494 



CURRENT RECTIFIERS 



CH. XIII] 



In order to get good projection when this current supply only is 

 available, a motor-generator set can of course be used, that is, the 

 2 5 cycle current is used as power to drive a direct current dynamo 

 ( 682). The 25 cycle current can be changed to direct current by 

 the use of a rectifier ( 683). Such current would of course be 

 pulsating although always in the same direction. As the authors 

 have never seen an arc supplied from a rectifier on 25 cycle current 

 we can rrake no recommendation except to examine one of these 

 machines in actual operation. If the arc should prove sufficiently 



FlG. 263. OSCILLOGRAMS OF THE POTENTIAL DIFFERENCE BETWEEN THE 



ANODE AND CATHODE. IN RELATION TO THE IMPRESSED ELECTROMOTIVE 



FORCE. 



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



Curve A Potential difference between anode and cathode. 



Note that during half of the wave this difference is equal to the full impressed 

 (line) voltage while during the other half wave the potential difference increases 

 until the voltage has reached the constant value of 14 volts. When this occurs 

 current is caused to flow through the arc and is used on the direct current side 

 of the rectifier. 



Curve B Impressed electromotive force, i. e., instantaneous value of the 

 line voltage. 



