104 SINGLE-PHASE CURRENTS. [Exp. 



upon L, which is a constant of the circuit depending upon its 

 form and dimensions, but also upon the frequency of the alter- 

 nating current supply. 



5. The preceding equations can be written 



The admittance Y of an alternating current circuit, defined as 

 the amperes per volt, is the reciprocal of impedance; Y = I -i-E. 

 The unit of admittance is commonly called the mho. 



6. Power factor, defined as the ratio of true power W to 

 apparent power or volt-amperes El, is always less than (or equal 

 to) unity. Power f actor = cos 6, where is the phase difference 

 between E and /; see Figs. 2 and 7 discussed later. In a circuit 

 with resistance R and reactance X, 



The subject will be most readily understood by considering: 

 first, circuits with R, only ; second, circuits with X, only ; and 

 finally circuits with both R and X. 



7. Series Circuit with Resistance Only. In an alternating 

 current circuit containing only a resistance R, the electromotive 

 force required to make flow a current /, is 



E* = RI, 



as in a direct current circuit. 



The current is in phase with the electromotive force. As the 

 electromotive force rises from zero to a maximum and falls again 

 to zero, the current i at each instant is proportional to the electro- 

 motive force e at that instant; e = Ri. The current is zero 

 when the electromotive force is zero, and is a maximum when 

 the electromotive force is a maximum. 



8. If E is represented as a vector, Fig. 5, the current I is 

 represented as a vector in the same direction or phase as E; 



