ost actual cases. All wires which are
wound into coils, such as electromagnets, possess inductance in a
greatly increased degree. A wire wound into a spiral, as indicated in
Fig. 32, possesses much greater inductance than when drawn out
straight. If iron be inserted into the spiral, as shown in Fig. 33,
the inductance is still further increased. It is for the purpose of
eliminating inductance that resistance coils are wound with double
wires, so that current passing through such coils turns in one
direction half the way and in the other direction the other half.

A simple test will enable the results of a series inductance in a line
to be appreciated. Conceive a very short line of two wires to connect
two local battery telephones. Such a line possesses negligible
resistance, inductance, and shunt capacity. Its insulation is
practically infinite. Let inductive coils such as electromagnets be
inserted serially in the wires of the line one by one, while
conversation goes on. The listening observer will notice that the
sounds reaching his ear steadily grow faint as the inductance in the
line increases and the speaking observer will notice the same thing
through the receiver in series with the line.

Both observations in this test show that the amount of current
entering and emerging from the line decreased as the inductance
increased. Compare this with the test with bridged capacity and the
loading of lines described later herein, observing the curious
beneficial result when both hurtful properties are present in a line.
The test is illustrated in Fig. 34.

The degree in which any current is opposed by inductance is termed the
reactance of that inductance. Its formula is

Inductive reactance = _L_[omega]

wherein _L_ is the inductance in henrys and [omega] is _2_[pi]_n_, or
twice 3.1416 times the frequency. To distinguish the two kinds of
reactance, that due to the capacity is called _capacity reactance_ and
that due to inductance is called _inductive reactance_.

All the foregoing leads to the generalization that the higher the
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 generalizat