ration: Fig. 106. Symbol of Impedance Coil]

Induction Coil. An induction coil consists of two or more windings
of wire interlinked by a common magnetic circuit. In an induction coil
having two windings, any change in the strength of the current flowing
in one of the windings, called the _primary_, will cause corresponding
changes in the magnetic flux threading the magnetic circuit, and,
therefore, changes in flux through the other winding, called the
_secondary_. This, by the laws of electromagnetic induction, will
produce corresponding electromotive forces in the secondary winding
and, therefore, corresponding currents in that winding if its circuit
be closed.

_Current and Voltage Ratios._ In a well-designed induction coil the
energy in the secondary, _i.e._, the induced current, is for all
practical purposes equal to that of the primary current, yet the
values of the voltage and the amperage of the induced current may vary
widely from the values of the voltage and the amperage of the primary
current. With simple periodic currents, such as the commercial
alternating lighting currents, the ratio between the voltage in the
primary and that in the secondary will be equal to the ratio of the
number of turns in the primary to the number of turns in the
secondary. Since the energy in the two circuits will be practically
the same, it follows _that the ratio between the current in the
primary and that in the secondary will be equal to the ratio of the
number of turns in the secondary to the number of turns in the
primary_. In telephony, where the currents are not simple periodic
currents, and where the variations in current strength take place at
different rates, such a law as that just stated does not hold for all
cases; but it may be stated in general that _the induced currents will
be of higher voltage and smaller current strength than those of the
primary in all coils where the secondary winding has a greater number
of turns than the primary_, and _vice versa_.

_Functions._ The function of the induction coil in telephony is,
therefore, mainly one of transformation, that is, either of stepping
up the voltage of a current, or in other cases stepping it down. The
induction coil, however, does serve another purpose in cases where no
change in voltage and current strength is desired, that is, it serves
as a means for electrically separating two circuits so far as any
conductive relation exists, and yet of allowing the fre