POWER AND POWER-FACTOR. 



67 



Instructions. Connect an alternating arc lamp and 

 choking coil in series. Join them to a source of alternat- 

 ing current in series with an ammeter and the series coil 

 of a wattmeter. Connect the shunt coil of the wattmeter 

 and a voltmeter across the terminals of the circuit. 

 Connect also a voltmeter across the arc. 



Adjust the choking coil* so that the lamp burns with 

 its correct current and voltage. Take several read- 

 ings at short intervals of the current and watts, and of 

 the voltages of the whole circuit and the arc. 



The readings should be tabulated as in the following 

 table. The watts taken by the arc (which forms a 

 non-inductive resistance) are numerically equal to the 

 product of the current and the voltage across the arc.f 



The watts lost in the choking coil and leads are the 

 difference between the total watts of the circuit and the 

 watts taken by the arc. 



The power saved by the use of the choking coil 

 instead of an equivalent resistance is the difference 

 between the total volt-amperes of the circuit, i.e., the 

 " apparent watts," and the volts registered by the watt- 

 meter, since with a non-inductive resistance in place of 

 the coil the total power taken would be the product of 

 current and voltage of the circuit. 



Determination of Power Absorbed in Circuit Containing Arc 

 Lamp and Choking Coil. 



The power factor of the circuit is the value of the 



W 

 fraction -frw~ an d the angle of lag is the angle whose 



. . W -i W 



cosine is or cos 



* Or if this cannot be done, adjust a resistance in the main circuit, so as 

 to alter the voltage of the circuit shown in the diagram. 



t The shunt coil and the series coil forming part of the mechanism of the 

 lamp will be inductive, and may modify this statement very slightly. 



