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
with the correlative generalization which follows the next thought
upon inductance.

Inductance of the Circuit. Inductance is the property of a circuit
by which change of current in it tends to produce in itself and other
conductors an electromotive force other than that which causes the
current. Its unit is the _henry_. The inductance of a circuit is one
henry when a change of one ampere per second produces an electromotive
force of one volt. Induction _between_ circuits occurs because the
circuits possess inductance; it is called _mutual induction_.
Induction _within_ a circuit occurs because the circuit possesses
inductance; it is called _self-induction_. Lenz' law says: _In all
cases of electromagnetic induction, the induced currents have such a
direction that their reaction tends to stop the motion which produced
them_.

[Illustration: Fig. 32. Spiral of Wire]

[Illustration: Fig. 33. Spiral of Wire Around Iron Core]

All conductors possess inductance, but straight wires used in lines
have negligible inductance in m