EXPRESSION FOR TOWER. 



30 



of the true power to the apparent power is called the power 

 factor. Thus 



Power factor 



true power 

 ~ apparent power 

 = cos0 



The expression for the mean power given to a circuit may be 

 deduced in the following simple manner. 



The instantaneous power given to the circuit is 



El sin pt sin (pt 0) 



VT 

 =~- . 2 sin pt sin (pt 0) 



(3) 



Now the average value of cos (2pt 9) taken over .a complete 

 period is zero, since for every positive value it has an equal 

 negative value. Also cos is constant; therefore the taking 

 the average value of the power over a complete period, we get 



El 



P = ^ {average value of cos average value of cos (Zpl 0)} 



El 



= COS0 



= El cos 



FIG. 8. 



In Fig. 8 curve 1 is the curve of the P.D. Curve 2 shows the 



