172 



ELEMENTS OF ELECTRICAL ENGINEERING. 



available for starting, the machine is brought up to speed by 

 operating the main direct-current generator as a motor, in which 

 case the small induction motor is not required for starting. 



(c) The synchronous converter. The device which is most fre- 

 quently used for converting alternating current into direct cur- 

 rent is the synchronous converter, which is described in Art. 85. 



(d) The mercury-arc rectifier * consists essentially of a highly 

 ^__ exhausted glass bulb, Fig. 



1 46, with two iron or graphite 

 electrodes connecting to ter- 

 minals AA, and the mercury- 

 pool electrode connecting to 

 terminal B. The terminal C 

 connects to an auxiliary pool 

 of mercury very near to the 

 pool which connects with ter- 



A I 



minal B. The arrangement 

 of the mercury -arc rectifier is 

 shown in Fig. 147, in which 



r\ 



Mercury Arc Rectifier Tube. 

 Fie- 146. 



J represents a storage bat- 

 tery which is being charged. 

 The full-line arrows represent 

 the flow of current during 

 one half of a cycle, and the 

 dotted arrows represent the 

 flow of current during the other half of a cycle. A slight 

 shake of the bulb causes the mercury to bridge over from 

 B to C, and the current starts to flow, making the mer- 

 cury pool B a cathode. Then current can flow readily from 

 either of the terminals A or A' to B t but it cannot flow out 



*See a paper on "The Electric Arc," by C. P. Steinmetz, Transactions of ike 

 International Electrical Congress, Vol. 2, pages 710 to 730, St. Louis, 1904; also 

 a paper on "The Mercury Arc Rectifier," by C. P. Steinmetz, Transactions of the 

 American Institute of Electrical Engineers, Vol. 24, pages 371 to 393, 1905 ; and a 

 paper by P. D. Wagoner, read before the National Electric Light Association at 

 Denver, Colo., in June, 1905. 



