262 



ELEMENTS OF ELECTRICAL ENGINEERING. 



battery. A solenoid, S, pulls on an iron plunger which is attached 

 to one end of the lever, //. Two lugs on this lever press on 

 the two piles of carbon plates, R and R f ', which constitute the 

 rheostat. The current in the field winding, F t is zero when 

 the resistances, R and R' , of the piles of carbon plates are equal, 

 the current through F is in one direction when R is greater than 

 R 1 ', and in the other direction when R is less than R f . Any 



Fig. 153. 



increase in the line current above its mean value produces a very 

 slight increase of current in 5 which increases the pull on the 

 lever, reduces R and increases R' , and so excites the field of the 

 booster as to cause the battery to discharge. In a similar manner 

 a decrease of line current below its mean value causes the bat- 



