frequency, the greater the opposition of an inductance to an
alternating current. If the frequency be zero, the reactance is zero,
_i.e._, the circuit conducts direct current as mere resistance. If the
frequency be infinite, the reactance is infinite, _i.e._, the circuit
is "open" to the alternating current and that current cannot pass
through it. Compare this with the correlative generalization following
the preceding thought upon capacity.

[Illustration: Fig. 34. Test of Line with Varying Serial Inductance]

Capacity and inductance depend only on states of matter. Their
reactances depend on states of matter and actions of energy.

In circuits having both resistance and capacity or resistance and
inductance, both properties affect the passage of current. The joint
reaction is expressed in ohms and is called _impedance_. Its value is
the square root of the sum of the squares of the resistance and
reactance, or, Z being impedance,

-------------------------
/ 1
Z = / R^{2} + ----------------
\/ C^{2}[omega]^{2}


and


--------------------------
Z = / R^{2} + L^{2}[omega]^{2}