ALTERNATORS. 



259 



force given to the external circuit A B would be zero, while 

 the electromotive force of the local circuit joining the machines 

 would be twice the voltage of either machine. 



The condition for switching in parallel is that of no 

 resultant voltage in the local circuit joining the machines 

 i.e., the machines must have : 



(1) Equal voltage. 



(2) Equal periodicity. 



(3) Equal phase. 



The simplest method of determining the condition is 

 shown in Fig. 123, which shows an incandescent lamp S L con- 

 nected in parallel with the switch S employed for connecting 

 the machines together. 



This lamp serves to show whether there is any resultant 

 voltage in the circuit joining the alternators, since it glows 



FIG. 123. USE OF LAMP AS SYNCHRONISER. 



when the machines are in series (i.e., joined -f- to ) and 

 ceases to glow when they are truly in parallel, and the 

 resultant voltage is consequently zero. 



If the two machines run at different speeds, the 

 lamp will appear alternately bright and dull, the changes 

 in the lamp occurring less frequently as the difference in 

 speed becomes less, and as the time taken for the faster- 

 running machine to catch up the slower machine becomes 

 greater. 



A voltmeter with a range equal to double the normal 

 voltage of the alternators may be used instead of the lamp, 

 if it is provided with a damping device to prevent the needle 



