SWITCHBOARDS AND SWITCHBOARD APPLIANCES. 337 



rotate at a speed which is equal to the difference in frequency of 

 two sources of alternating current. Figure 288 is an external 

 view and Fig. 289 shows the essential features of the dial syn- 

 chronizer of Mr. Paul M. Lincoln. Suppose that an alternator 

 A is to be started up, brought into synchronism with alternators 



Fig. 288- 



Fig. 289. 



already in operation, and connected to the bus-bars in parallel 

 with the other alternators. The synchronizer being connected 

 as explained below, its pointer does little more than vibrate at 

 first ; but, when the frequency a of the alternator A is nearly 

 as great as the bus-bar frequency b, the pointer begins to rotate 

 at a speed equal to b a, becomes stationary when a = b y and 

 23 



