(I) the current in a circuit (Fig. 1)
containing an inductance L and a total resistance RT , to
which an AC emf 
is applied is
(1)
In this equation 
and 
is called the impedance of the circuit. Note that Z
depends on (1) the applied frequency 
; (2) the total
resistance RT=RL+R , which is the sum of the resistance of
the coil (RL) and the other resistance (R) in the
circuit; (3) the inductance L.
The amplitude of the current 
. Thus from equation (1)
we can write
(2)
A plot of 
vs. 
will be a straight line with slope 
.