The condenser or other capacity acts as an effective barrier to the
steady flow of direct currents. Applying a fixed potential causes a
mere rush of current to charge its surface to a definite degree,
dependent upon the particular conditions. The condenser does not act
as such a barrier to alternating currents, for it is possible to talk
through a condenser by means of the alternating voice currents of
telephony, or to pass through it alternating currents of much lower
frequency. A condenser is used in series with a polarized ringer for
the purpose of letting through alternating current for ringing the
bell, and of preventing the flow of direct current.

The degree to which the condenser allows alternating currents to pass
while stopping direct currents, depends on the capacity of the
condenser and on the frequencies of alternating current. The larger
the condenser capacity or the higher the frequency of the
alternations, the greater will be the current passing through the
circuit. The degree to which the current is opposed by the capacity is
the reactance of that capacity for that frequency. The formula is

Capacity reactance = 1 /_C_[omega]

wherein _C_ is the capacity in farads and [omega] is 2[pi]_n_, or
twice 3.1416 times the frequency.

All the foregoing leads to the generalization that the higher the
frequency, the less the opposition of a capacity to an alternating
current. If the frequency be zero, the reactance is infinite, _i.e._,
the circuit is open to direct current. If the frequency be infinite,
the reactance is zero, _i.e._, the circuit is as if the condenser were
replaced by a solid conductor of no resistance. Compare this statement