ELEMENTS OF ELECTRICAL ENGINEERING. 



8. Average values and effective values of alternating electromo- 

 tive force and current. It is customary to speak of an alternating 

 electromotive force as so many volts and of an alternating current 

 as so many amperes, although a given alternating electromotive 

 force or current is a rapidly varying quantity. Thus, an alternat- 



iinic 



Fig. 28. Circuit having resistance and containing a condenser. 



ing current which would be specified as 10 amperes is, of course, 

 a current which is alternating rapidly in direction and varying 

 rapidly in value from zero to a more or less definite maximum. The 

 specified value of an alternating electromotive force or current 

 always refers to the square-root-of-the-average-value-of-the-square 



Axis 



of 



time 



Fig. 29. Coil of wire wound on an iron core. 



of the electromotive force or current, the so-called effective value, 

 as explained below. 



The simple average of the successive values of an alternating 

 electromotive force or current is always zero. This is true even 

 in a case like that shown in Fig. 25, and the only exception is in 



