A condenser is a device composed of two or more conductors insulated
from each other by a medium called the _dielectric_. A pair of metal
plates separated by glass, a pair of wires separated by air, or a pair
of sheets of foil separated by paper or mica may constitute a
condenser. The use of condensers as pieces of apparatus and the
problems presented by electrostatic capacity in lines are discussed in
other chapters.

Measurements of Telephone Currents. It has been recognized in all
branches of engineering that a definite advance is possible only when
quantitative data exists. The lack of reliable means of measuring
telephone currents has been a principal cause of the difficulty in
solving many of its problems. It is only in very recent times that
accurate and reliable means have been worked out for measuring the
small currents which flow in telephone lines. These ways are of two
general kinds: by thermal and by electromagnetic means.

_Thermal Method_. The thermal methods simply measure, in some way, the
amount of heat which is produced by a received telephone current. When
this current is allowed to pass through a conductor the effect of the
heat generated in that conductor, is observed in one of three ways: by
the expansion of the conductor, by its change in resistance, or by the
production of an electromotive force in a thermo-electric couple
heated by the conductor. Any one of these three ways can be used to
get some idea of the amount of current which is received. None of them
gives an accurate knowledge of the forms of the waves which cause the
reproduction of speech in the telephone receiver.

[Illustration: Fig. 14. Oscillogram of Telephone Currents]