DYNAMOS. 717 



We may determine this fictive resistance y and the electro- 

 motive force E by two experiments, in which the inductors are 

 excited by an external current I, by measuring the induced currents 

 obtained for the same velocity with external resistances x and x. 

 The equations 



E = *'(R + * +7) = *"( R + *'+.?) 

 give 



i'x' ix i' 



y = -- R, E = z - (x -x). 



i-i' ' i-i' 



1271. A knowledge of the function <(!) enables us to deter- 

 mine all the properties of the machine. 



In a series of researches on dynamos, Frohlich has called this 

 quantity the active magnetism ; he found that if the angle of lead 

 is unchanged, it may be represented by an expression of the form 



in which A and B are constants to be determined experimentally. 



Expressed geometrically by taking intensities for the abscissae, 

 this equation represents a hyperbola passing through the origin, 

 and having a horizontal asymptote. It only holds really for mean 

 strengths of the current, in regions in which the curvature of the 

 characteristic is pretty large. 



When the machine acts as an electromotor, and we add to 

 the circuit an external resistance x the current is 



or 



BI- 



B 



In order that the machine may prime itself, we must have 



The minimum velocity for an external resistance null, is then equal 

 to AR. 



