CHAPTER X. 



Choking Coils for Non-inductive and Inductive Circuits Design of 

 Choking Coils. 



IMPEDANCE COILS. 



56. Impedance coils, or choking coils, as they are often called, 

 are simply coils having low ohmic resistance and high self-induc- 

 tion. They are formed by winding a coil of copper wire round a 

 laminated iron core, and are used for the purpose of absorbing a 

 portion of the pressure between constant potential alternating 

 current mains when it is desired to place between the mains any 

 apparatus which requires a voltage less than that between the 

 mains. 



For example, a single open arc lamp requires a pressure of 

 about 40 volts, and if such a lamp is to be run from 100-volt 

 mains, the pressure must be by some means lowered to suit its 

 requirements. 



An obvious means of accomplishing this would be to place a 

 non-inductive resistance in series with the lamp such that about 

 60 volts would be required to drive the current against the resist- 

 ance, but this is an extremely wasteful device. To illustrate the 

 point, suppose that the arc lamp required a current of 10 amperes 

 at a pressure of 40 volts, and that only 100 volts was available. 

 A resistance of 6 ohms placed in series would then absorb 60 volts, 

 leaving the necessary 40 volts to run the lamp. The power used 

 in running the lamp would be 400 watts, while that wasted in the 

 resistance would be 600 watts. If, as is often the case, the con- 

 sumer was charged so much per ampere-hour, the waste energy 

 does not affect him, since in either case he pays too much for the 

 energy he uses ; if, on the other hand, he is charged per kilowatt- 

 hour, he has to pay for a single lamp for as much energy as would 



