insulating material. The larger the surfaces, the
closer they are together; and the higher the specific inductive
capacity of the insulator, the greater the capacity of the device. An
insulator used in this relation to two conducting surfaces is called
the _dielectric_.

[Illustration: Fig. 28. Simple Condenser]

[Illustration: Fig. 29. Condenser Symbols]

Two conventional signs are used to illustrate condensers, the upper
one of Fig. 29 growing out of the original condenser of two metal
plates, the lower one suggesting the thought of interleaved conductors
of tin foil, as for many years was the practice in condenser
construction.

With relation to this property, a telephone line is just as truly a
condenser as is any other arrangement of conductors and insulators.
Assume such a line to be open at the distant end and its wires to be
well insulated from each other and the earth. Telegraphy through such
a line by ordinary means would be impossible. All that the battery or
other source could do would be to cause current to flow into the line
for an infinitesimal time, raising the wires to its potential, after
which no current would flow. But, by virtue of electrostatic capacity,
the condition is much as shown in Fig. 30. The condensers which that
figure shows bridged across the line from wire to wire are intended
merely to fix in the mind that there is a path for the transfer of
electrical energy from wire to wire.

[Illustration: Fig. 30. Line with Shunt Capacity]

A simple test will enable two of the results of a short-circuiting
capacity 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 condensers be bridged across the line,
one by one, while conversation goes on. The listening observer will
notice that the sounds reaching his ear steadily grow less loud as the
capacity across the line increases. The speaking observer will notice
that the sounds he hears through the receiver in series with the line
steadily grow louder as the capacity across the line increases. Fig.
31 illustrates the test.

The speaker's observation in this test shows that increasing the
capacity across the line increased the amount of current entering it.
The hearer's observation in this test shows that increasing the
capacity across the line decreased the amount of energy