

 ties there is only one system of units in use, the C.G.S. electromagnetic 

 system ; magnetic units of the practical system would be of inconvenient 

 size, they have no names and are never used. 



The unit of magnetic flux is the maxwell, which is one C.G.S. line of 

 force. The unit of flux density is the gauss, which is one maxwell per 

 square centimeter. The unit of magnetomotive force is the gilbert, which 

 is (io-f-4"0 ampere-turn. The unit of reluctance is the oersted, which 

 is a reluctance through which a magnetomotive force of one gilbert pro- 

 duces a flux of one maxwell. The maxwell and the gauss are author- 

 ized by International Electrical Congress, but not the gilbert and the 

 oersted. 



Analogous to Ohm's Law (current = electromotive force -H resistance), 

 we have the corresponding law for the magnetic circuit: flux (maxwells) 

 = magnetomotive force (gilberts) -i- reluctance (oersteds). 



f A. I. E. E. Standardization Rules, 57, 58. 



