CH. XIII] 



CURRENT RECTIFIERS 



489 



CURRENT RECTIFIERS 



681 . While alternating current is mo/e cheaply generated and 

 transmitted, especially if the distance is great, the available light 

 given by the alternating arc is much inferior to that given by a 

 direct current, as can be seen by consulting the table of available 

 candle-powers for different amperages ( 756). On this account 

 and from the noiseless character of the direct current arc, efforts 

 have been made to utilize alternating current to get direct current. 



Up to the present time two methods of doing this for projection 

 purposes have proven themselves successful : 



FIG. 258. MOTOR-GENERATOR SET. 

 (Cut loaned by the General Electric Co.). 



The alternating current motor is at the left, the direct current generator is 

 at the right. The two armatures are mounted on the same shaft. 



682. Motor-generator sets. This is an indirect way of 

 getting direct current from alternating. It consists of an alternat- 

 ing current motor and a direct current dynamo attached to the 

 same shaft. The alternating current is not converted into direct 

 current but is used to furnish mechanical power which drives the 

 direct current dynamo just as it could be driven by a water-wheel, 

 a gas or other engine. 



The efficiency of a motor-generator is about 60%. 



If the dynamo is specially designed for the purpose, the arc lamp 

 can be connected directly to it without using a rheostat so that 

 there is no loss from this cause as must be the case when the rheo- 

 stat is used. (See above 680). 



