5 6 



ELEMENTS OF ELECTRICAL ENGINEERING. 



machines A and B are practically in synchronism [condition 

 (a)] and the switch s, Fig. 131, is closed when the lamp is at its 

 minimum of brightness [condition (^)] . 



In practice the voltages of machines A and B are much too 

 large to permit of the use of a lamp as shown in Fig. 131, and 

 two step-down transformers T and T' are used as shown in 

 Fig. 132, the lamp / being connected in circuit with the two 



Fig. 132. 



secondary coils of the two transformers. With this arrangement 

 the connections of one of the transformers may be reversed so 

 that the lamp may be at its maximum brightness when the elec- 

 tromotive forces of machines A and B are opposed, that is 

 when condition (ft) is right for closing the double-pole switch ss f . 

 The dial synchronizer is a device in which a pointer is caused 

 to rotate at a speed which is equal to the difference of the fre- 

 quencies of machines A and B, Fig. 132. The dial synchro- 

 nizer is now extensively used and one form of this synchro- 

 nizer is described in Chapter XVI. The dial indicates approxi- 

 mate synchronism (condition a) by slowness of speed of the 

 pointer, and it indicates the proper phase relation (condition b) 

 by the position of the pointer on the dial. The chief advantage 

 of the dial synchronizer is that it shows whether the speed of 

 machine B is too great or too small, whereas the sychronizing 

 lamp behaves in exactly the same way whether the speed of 

 machine B be too great or too small. 



