THE ALTERNATING CURRENT GENERATOR 259 



S in continual vibration at a rate of several vibrations per 

 second. 



Action of the Generator Solenoid. Now consider the 

 action of the generator solenoid A'. Its winding is con- 

 nected across the terminals of the generator (through a 

 potential transformer for high-voltage machines) and 

 the action of its magnetic pull is to raise plunger P f . The 

 plunger P' is suspended from a lever L' and counterbalanced 

 to any desired degree by the adjustable weight D. When 

 the regulator is in adjustment and the voltage of the gen- 

 erator is normal, P f (and hence C" which is carried on the 

 same lever as P') occupies a certain mean position. Voltage 

 on the alternator above normal will lift P f above its normal 

 position and so depress C' below its normal position. A 

 decrease in alternator voltage will have the opposite 

 effects. 



But if C f is lowered it will require less pull on P to make 

 the contacts C, C' open, so they will open on less than normal 

 exciter voltage, and when they open the exciter voltage 

 falls as explained before. But if the exciter voltage falls, 

 a corresponding decrease in the alternator voltage must 

 occur. Hence the total action of the regulator is to bring 

 the alternator voltage back to normal if it tends to rise; 

 also if the alternator voltage falls an increase in the exciter 

 voltage (and hence in the alternator voltage) must take 

 place. 



The Tirrill regulator has been so perfected that it is 

 possible by its use to maintain the voltage of an alternator 

 constant to within one-quarter of one per cent. -4*~*- 



The Regulator may be used to Compound an Alternator. 

 It is many times desired to have the alternator voltage 

 increase with the load so as to overcome the line drop and 

 give constant voltage at some distant point on the line. 

 This is easily done by winding on A' another coil and con- 

 necting it to a current transformer in the generator line, 

 the connection to be so made that the action of this second 



