62 



POWER AND POWER-FACTOR 



Take simultaneous readings of voltmeter, ammeter, and 

 wattmeter for each position of the' voltmeter switch. Repeat 

 the readings with several different values of the current 

 obtained by altering the resistance R t outside the portion of 

 the circuit on which the measurement is being made. If it 

 is desired to determine the power-factor in a definite portion 

 of the circuit, the resistances must of course not be 

 altered in the portion of the circuit in which the watts 

 are being measured, since this would at once alter the 

 power-factor. In cases where the power-factor of a motor, 

 coil, or other piece of apparatus is required, the current 

 should always be varied to give the range of readings 

 likely to occur subsequently, since the power-factor will 

 probably vary with the current on account of iron saturation. 



For the purpose of forming a suitable experiment, it is 

 convenient to keep r constant and to determine the power- 

 factor of the circuit for various values of R. 



Enter the readings under headings as follows : 



Determination of Power-factor in an Inductive Circuit. 



The power-factor in the non-inductive portion of the circuit 

 should, of course, be constant and equal to unity. Its 

 measurement is introduced into the experiment chiefly in 

 order to illustrate the difference between the power-factors 

 in the two cases. In practical measurements the readings 

 taken as just described on the non-inductive circuit form a 

 most valuable check on the correctness of the readings of 

 the instruments, and the extra connections and time involved 

 are well repaid by the extra certainty of the results whenever 

 a non-inductive resistance forms a part of the circuit. It is 

 therefore strongly recommended that the watts and volt- 

 amperes in a non-inductive portion of the circuit should be 



