170 ALTERNATING CURRENTS 



each switch blade and its clip is nearly zero and the two alterna- 

 tors are in opposition so far as their local series circuits are con- 

 cerned. Two points across which the potential difference is 

 zero may be connected without any resulting disturbance, so 

 that the switch may now be safely closed and the two alternators 

 are in parallel. 



The disadvantage of this method is that lamps are dark even 

 although a very considerable voltage may exist across their 

 terminals, and the machines may be connected in parallel there- 

 fore when considerable voltage difference exists between them. 

 This may do no harm with slow-speed or small-capacity units, 

 but with high-speed turbo-units, which have little armature 

 reactance and are quite "sensitive," there may be considerable 

 disturbance if there exists a substantial phase difference at the 

 time of connecting in parallel. Another objection to this 

 " three dark" method is that the lamps do not show whether the 

 incoming machine is fast or slow. 



The foregoing difficulties may be in part eliminated if the con- 

 nections of two of the lamps, as 1 and 2, Fig. 169, be crossed. 



Fia. 169. Connections for "2 light and 1 dark" method of synchronizing 



with lamps. 



When the machines are in synchronism, 1 and 2 are bright and 3 

 is dark. As one of the bright lamps is increasing and one is 

 decreasing in brilliancy near the point of synchronism, it is possi- 

 ble to determine very accurately the instant at which the switch 

 should be closed. This is called the Siemens-Halske or " two 

 bright and one dark" method. By noting the sequence of 

 brightness of the lamps, it can be determined whether the incom- 

 ing machine is fast or slow. 



The best method is the use of the synchronism indicator or 

 synchroscope described in Chap. Ill, page 71. Such an instru- 



