108 



Trans. Acad. Sci. of St. Louis. 



la. 



Dynamo and Motor Surfaces. 



The relations between the current, electromotive force and 

 speed of dynamo-electric machines have been extensively 

 studied ever since Dr. John Hopkinson* first examined the 

 subject. M. Marcel Deprez f called the relation between the 

 current and electromotive force when the speed was constant 

 the characteristic of the machine. This name has been 

 universally adopted. In this paper I will call the relation 



80 



Fig. 1 



Internal Characteristic of Shunt Dynamos with Scales Changed. 



120 



between the three variables — current, electromotive force, 

 and speed — the characteristic surface of the machine and 

 the algebraic expression the characteristic equation. 



* Proc. Inst. Mech. Eng., p. 238. 1879. 

 f La Lumiere Electrique Dec. 3, 1881. 



