344 ELEMENTS OF ELECTRICAL ENGINEERING. 



T' into contact again, after which the exciter voltage rises slightly 

 causing the points T and T' to separate, and so on. The 

 result is that the exciter voltage is kept oscillating very slightly 

 above and below the value necessary to hold the lever L in the 

 position which barely gives contact between T and T' . If the 

 terminal T f is permanently lowered, the exciter voltage is 

 reduced until T and T 1 come into contact and the above- 

 described oscillations begin. If the point T' is permanently 

 raised, the exciter voltage is increased until the above-de- 

 scribed oscillations begin. Therefore, all that is needed to vary 

 the field excitation of the alternator is to cause the point T 1 

 to move up when the field excitation is to be increased, and 

 to cause it to move down when the field excitation is to be 

 decreased. 



This movement of the terminal T 1 is accomplished by means 

 of an A.C. solenoid and a plunger P r . The plunger P 1 is 

 counterpoised by the weight W, and the end of P' is near the 

 middle of the solenoid so that an extremely slight change of the 

 alternating voltage which acts on the solenoid will cause P to 

 move up or down through a considerable distance. Then any 

 very slight reduction of the alternating voltage lowers P 1 and 

 raises the point T 9 which causes an increased excitation of the 

 alternator as above described ; and any very slight increase of the 

 alternating voltage raises P' and lowers the point T' which 

 causes a decreased excitation of the alternator. 



If it is desired to maintain a constant voltage between the alter- 

 nating-current bus bars in the station, the A.C. solenoid is con- 

 nected exactly as an ordinary voltmeter would be (through a 

 potential transformer to the high -voltage alternating-current bus 

 bars). 



If it is desired to cause the station voltage to increase with 

 increase of current output so as to compensate * for line drop of 



*This compensation is not theoretically correct because a variation of the power 

 factor of the receiving circuit with given value of current outpiit of the station affects 

 the line drop. See Art. 152 on the compensated voltmeter. 



