PHASE METERS 



543 



will stand at the reference mark. If they are running at the same 

 frequency, but are not in the proper phase relation, the pointer 

 will come to rest at a position depending on the phase differ- 

 ence of the machines. If the frequencies are nearly, but not 

 exactly the same, the pointer will "beat," or move back and 

 forth over the scale. This arrangement is supplemented by a 

 synchronizing lamp, as indicated. The lamp is behind the pointer 

 and the transparent scale and is arranged to be bright when the 

 machines are in synchronism. The incoming machine is to be 

 connected to the bus-bars when the dark pointer coincides with 

 the reference mark and both appear on the light field due to the 

 glowing of the synchronizing lamp. 



Hartmann and Braun Synchroscope. The firm of Hartmann 

 and Braun has devised a synchroscope based on the vibrating 

 reed frequency meter, page 548. This instrument, with a dia- 

 gram of its circuits, is shown in Fig. 327. 



FIG. 327. Hartmann and Braun synchroscope. 



The frequency at the bus-bars and that of the incoming ma- 

 chine are shown on the two vertical banks of reeds. On closing 

 the switch, the upper set of reeds is put in series with the two 

 machines and will be acted upon by the net voltage around the 

 machine circuit. The synchroscope circuits are so arranged that 

 when both machines are running at the normal frequency, shown 

 on banks I and II, and are in the proper phase relation, the 

 reed in the upper set which corresponds to the normal frequency 

 will vibrate continuously with its maximum amplitude. 



Phasing Lamps. Before the invention of such synchron- 

 izing devices as have been described, it was customary to 



