544 USE OF BALLAST WITH ARC LAMPS [Cn. XIII 



746. Line voltage exactly equal to arc voltage. It would 

 appear that it might be desirable to use a line voltage of exactly 

 what is required by the arc and omit the rheostat. Suppose in the 

 above example that this were done by using a line voltage of 50 

 volts. Now as the arc voltage is constantly varying owing to slight 

 irregularities in the carbons, to the wearing away of the carbons and 

 to other causes, it is evident that for an instant the arc voltage 

 might drop below 50 volts or it might rise above 50 volts. If the 

 arc voltage should rise above 50 volts, the arc would immediately go 

 out as the supply is but 50 volts, and if the arc voltage should drop 

 slightly below this value, the current would rapidly increase. The 

 result would be that the arc would either go out or else would act 

 like a short circuit. In this example the conditions are unstable; 

 that is, no definite current can be maintained. 



747. Intermediate voltage. In practice an intermediate 

 voltage is sometimes used, that is, dynamos to be used for projector 

 arcs are sometimes designed for about 70 volts. Here the arc is 

 sufficiently stable for practical purposes but requires more atten- 

 tion than with the higher supply voltage. Taking the above 

 example. The arc voltage at 50 volts leaves 20 volts across the 

 rheostat. To give 10 amperes requires 20/10 = 2 ohms resistance. 

 If now the electrodes are brought in contact to start the arc the 

 current will be limited only by the resistance in the rheostat and 

 the current will be 70/2 =35 amperes. If the arc gets long enough 

 to take 60 volts, the difference to be taken up in the rheostat is but 

 10 volts, and the current will drop off to 10/2 = 5 amperes. This, 

 therefore, means that with the smaller margin between the line 

 voltage and the arc voltage, the arc becomes less stable. 



748. Ballast with alternating current. With alternating 

 current, an inductor (choke-coil) is often used instead of a rheostat. 

 This behaves as a ballast in a somewhat similar way to the rheostat 

 but to explain the exact process of regulation would require a more 

 exhaustive discussion of alternating currents than is justified in 

 this book, but see 736. 



