CH. XIII] RHEOSTATS AND OTHER BALLAST 533 



With an inductor. Let the line voltage be no and the amper- 

 age 55 as shown by the ammeter; the voltage across the arc will 

 be 34 volts. The power consumption will be volts times amperes, 

 that is, in this case, 34 x 55 = 1870 watts or 1.87 kilowatts. As 

 the inductor does not absorb an appreciable amount of energy, 

 the 1.87 kilowatts represents the energy needed to produce the 

 arc light. 



(B) With a rheostat. If now a rheostat is used, the watt- 

 meter will record not only the energy required to maintain the arc 

 light, but also the energy wasted in heating the rheostat. 



For example, suppose as above that the line voltage is no, the 

 amperage 55, and the voltage across the arc is 34. Then as before 

 the arc light requires 34 x 55 = 1870 watts or 1.87 kilowatts. 



But the difference between the 34 volts at the arc and the no 

 volts in the line (76 volts) is used in heating the rheostat. 



The energy used in heating the rheostat is then 76x55 = 4180 

 watts or 4.18 kilowatts. Both this wasted energy as well as the 

 actual energy used in the arc will be recorded on the wattmeter 

 and the user of the arc lamp will have to pay for 1.87 + 4.18 or 6. 05 

 kilowatts to run his lamp instead of the 1.87 kilowatts when the 

 inductor is used. That is it will cost more than three times as 

 much to run the arc lamp with a rheostat as with an inductor or 

 choke-coil. 



STATIONARY TRANSFORMER FOR ALTERNATING CURRENT 



739. Transformer. A transformer is a device for changing 

 the voltage of an alternating electric current. This change may 

 be an increase in the voltage step-up transformer, or a decrease 

 in the voltage step-down transformer. The device consists in a 

 soft -iron ring wound with coils of insulated wire. In the simplest 



738a There is no simple method of economizing with direct current 

 comparable with the use of an inductor with alternating current. Sometimes 

 when one must draw on a current at 220 volts pressure there is used a motor 

 generator set. The motor is driven by the 220 volts current and the genera- 

 tor produces current at 60 to 70 volts pressure. At this voltage only a 

 limited amount of resistance is necessary ( 747), and there is some saving, 

 but not so much as by using an inductor with alternating current. 



