544 ELECTRICAL MEASUREMENTS 



depend upon synchronizing lamps. With small low-voltage 

 machines it is sufficient to place an incandescent lamp across the 

 gap of the single pole switch by which the incoming machine is 

 to be connected to the bus-bars. If, when the voltage has been 

 adjusted, the incoming machine is not running at ,the proper 

 frequency, the lamp will be alternately light and dark. As the 

 frequency of the incoming machine is brought toward its proper 

 value, the flicker of the lamp becomes slower and slower. The 

 proper time for closing the switch is when the lamp re- 

 mains dark, for then the voltages on the machine circuits are in 

 opposition. 



To Bus Bars 



To Incoming Machine 



FIG. 328. Arrangement of phasing lamp actuated by two transformers. 



When high voltage machines are used, it becomes necessary 

 to employ transformers. They may be connected so that the 

 proper time for closing the main switch is shown either when the 

 lamp is dark or when it is at full brilliancy. The latter is the 

 better practice as it avoids mistakes due to the failure of the lamp. 

 The two transformers may be combined into one with two pri- 

 maries wound on two different branches of the magnetic circuit 

 and a single secondary wound on a third branch as shown in 

 Fig. 326. 



A fault of these arrangements of phasing lamps is that they 

 give no indication as to whether the speed of the incoming 

 machine should be increased or diminished. 



Siemens and Halske Arrangement of Phasing Lamps. The 

 operation of the arrangement will be understood from the sim- 

 plified diagram, Fig. 329, where only three lamps are shown, and 

 the transformers necessary on a high voltage system are omitted. 



The dotted connection simply denotes that the two neutral 

 points are at the same potential. The noticeable feature of the 



