CH. XIII] 



CURRENT RECTIFIERS 



495 



free from flicker the rectifier would doubtless answer perfectly in 

 all other particulars. There is no doubt about the motor-genera- 

 tor; it will give perfect direct current for projection. 



685. Need of apparatus designed for the frequency used. 



All alternating current apparatus is designed to work with one 

 frequency only, that is a transformer, for example, if designed for 

 use on 60 cycle current will not work satisfactorily on either 135 

 or 25 cycles. Hence, in ordering apparatus for alternating current 

 it is necessary to ascertain and specify the frequency as well as the 

 voltage and other particulars of the supply. This information 

 can be furnished by the power company. 



FlG. 264. OSCILLOGRAMS OF THE ANODE CURRENTS. 



(Cut loaned by the General Electric Co.; made from the original photograph). 



Curve A Portion of the current furnished by one anode. 



Curve B Portion of the current furnished by the other anode. 



Note that from a single anode, current flows in one direction only, the mer- 

 cury arc acting as a valve which prevents the current from flowing in the 

 opposite direction. When current ceases in one anode the other anode com- 

 mences to furnish the current. 



