68 POWER AND POWER- FACTOR. 



The watts spent in heating the coil are chiefly spent 

 in heating the windings ( = (?-' R watts). Power will also 

 be spent in hysteresis of the core and in producing eddy 

 currents in the metal of the core, screws, &c. The copper 

 and iron losses can be separated by making a careful 

 measurement of the resistance of the winding of the coil 

 at the working temperature and calculating the power 

 lost in this resistance. 



The relation between the voltages in a circuit com- 

 posed of an arc lamp and choking coil is so important 

 that another experiment is given by way of illustra- 

 tion. 



EXPERIMENT X. DETERMINATION OF VOLTAGE OF 

 CHOKING COIL REQUIRED BY AN ARC LAMP. 



DIAGRAM OF CONNECTIONS. 



Same as Fig. 13 (page 29), with substitution of the arc 

 for the non-inductive resistance R. 



Instructions. Connect the arc lamp in series with a 

 variable choking coil and an ammeter to an alternating 

 current circuit of the given voltage. Connect voltmeters 

 to read respectively the voltage across the arc, coil, and 

 main circuit. 



Adjust the coil until the current and voltage of the 

 lamp are those required, then read the voltage at the 

 terminals of the coil. 



It will be noticed that the total voltage of the circuit 

 is less than the sum of the voltages of the coil and arc. 

 This is due to the fact that the voltage in the coil is not 

 in phase with the voltage of the arc, because it is the 

 resultant of an energy voltage and voltage of self-induc- 

 tion. The energy voltage is in phase with the voltage of 

 the arc, since the arc itself has no self-induction. 



The three voltmeter readings should be drawn as the 

 three sides of a triangle, of which the total voltage 

 forms the hypotenuse (see Fig. 31), in which the values 

 chosen represent those of the circuit in which the readings 

 in the table (page 67) were taken. 



