in the turns of the secondary
winding. The key of Fig. 13 being closed, current flows upward in the
primary winding as drawn in the figure, inducing a downward impulse of
current in the secondary winding and its circuit as noted at the right
of the figure. On the key being opened, current ceases in the primary
circuit, inducing an upward impulse of current in the secondary
winding and circuit as shown. During other than instants of opening
and closing (changing) the local circuit, no current whatever flows in
the secondary circuit.

[Illustration: Fig. 13. Induction-Coil Action]

It is by these means that telephone transmitters draw direct current
from primary batteries and send high-potential alternating currents
over lines; the same process produces what in Therapeutics are called
"Faradic currents," and enables also a simple vibrating contact-maker
to produce alternating currents for operating polarized ringers of
telephone sets.

Detrimental Effects of Capacity. Electrostatic capacity plays an
important part in the transmission of speech. Its presence between the
wires of a line and between them and the earth causes one of the
losses from which long-distance telephony suffers. Its presence in
condensers assists in the solution of many circuit and apparatus
problems.

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