108 TRANSACTIONS OF THE [dEC. 21, 



—— =: constant = — ^ 

 H H, 



Let Rs = resistance of shunt field coils. 



" Ns = number of turns of shunt field coils. 



" Rd = resistance of series field coils. 



'< Nd = number of turns of series fieki coils. 



" E = potential at terminals of motor. 



" I = intensity of current through series coils. 



" R = resistance of armature. 

 E — Rdl = potential at shunt terminals = Es. 



= amperes in shunt coils. 1 



^ -^^j I = amperes in series coils. 



E-Rdl-e ^ «^ '' armature. [ 



ii . J 



from the first equation wehave 



0, 



Es /Es— e Es\ tis ^^, /Es— e, E 



- R^) N^R^- 



Ns R + Rs 



^^ Es ,,,/Es— e i^s\ ^^ bs ^^, /Jb^s— e, i^Js\ 



Eliminating^^ _ _ ^^-^ 



The magnetizing currents in shunt and series windings are sent 

 in opposite directions, and the number of shunt windings is to 

 the number of series windings, as the sum of the resistances of 

 the series windings and the armature is to the resistance of the 

 armature. 



This condition produces a magnetic field whose intensity is 

 directly proportional to the counter electro-motive force, provided 

 the magnets have not reached saturation. 



Mr. Sprague, by ingenious devices, causes the currents to act 

 together to start the motor with a very strong effort, and, once 

 started, reverses one current and sets the contrary currents in the 

 field coils to balancing each other, so as to produce a constant 

 speed. 



For constant potential circuits, this motor will not govern if 

 its theoretical efficiency is less than 50^. On the other hand for 

 constant current circuits such as are used for arc lighting, this 

 motor will not govern if the theoretical efficiency is greater than 

 50 fo. We need not discuss it. 



To avoid s})arking at the brushes, Mr. Sprague has added a 

 third series coil which causes, in the case of dynamos having con- 

 sequent poles, a counter distortion of the poles of the field mag- 



