coil or transformer in whose primary circuit a direct current is
increased and decreased. An increase of current in the local winding
produces an impulse of _opposite_ direction in the turns of the
secondary winding; a decrease of current in the local winding produces
an impulse of _the same_ direction 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.