492 CURRENT RECTIFIERS [Cn. XIII 



The small electrode in the bottom of the tube, at C is used in starting the 

 tube. In starting, the tube is first rocked making and breaking a mercury 

 contact. A small amount of current flows through between C and B and starts 

 the arc going, after which it will continue to burn as long as B is the cathode, 

 but if the arc is extinguished even for an instant, it will go out and the tube 

 must be tilted again before it will work. 



pulsating, but the current is always in one direction and the pulsa- 

 tions are so slight that the crater of the positive carbon remains 

 almost as constant as with the direct current furnished by a motor- 

 generator set. 



Both the motor-generator set and the mercury arc rectifier are 

 necessarily expensive. For a small plant to be used much of the 

 time for the arc lamp, and where power is needed for other pur- 

 poses, like the lighting of the house, pumping water, running 

 machinery, etc., etc., it would be cheaper to install one of the 

 modern forms of engines. The cost of running these is relatively 

 very little, much less than for the current supplied to the rectifier 

 or for the motor-generator set. It is also very easy to care for the 

 modern engine used with the generator. 



By adapting the generator set for low voltages (60 volts) it is 

 possible to connect the arc lamp directly without a rheostat, thus 

 saving the energy wasted by heating the rheostat. A rheostat 

 may also be used but if so it is called upon to give very slight reduc- 

 tion in voltage, and therefore uses up but little energy. 



PROJECTION WITH 135 CYCLE AND 25 CYCLE CURRENT 



684. In most places where alternating current is used for 

 lighting, the supply has a frequency of 60 cycles per second, and 

 in this chapter it has generally been assumed that the alternating 

 current has this frequency. There are, however, places in which 

 the supply has a frequency of 135 cycles per second and there are 

 others, especially small towns in the neighborhood of large hydro- 

 electric plants, in which the supply has a frequency of 25 cycles. 

 The authors of this book have had practically no experience with 

 other frequencies than 60 cycles. We have reason to believe how- 

 ever, that with 135 cycle current the arc will give as good results as 

 with 60 cycles and will perhaps have less tendency to show a flicker, 



