532 RHEOSTATS AND OTHER BALLAST [Cn. XIII 



736. Reactors, inductors, choke-coils, economy-coils, com- 

 pensator-coils, etc. When alternating current is used the wasteful 

 method of current control by means of a resistor or rheostat where 

 so much electrical energy is transformed into heat should be 

 avoided whenever possible. 



In place of a rheostat such as is described above ( 723 + ) an 

 inductor is used. This consists of a soft -iron core around which is 

 wound a coil of insulated wire. The alternating current passes 

 through this coil; this alternately magnetizes and demagnetizes 

 the soft-iron core and limits the flow of the current. But the 

 energy is not dissipated, for the energy used in magnetizing the 

 core is given up again when the core is demagnetized. It is true 

 that a small amount of the energy is wasted in heating the appar- 

 atus, but the amount is so small (5% to 8%) as compared with that 

 lost in a rheostat that it is negligible. 



Variable amperage can be obtained with an inductor by having 

 the soft-iron core movable so that a greater or less amount of it 

 will be within the coil. 



The more of the soft -iron core within the coil the greater will be 

 the inductance and hence the less the amperage; and conversely, 

 the less of the soft-iron core within the coil the less will be the 

 inductance and the greater the amperage. In fig. 197 the core 

 is only partly inserted in the coil and a medium amount of current 

 is therefore allowed to flow. 



1 



737. Wiring the inductor and transformer. The inductor is 



inserted along one wire (in series) 

 exactly as the rheostat is inserted 

 (fig. 289). With a special arc 

 lamp transformer the line is con- 

 nected to the primary of the trans- 

 former and the arc lamp is con- 

 FIG. 289. INDUCTOR IN SERIES WITH j , ,, , .^, 



AN ARC LAMP. nected to the secondary without 



G Dynamo. the use of resistance (fig. 290). 



=t Alternating current circuit. 



A Arc lamp with right-angle car- 738. Comparison of the 



bons - T ... amount of energy used with an 



L Inductor to serve as ballast with 



alternating current. inductor and with a rheostat. (A) 



