GRAPHICAL PROPERTIES OF THE CHARACTERISTICS. 



725 



resistance which represents the effects of self-induction, and that the 

 angle of lead of the brushes is unaltered ; for a self-exciting machine 

 with entire current, calling x the useful external resistance, we have 

 the equation 



1 = 



which may be written 



As the characteristic is given by a curve E x as a function of I 

 (Fig. 256), we take OA = i and AB = ; that is to say, that we 



A 



Fig. 256. 



draw a straight line OB in a direction a defined numerically by the 



condition tan a = . The point M at which this right line cuts 

 n 



the characteristic, gives the electromotive force of the current 

 E = n x MP, and the corresponding intensity OP of the current. 



In order that the machine may produce a current in the given 

 conditions, the right line OB must meet the curve that is to say, 

 the angle a must be less than the angle 5 of the tangent OT at the 

 origin. 



If the inductors have an appreciable remanent magnetism, the 

 curve E! no longer passes through the origin ; we may consider this 

 remanent magnetism as equivalent to that which an initial current / 

 would produce, and the characteristic is sensibly represented by 



